This report contains the collective views of an
    international group of experts and does not
    necessarily represent the decisions or the stated
    policy of either the World Health Organization
    or the United Nations Environment Programme

    Published under the joint sponsorship of
    the United Nations Environment Programme
    and the World Health Organization

    World Health Organization Geneva, 1980

    ISBN 92 4 154072 9

    (c) World Health Organization 1980

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         1.1. Summary
               1.1.1. Introduction
               1.1.2. Noise measurement
               1.1.3. Effects of noise
                Interference with communication
                Hearing loss
                Disturbance of sleep
                Effects on performance
                Miscellaneous effects
               1.1.4. Summary of recommended noise exposure limits

         1.2. Recommendations for further studies


         2.1. Physical properties and measurements

         2.2. Sound perception and its measurement
               2.2.1. Loudness and loudness level
               2.2.2. Calculation and measurement of loudness level
               2.2.3. Sound level and noise level
               2.2.4. The time factor
               2.2.5. Noise exposure scales
               2.2.6. Equivalent continuous sound pressure level
               2.2.7. Level distribution

         2.3. Sources of noise
               2.3.1. Industry
               2.3.2. Road traffic
               2.3.3. Rail traffic
               2.3.4. Air traffic
               2.3.5. Sonic booms
               2.3.6. Construction and public works
               2.3.7. Indoor sources
               2.3.8. Miscellaneous sources


         3.1. Noise-induced hearing loss
               3.1.1. Hearing impairment
                Hearing level, noise-induced threshold
                                 shift, and hearing impairment
                Noise-induced temporary threshold shift
                Noise-induced permanent threshold shift

                Incidence of noise-induced permanent
                                 hearing loss
               3.1.2. Relation between noise exposure and hearing loss
                Laboratory studies
                Occupational hearing loss
                Factors that may influence the incidence
                                 of noise-induced permanent threshold
                Combined effects of intensity and
                                 duration of noise exposure
                Estimation of hearing impairment risk
                The importance of high-frequency hearing
               3.1.3. Effects of impulsive noise
               3.1.4. Infrasound and ultrasound

         3.2. Interference with communication
               3.2.1. Masking and intelligibility
               3.2.2. Speech interference indices
                Articulation index
                Speech Interference Level
                A-weighted sound pressure level
               3.2.3. Perception of speech out-of-doors
               3.2.4. Indoor speech communication

         3.3. Pain

         3.4. Sleep
               3.4.1. Nature of sleep disturbance
               3.4.2. Influence of noise characteristics
               3.4.3. Influence of age and sex
               3.4.4. Influence of previous sleep deprivation,
                       adaptation, and motivation
               3.4.5. Long-term effects of sleep disturbance by noise

         3.5. Nonspecific effects
               3.5.1. The stress response
               3.5.2. Circulatory system responses
               3.5.3. The startle reflex and orienting response
               3.5.4. Effects on equilibrium
               3.5.5. Fatigue

         3.6. Clinical health effects
               3.6.1. Background
               3.6.2. General health
               3.6.3. Mental health

         3.7. Annoyance
               3.7.1. Definition and measurement
               3.7.2. Instantaneous noise dose
               3.7.3. Long-term noise dose
                Aircraft noise
                Road traffic noise

                General environmental noise
               3.7.4. Correlation between noise exposure and annoyance
               3.7.5. Overt reaction

         3.8. Effects on task performance
               3.8.1. Noise as a distracting stimulus
               3.8.2. Effects on tasks involving motor or monotonous
               3.8.3. Effects on tasks involving mental activities


         4.1. Environmental noise

         4.2. Populations affected

         4.3. Specific health criteria
               4.3.1. Physical injury
               4.3.2. Hearing loss
               4.3.3. Nonspecific health effects
               4.3.4. Interference effects

         4.4. General health, welfare, and annoyance criteria


         5.1. Noise control at source

         5.2. Control of sound transmission

         5.3. Reduction in length of exposure

         5.4. Education of workers

         5.5. Ear protection

         5.6. Audiometry



        While every effort has been made to present information in the
    criteria documents as accurately as possible without unduly delaying
    their publication, mistakes might have occurred and are likely to
    occur in the future. In the interest of all users of the environmental
    health criteria documents, readers are kindly requested to communicate
    any errors found to the Division of Environmental Health, World Health
    Organization, Geneva, Switzerland, in order that they may be included
    in corrigenda which will appear in subsequent volumes.

        In addition, experts in any particular field dealt with in the
    criteria documents are kindly requested to make available to the WHO
    Secretariat any important published information that may have
    inadvertently been omitted and which may change the evaluation of
    health risks from exposure to the environmental agent under
    examination, so that the information may be considered in the event of
    updating and re-evaluation of the conclusions contained in the
    criteria documents.



    Dr H. E. von Gierke, Department of the Air Force, Aerospace Medical
         Research Laboratory, Wright Patterson Air Force Base, OH, USA

    Dr E. Gros, Institute for Hygiene and Occupational Medicine,
         University Clinic, Essen, Federal Republic of Germany

    Professor L. L. Karagodina, F. F. Erisman Research Institute of
         Hygiene, Moscow, USSR  (Vice-Chairman) 

    Professor G. E. Lambert, Médecin Inspecteur du Travail Région Midi-
         Pyrénées, Cité Administrative, Toulouse, France

    Professor J. B. Ollerhead, Department of Transport Technology,
         University of Technology, Loughbrough, Leicester, England

    Dr Y. Osada, The Institute of Public Health, Tokyo, Japan

    Professor B. Paccagnella, Institute of Hygiene, University of Padua,
         Verona, Italy

    Dr P. Rey, Institute of Social and Preventive Medicine, University of
         Geneva, Geneva, Switzerland

    Professor R. Rylander, Department of Hygiene, University of
         Gothenburg, Gothenburg, Sweden  (Rapporteur) 

    Professor W. J. Sulkowski, Institute of Occupational Medicine, Lodz,

    Ms A. Suter, Office of Noise Abatement and Control, United States
         Environmental Protection Agency, Washington DC, USA  (Rapporteur)

    Representatives of other organizations

    Dr G. H. Coppée, International Labour Organisation, Geneva,

    Dr W. Hunter, Commission of the European Communities, Luxembourg

    Dr A. Alexandre, Organisation for Economic Co-operation and
         Development, Paris, France

    Mr L. Nielsen, International Organization for Standardization,
         Hellerup, Denmark


    Ms G. Vindevogel, Ministry of Public Health and Family, Brussels,

    Mr L. Baekelandt, Ministry of Public Health and Family, Brussels,


    Ms B. Goelzer, Scientist, Office of Occupational Health, World Health
         Organization, Geneva, Switzerland

    Dr H. W. de Koning, Scientist, Control of Environmental Pollution and
         Hazards, World Health Organization, Geneva, Switzerland

    Dr V. Krichagin, Environment and Occupational Health, WHO Regional
         Office for Europe, Copenhagen, Denmark

    Dr J. Lang, National Institute for Research on Heat and Noise
         Technology, Vienna, Austria  (Temporary Adviser) 

    List of abbreviations and symbols used in this document

    AI             articulation index
     c             speed of sound
    CNEL           community noise equivalent level
    CNR            composite noise rating
     f             frequency
     I             sound intensity
    Ldn            day-night average-sound level
    Le             aircraft exposure level
    Leq            equivalent continuous sound pressure level
    Lp or SPL      sound pressure level
    Lp(A)          A-weighted sound pressure level
    LPN            mean peak perceived noise level
    NEF            noise exposure forecast
    NI             noiseness index
    NIPTS          noise-induced permanent threshold shift
    NITS           noise-induced threshold shift
    NITTS          noise-induced temporary threshold shift
    NNI            noise and number index
    NPL            noise pollution level
    p              root mean square pressure
    p2             mean square sound pressure
    P              sound power
    PNL            perceived noise level
    SIL            speech interference level
    SPL or Lp      sound pressure level
    TNEL           total noise exposure level
    TNI            traffic noise index
    WECPNL         weighted equivalent continuous perceived noise level
    lambda         wavelength


        A WHO Task Group on Environmental Health Criteria for Noise met
    in Brussels from 31 January to 4 February 1977. Dr. H. W. de Koning,
    Scientist, Control of Environmental Pollution and Hazards, Division of
    Environmental Health, WHO, opened the meeting on behalf of the
    Director General and expressed the appreciation of the Organization to
    the Government of Belgium for having made available the necessary
    financial support for the meeting. On behalf of the Government, the
    Group was welcomed by Professor Lafontaine, Director of the Institute
    for Hygiene and Epidemiology, Brussels. The Task Group reviewed and
    revised the second draft criteria document and made an evaluation of
    the health risks from exposure to noise.

        The first draft of the criteria document was prepared by a study
    group that met in Geneva from 5-9 November 1973. Participants of the
    Group included: Dr. T. L. Henderson and Professor G. Jansen (Federal
    Republic of Germany); Dr A. F. Meyer (USA); Professor J. B. Ollerhead
    (United Kingdom, Rapporteur); Professor P. Rey (Switzerland,
    Chairman); Professor R. Rylander (Sweden); Professor W. J. Sulkowski
    (Poland); Dr A. Annoni, Mr E. Hellen, and Mr B. Johansson
    (Consultant), International Labour Organisation (ILO); Dr A.
    Alexandre, Organisation for Economic Co-operation and Development
    (OECD); Dr A. Berlin, Commission of the European Communities (CEC);
    Professor L. A. Saenz, Scientific Committee on Problems of the
    Environment (SCOPE); Mr H. J. Gursahaney, International Civil Aviation
    Organization (ICAO); Dr M. Suess, World Health Organization Regional
    Office for Europe; and Dr G. Cleary and Dr G. E. Lambert, World Health
    Organization, Geneva. Certain sections of the first draft were later
    completed with the assistance of Dr A. Alexandre (OECD), Dr D. E.
    Broadbent (UK), Professor G. Jansen (FRG), and Professor W. D. Ward

        The second draft was prepared by the Secretariat after comments
    had been received from the national focal points for the WHO
    Environmental Health Criteria Programme in Czechoslovakia, Federal
    Republic of Germany, Finland, Greece, Japan, New Zealand, Poland,
    Sweden, Thailand, United Kingdom, USSR, and USA, and from the
    International Labour Organisation, Commission of the European
    Communities, the Organisation for Economic Co-operation and
    Development, the International Civil Aviation Organization, and the
    International Organization for Standardization. Many comments were
    also received from individual experts and commercial concerns
    including E. I. Du Pont de Nemours & Company, Wilmington, Delaware,
    USA, whose contributions are gratefully acknowledged.

        The Secretariat particularly wishes to thank Dr D. Hickish, Ford
    Motor Company Limited, Brentwood, Essex, England, Dr G. E. Lambert,
    Professor J. B. Ollerhead, Professor P. Rey, Professor R. Rylander,
    and Ms A. Suter for their most valued help in the final phases of the
    preparation of the document.

        This document is based primarily on original publications listed
    in the reference section and every effort has been made to review all
    pertinent data and information available up to 1978. In addition,
    reference has often been made to the various publications on noise of
    the International Organization for Standardization that include the
    international standards for noise assessment (ISO, 1971; 1973a;
    1975a). The following reviews and criteria documents have been
    referred to: Burns & Robinson (1970), Karagodina et al. (1972), Burns
    (1973), NIOSH (1973a), US Environmental Protection Agency (1973a), ILO
    (1976), Thiessen (1976), Rylander et al. (1978), and Health and
    Welfare, Canada (1979).

        Details of the WHO Environmental Health Criteria Programme
    including some terms frequently used in the document may be found in
    the general introduction to the Environmental Health Criteria
    Programme published together with the environmental health criteria
    document on mercury (Environmental Health Criteria 1, Mercury, World
    Health Organization, Geneva, 1976) and now available as a reprint.


    1.1  Summary

    1.1.1  Introduction

        Noise can disturb man's work, rest, sleep, and communication; it
    can damage his hearing and evoke other psychological, physiological,
    and possibly pathological reactions. However, because of their
    complexity, their variability, and the interaction of noise with other
    environmental factors, the adverse health effects of noise do not lend
    themselves to a straightforward analysis.

        Probably the most important issue is the industrial noise
    problem, and a need for noise control and hearing conservation
    programmes is widely recognized. Road traffic is the main source of
    community noise that may disturb large segments of the urban
    population. Also of worldwide concern is aircraft noise, which can
    significantly affect the mode of life of people living in the vicinity
    of airports.

    1.1.2  Noise measurement

        Sound is produced by the vibration of bodies or air molecules and
    is transmitted as a longitudinal wave motion. It is, therefore, a form
    of mechanical energy and is measured in energy-related units. The
    sound output of a source is measured in watts and the intensity of
    sound at a point in space is defined by the rate of energy flow per
    unit area, measured in watts per ms. Intensity is proportional to the
    mean square of the sound pressure and, as the range of this variable
    is so wide, it is usual to express its value in decibels (dB)a.
    Because the effects of noise depend strongly upon frequency of sound
    pressure oscillation, spectrum analysis is important in noise


    a  decibel = a measure on a logarithmic scale of a quantity such as
       sound pressure, sound power, or intensity with respect to a
       standard reference value (0.0002 microbars for sound pressure,
       10-12W for sound power, and 10-12W/m2 for intensity). Thus,
       for example, when the sound intensity increases by a factor of
       1.26 (= 100.1), it is said to have increased by 1 decibel (dB);
       1 Bel equals 10 dB or a factor of 10 in intensity. The standard
       reference values are implied throughout this document unless
       otherwise stated.

        The perceived magnitude of sound is defined as loudness and its
    decibel equivalent is known as the loudness level. The loudness is a
    function of both intensity and frequency, and various procedures exist
    by which it may be estimated from physical measurements. The simplest
    methods involve the measurement of the sound pressure level (SPL)
    through a filter or network of filters that represent the frequency
    response of the ear. Despite the existence of other slightly more
    accurate but more complex techniques, the A-weighted sound pressure
    level scale is gaining widespread acceptance and is revommended for
    general use.b Whatever procedure is used, such frequency-weighted
    measurements are referred to simply as sound (or noise) levels.

        Measurements of sound level may be averaged over two distinctly
    different periods of time. Steady sound levels and instantaneous
    levels of variable sounds are measured on a very short time scale of
    1 second or less. Variable sounds can be measured with a much longer
    average time, over periods of hours if necessary, and are expressed in
    terms of the equivalent continuous sound pressure level (Leq). This
    convenient measure of average noise exposure using the A-weighting
    correlates reasonably well with many human responses to noise and is
    recommended for general use.

        Many noise indices have been developed for predicting human
    reaction to various noise levels. Some of these incorporate non-
    acoustic factors that influence the reaction. Although the use of such
    indices is not to be discouraged, it is desirable to adopt a uniform
    approach to noise measurement, whenever possible.

    1.1.3  Effects of noise  Interference with communication

        Although there appears to be no firm evidence, it is believed
    that interference with speech in occupational situations may lead to
    accidents due to inability to hear warning shouts etc. In offices,
    schools, and homes, speech interference is a major source of
    annoyance. Many attempts have been made to develop a single index of
    such interference, based on the characteristics of the masking noise,
    that directly indicates the degree of interference with speech
    perception. Such indices involve a considerable degree of
    approximation. The following are the three most widely used:


    b  To obtain a single number representing the sound level of a noise
       containing a wide range of frequencies in a manner representative
       of the ear's response, it is necessary to modify the effects of
       the low and high frequencies with respect to the medium
       frequencies. The A-filter is one particular frequency weighting
       and, when this is used, the resulting sound level is said to be

         Articulation index (AI). This is the most complicated index,
    since it takes into account the fact that some frequencies are more
    effective in masking speech than others. The frequency range from 250
    to 7000 Hz is divided into 20 bands. The difference between file
    average speech peak level in each of these bands is calculated and the
    resulting numbers combined to give a single index.

         Speech interference level (SIL). SIL was designed as a
    simplified substitute for the AI. It was originally defined as the
    average of the now obsolete octave-band SPLs in the 600-1200,
    1200-2400, and 2400-4800 Hz octaves. At the present time, SIL, based
    upon the octave band levels at the preferred frequencies of 500,
    1000, 2000, and 4000 Hz, is considered to provide a better estimate of
    the masking ability of a noise. As SIL does not take the actual
    speech level into account, the associated masking effect depends upon
    vocal effort and speaker-to-listener distance.

         A-weighted sound level. This is also a convenient and fairly
    accurate index of speech interference.

        It is usually possible to express the relationship between noise
    levels and speech intelligibility in a single diagram, based on the
    assumptions and empirical observations that, for speaker-to-listener
    distances of about 1 m:

        (a) speech spoken in relaxed conversation is 100% intelligible in
    background noise levels of about 45 dB(A), and can be understood
    fairly well in background levels of 55 dB(A); and

        (b) speech spoken with slightly more vocal effort can be
    understood well, when the noise level is 65 dB(A).

        For outdoor speech communication, the "inverse square law"
    controls speech transmission over moderate distances, i.e., when the
    distance between speaker and listener is doubled, the level of the
    speech drops by approximately 6 dB. This relationship is less likely
    to apply indoors, where speech communication is affected by the
    reverberation characteristics of the room.

        In cases where the speech signals are of paramount importance,
    e.g., in classrooms or conference rooms, or where listeners with
    impaired hearing faculties are involved, e.g., in homes for aged
    people, lower levels of background noise are desirable.  Hearing loss

        Hearing loss can be either temporary or permanent. Noise-induced
    temporary threshold shift (NITTS) is a temporary loss of hearing
    acuity experienced after a relatively short exposure to excessive
    noise. Pre-exposure hearing is recovered fairly rapidly after
    cessation of the noise. Noise-induced permanent threshold shift
    (NIPTS) is an irreversible (sensorineural) loss of hearing that is

    caused by prolonged noise exposure. Both kinds of loss together with
    presbyacusis, the permanent hearing impairment that is attributed to
    the natural aging process, can be experienced simultaneously.

        In the quantification of hearing damage, it is necessary to
    differentiate between NIPTS, hearing level (the audiometric level of
    an individual or group in relation to an accepted audiometric
    standard), and hearing impairment.

        NIPTS is the hearing loss (i.e., the reduction of hearing level)
    attributable to noise exposure alone, disregarding losses due to
    aging. NIPTS occurs typically at high frequencies, usually with a
    maximum loss at around 4000 Hz. Noise-induced hearing loss occurs
    gradually, usually over a period of years. Once there is considerable
    hearing loss at a particular frequency, the rate of loss usually
    diminishes. Audiometrically, noise-induced losses are similar to
    presbyacusis. Hearing loss due to prolonged excessive noise exposure
    is generally associated with destruction of the hair cells of the
    inner ear. The severity of hearing loss is correlated with both the
    location and the extent of damage in the organ of Corti.

        "Hearing impairment" is usually defined as the hearing level at
    which individuals begin to experience difficulties in everyday life.
    It is assessed in terms of difficulty in understanding speech. The
    amount of loss at the speech frequencies has been used as a basis for
    compensation and varies from one country to another. The unweighted
    average of the losses, in dB, at 500, 1000, and 2000 Hz that is widely
    used for assessing noise-induced hearing impairment, is somewhat
    misleading since most hearing loss usually occurs at 2000 Hz and
    above. Consequently, there is an increased tendency to include the
    frequencies of 3000 and 4000 Hz in damage assessment formulae.

        Attempts have been made to establish the levels of noise that are
    permanently damaging to the ear and to identify individual
    susceptibility to NIPTS on the basis of NITTS measurements. However,
    the validity of the connection between NITTS and NIPTS has not been

        There is also some disagreement concerning the relationship
    between the relative ear-damaging capacity of the noise level and its
    duration. However, the hypothesis that the hearing damage associated
    with a particular noise exposure is related to the total energy of the
    sound (i.e., the integrated product of intensity and time) is rapidly
    gaining favour for practical purposes. Thus, noise should preferably
    be described in terms of equivalent continuous sound level, Leq,
    measured in dB(A). For occupational noise, the level should be
    averaged over the entire 8-h shift (Leq (8-h)).

        Available data show that there is considerable variation in human
    sensitivity with respect to NIPTS. The hazardous nature of a noisy
    environment is therefore described in terms of "damage risk". This may
    be expressed as the percentage of people exposed to that environment

    who are expected to suffer noise-induced hearing impairment after
    appropriate allowance has been made for hearing losses due to other
    causes. It is now accepted that this risk is negligible at noise
    exposure levels of less than 75 dB(A) Leq (8=h) but increases with
    increasing levels. Based on national judgements concerning "acceptable
    risk", many countries have adopted industrial noise exposure limits of
    85 dB(A) + 5dB(A) in their regulations and recommended practices.

        The exposure to ototoxic drugs such as certain aminoglycosidic
    antibiotics however, can lower the threshold below which noise can
    damage the ear.

        It is not yet clear whether the damage risk rules already
    mentioned can be extended to the very short durations of impulsive
    noise. Available evidence indicates that a considerable risk exists,
    when impulsive sound levels reach 130-150 dB, depending upon the
    temporal characteristics of the impulse.

        Although there is a fairly wide range of individual variability,
    especially for high frequency stimuli, the threshold of pain for
    normal ears is in the region of 135-140 dB sound pressure level. Aural
    pain should always be considered to be an early warning sign of
    excessive noise exposure.

        Wherever possible, problems of noise control should be tackled at
    source, i.e., by reducing the amount of noise produced. An acceptable
    alternative is to isolate people from the noise by the use of noise
    insulation, including soundproof enclosures, partitions, and acoustic
    barriers. If this is not possible, the risk can also be minimized by
    limiting the duration of exposure. Only in cases where these control
    measures are impracticable should personal ear protection be
    considered. These devices can and do provide useful protection but
    inherent problems include those of proper fitting and use, and a
    degree of discomfort.

        If there is any risk of hearing damage, pre-employment and
    follow-up audiometric examinations of workers should be carried out to
    detect changes in hearing acuity that might indicate possible
    development of NIPTS, in order to initiate preventive action.  Disturbance of sleep

        Noise intrusion can cause difficulty in falling asleep and can
    awaken people who are asleep. Detailed laboratory studies of the
    problem have been made by monitoring electroencephalographic (EEG)
    responses and changes in neurovegetative reactions during sleep.

        Studies have indicated that the disturbance of sleep becomes
    increasingly apparent as ambient noise levels exceed about 35 dB(A)
    Leq. It has been found that the probability of subjects being
    awakened by a peak sound level of 40 dB(A) is 5%, increasing to 30% at
    70 dB(A). Defining sleep disturbance in terms of EEG changes, the

    probability of disturbance increases from 10% at 40 dB(A) to 60% at
    70 dB(A). It has also been observed that subjects who sleep well
    (based on psychomotoric activity data) at 35 dB(A) Leq complain
    about sleep disturbance and have difficulty in falling asleep at
    50 dB(A) Leq and even at 40 dB(A) Leq. Weak stimuli that
    are unexpected can still interfere with sleep.

        Within a population, differences in sensitivity to noise occur
    related, for example, to age and sex. Adaptation has been observed
    only when noise stimuli are of low intensity. Even though sleep is
    more disturbed by noise rich in information, habituation to such noise
    has been observed. Based on the limited data available, a level of
    less than 35 dB(A) Leq is recommended to preserve the restorative
    process of sleep.  Stress

        Noise produces different reactions along the hypothalamo-
    hypophyseal-adrenal axis including an increase in adenocorticotropic
    hormone (ACTH) release and an elevation of corticosteroid levels. Some
    of these reactions have been elicited in an acute form in laboratory
    experiments at rather moderate levels of noise.

        Effects on the systemic circulation such as constriction of blood
    vessels have been produced under laboratory conditions and a high
    incidence of circulatory disturbances including hypertension has been
    found in noise-exposed workers. A tendency for blood pressure to be
    higher in populations living in noisy areas around airports has been
    suggested but no conclusive evidence of this has been presented.

        Noise affects the sympathetic division of the autonomic nervous
    system. Eye dilation, bradycardia, and increased skin conductance are
    proportional to the intensity of noise above 70 dB SPL, without
    adaptation to the stimulus.

        Other sympathetic disturbances, such as changes is
    gastrointestinal motility, can be produced by intense sound. Medical
    records of workers have shown that, in addition to a higher incidence
    of hearing loss, noise-exposed groups have a higher prevalence of
    peptic ulcer; however, a causal relationship has not been established.

        More studies are required to determine the long-term health risks
    due to the action of noise on the autonomic nervous system.  Annoyance

        Noise annoyance may be defined as a feeling of displeasure evoked
    by a noise. The annoyance-inducing capacity of a noise depends upon
    many of its physical characteristics including its intensity, spectral
    characteristics, and variations of these with time. However, annoyance

    reactions are sensitive to many nonacoustic factors of a social,
    psychological, or economic nature and there are considerable
    differences in individual reactions to the same noise.

        Attempts to define criteria linking noise exposure and annoyance
    have led to the development of many methods for the measurement of
    both variables. In social surveys, questionaires are used to assess
    the annoyance felt by an individual in response to various types of
    noise. Much research has been aimed at the definition of suitable
    questions through which annoyance reactions could be quantified.

        In the search for a suitable noise index, numerous noise and some
    nonacoustic variables were assembled in various ways to discover which
    combinations were most closely correlated with annoyance reactions.
    The resulting diverse indices were given such names as composite noise
    rating (CNR), community noise equivalent level (CNEL), noise and
    number index (NNI), and noise pollution level (NPL) among many others.
    In fact, many experts consider that, in terms of annoyance prediction
    ability, there is little practical difference between the various
    indices and that an appropriate index should be selected for the
    convenience with which it can be measured or calculated. For this
    reason, variants of the equivalent continuous A-weighted sound
    pressure level (Leq) are being widely adopted for general use. These
    are conveniently applied to noise exposure patterns of all kinds, from
    multiple sources if necessary, and are reasonably well correlated both
    with annoyance and with other specific effects of noise.

        Whatever noise scale is used to express noise exposure, it must
    be recognized that, at any level of noise annoyance, reactions will
    vary greatly because of psychosocial differences. A useful technique
    for accommodating the possible extent of individual variation is the
    use of a criterion curve showing the percentage of persons who will be
    annoyed as a function of noise level.

        Such curves have been derived for a variety of noise conditions
    but mainly for those concerned with aircraft or road traffic noise. On
    the basis of these, it can be concluded that, in residential areas
    where the general daytime noise exposure is below 55 dB(A) Leq,
    there will be few people seriously annoyed by noise. This is
    recommended as a desirable noise exposure limit for the general
    community, even though it will be difficult to achieve in many urban
    areas. Some residents may consider this level too high, especially as
    substantially lower levels currently prevail in many suburban and
    rural areas.

        Criteria relating noise exposure and complaint potential have
    found widespread application for environmental control purposes in
    some countries. However, the scientific basis for such criteria is
    rather fragmentary and surveys have indicated that the correlation
    between noise exposure and individual complaint behaviour is low. This
    may be explained in terms of the strong influence of psychosocial
    factors.  Effects on performance

        The effect of noise on the performance of tasks has mainly been
    studied in the laboratory and, to some extent, in work situations,
    but, there have been few, if any, detailed studies of the effects of
    noise on human productivity in real-life situations. It is evident
    that when a task involves auditory signals of any kind, noise at an
    intensity sufficient to mask or interfere with the perception of these
    signals will interfere with the performance of the task.

        Noise can act as a distracting stimulus, depending on how
    meaningful the stimulus might be, and may also affect the psycho-
    physiological state of the individual. A novel event, such as the
    start of an unfamiliar noise will cause distraction and interfere with
    many kinds of tasks. Impulsive noise (such as sonic booms) may produce
    disruptive effects as the result of startle responses which are more
    resistant to habituation.

        Noise can change the state of alertness of an individual and may
    increase or decrease efficiency.

        Performance of tasks involving motor or monotonous activities is
    not always degraded by noise. At the other extreme, mental activities
    involving vigilance, information gathering, and analytical processes
    appear to be particularly sensitive to noise. It has been suggested
    that, in industry, the most likely indicator of the effects of noise
    on performance would be an increase in accidents attributable to
    reduced vigilance.  Miscellaneous effects

        Certain noises, especially impulsive ones, may induce a startle
    reaction. This consists of contraction of the flexor muscles of the
    limbs and the spine, a contraction of the orbital which can be
    recorded as an eye blink, and a focusing of attention towards the
    location of the noise. The startle reflex to acoustic stimulation has
    been observed in the 27-28 week fetus in utero as a change in the
    pulse rate.

        It has been suggested that observed noise-induced equilibrium
    effects are due to the noise stimulating the vestibular apparatus, the
    receptors of which are part of the inner ear structure.

        Although there is no clear evidence of a direct relationship
    between noise and fatigue, noise can be considered as an environmental
    stress which, in conjunction with other environmental and host
    factors, may induce a chronic fatigue that could lead to non-specific
    health disorders.

    1.1.4  Summary of recommended noise exposure limits

        The equivalent continuous A-weighted sound pressure level Leq
    is recommended for use as a common measure of noise exposure. The
    measurement period should be related to the problem under study, for
    example in the case of occupational noise, Leq (8-h) would be
    measured for a complete 8-h shift.

        For the working environment, there is no identifiable risk of
    hearing damage in noise levels of less than 75 dB(A) Leq (8-h). For
    higher levels, there is an increasing predictable risk and this must
    be taken into account when setting occupational noise standards.

        In other occupational and domestic environments, acceptable noise
    levels can be established on the basis of speech communication
    criteria. For good speech intelligibility indoors, background noise
    levels of less than 45 dB(A) Leq are required.

        At night, sleep disturbance is the main consideration and
    available data suggest a bedroom noise limit of 35 dB(A) Leq.

        Data from surveys of community noise annoyance lead to the
    recommendation that general daytime outdoor noise levels of less than
    55 dB(A) Leq are desirable to prevent any significant community
    annoyance. This is consistent with speech communication requirements.
    At night, a lower level is desirable to meet sleep criteria; depending
    upon local housing conditions and other factors this would be in the
    order of 45 dB(A) Leq.

    1.2  Recommendations for Further Studies

        Considerable research aimed at improving the scientific basis and
    application of environmental health criteria for noise is in progress
    in many countries. However, there are certain areas where present
    national and international efforts do not appear adequate. Thus,
    further studies should include:

        (a) The identification of long-term health effects due to high
    level industrial noise and lower level general environmental noise.
    The potential contribution of noise stress to the general morbidity of
    the population, the ability of people to adapt to environmental noise,
    and the possibilities of noise-induced disease must be established not
    only for the working population, but also for the more vulnerable
    population segments, including the elderly, pregnant women, people
    undergoing medication, particularly with ototoxic drugs such as
    salicylates, quinine, and certain antibiotics, and those generally
    under stress. The possibility that the disturbance of sleep by noise
    can result in definite health impairment should be examined as part of
    these investigations.

        (b) Studies on young people over many years prior to, and during,
    occupational noise exposure to find out to what extent changes in
    hearing acuity during adolescence are attributable to normal growth or
    to environmental conditions, to learn about noise susceptibility in
    childhood, and to obtain data on the progressive effects of noise
    (including high-level music and other leisure-time sounds) on the
    "normal" hearing level of the population. Monitoring of the total
    noise exposure of these groups over the whole observation period would
    be part of these studies. Similar studies in nonindustrialized
    countries would be of particular value.

        (c) Work on the development of sensitive hearing tests and on
    tests to evaluate the problem of individual susceptibility to noise,
    since pure tone audiometry is only a crude technique for measuring
    hearing acuity and for detecting pathological damage.

        (d) Longitudinal studies of communities exposed to major changes
    in environmental noise to refine existing dose-response (noise-
    annoyance) relationships and to include the effects of adaptation and
    societal changes on public reaction to noise. Attention should be
    given to the study of the response of specially vulnerable segments of
    the population.

        The methods of study should be internationally uniform, as far as
    is feasible, to allow pooling of data and broader interpretation of
    the results.


        Noise is considered as any unwanted sound that may adversely
    affect the health and well-being of individuals or populations.

        Physically, sound is a mechanical disturbance propagated as a
    wave motion in air and other elastic or mechanical media such as water
    or steel.

        Physiologically, sound is an auditory sensation evoked by this
    physical phenomenon. However, not all sound waves evoke an auditory
    sensation: for example, ultrasound has a frequency too high to excite
    the sensation of hearing.

        The physical properties and perception of sound or noise are
    expressed and measured in different concepts and units.

    2.1  Physical Properties and Measurements

        Sound waves involve a succession of compressions and rarefactions
    of an elastic medium such as air. These waves are characterized by the
    amplitude of pressure changes, their frequency, and the velocity of
    propagation. The speed of sound  (c), the frequency  (f), and
    the wavelength (lambda), are related by the equation

    lambda = c/f

        A mechanical energy flux accompanies a sound wave, and the rate
    at which sound energy arrives at, or passes through, a unit area
    normal to the direction of propagation is known as the sound
    intensity,  I. In a free sound field, the sound intensity is related
    to the root mean square a sound pressure,  p, and the density of the
    medium,  q, by the expression

     I =        
          q c

    Sound intensity is normally measured in watts per square metre
    (W/m2). The total sound energy emitted by a source per unit time is
    known as the sound power, P, and is measured in watts.

        Sound intensities of practical interest cover a very large range
    and are therefore measured on a logarithmic scale. The relative
    intensity level of one sound with respect to another is defined as 10
    times the logarithm (to the base 10) of the ratio of their


    a The square root of the mean value of the squares of the
      instantaneous values of a quantity. For a periodic variation, the
      mean is taken over one period.

    intensifies. Levels defined in this way are expressed in decibels
    (dB). Any acoustic quantity that is related to sound energy, e.g.,
    power, intensity, or mean square pressure, may be expressed as a
    decibel level. To establish an absolute level, a reference value must
    be agreed. Thus, the sound pressure level of a sound with a mean
    square sound pressure p2 is:

    Table 1.  Table for combining intensity levels

    Excess of stronger       Add to the stronger to get
       component                   combined level

           d8                          d8
            0                         3.0
            1                         2.5
            2                         2.0
            3                         1.8
            4                         1.5
            5                         1.2
            6                         1.0
            7                         0,8
            8                         0.7
            9                         0.6
           10                         0.5

                    p   2
    Lp = 10 log10 (   )  dB

    where the reference pressure pref has an internationally agreed
    value of 20 micropascals (µPa) (ISO, 1959). The reference values for
    sound power level and sound intensity level are 10-12 watts and 
    10-12 W/m2, respectively (ISO, 1963). Sound levels are expressed in
    decibels (dB) relative to the international standard reference
    quantities, unless otherwise stated (dB re: 20 µPa).

        Whereas sound intensities or energies are additive,b sound
    pressure levels (SPL) (in decibels) have to be first expressed as mean
    square pressures, and then added. The summation of sound pressure
    levels can be easily performed by using the following equation:

                      Lp1      Lp2       Lp3
    Lp = 10 log10 [         +         +       .... ] dB
                    1010     1010      1010


    b Such combinations of decibel values may be simplified by using
      Table 1.

        A simple example will illustrate the use of this equation. If two
    sound sources of 80 dB SPL each have to be combined, then

         L = 10 log10 [108 + 108]

           = 10 log10 2 + 80 = 10 X 0.301 + 8 = 83 dB

        It is only when two sources generate similar levels that there is
    a significant increase in level when the sources are combined. The
    example just quoted gave a 3 dB increase. If there is any difference
    in the original, independent levels, the combined level will exceed
    the higher of the two levels but by less than 3 dB. When the
    difference between the two original levels exceeds 10 dB, the
    contribution of the quieter source to the combined noise level is

        Sound is measured with a microphone that generates a voltage
    proportional to the acoustic pressure acting upon it. This signal can
    be measured and analysed using conventional electronic
    instrumentation. A sound level meter is usually a portable, self-
    contained instrument incorporating a microphone, amplifiers, a
    voltmeter and attenuators, the whole of which carl be calibrated to
    read sound pressure levels directly. Intensity levels and power levels
    can be derived from sound pressure level measurements if required.

        The sound at a given location can be completely described in
    terms of the history of the sound pressure fluctuation. If this
    fluctuation is periodic, its fundamental frequency is the number of
    repetitions per second, expressed in hertz (Hz). Most real periodic
    cycles are quite complex and consist of a component at the fundamental
    frequency and components at multiples of this basic frequency, known
    as harmonics.

        The simplest kind of sound, known as a pure tone, has a
    sinusoidal pressure cycle that is completely defined in terms of a
    single frequency and pressure amplitude (a more precise definition
    would also include phase which effectively defines the starting point
    in time, but this is usually of little or no interest).

        Pure tones are relatively rare -- perhaps the nearest
    approximation is the sound of a tuning fork. Most musical sounds are
    periodic but contain many harmonics. Analytically these may be
    expressed as a sum of harmonically related components. This assembly
    is known as the frequency spectrum of the sound, and it specifies how
    the energy in the periodic sound is concentrated at certain discrete
    frequencies. The frequency distribution of sound energy is measured by
    electronic filters.

        Although some kinds of machinery produce sound that is largely
    periodic, most noise is nonperiodic, i.e., the sound pressure does not
    oscillate with time in any regular or predictable way. Such sound is
    said to be random. Examples of random sound include the roar of a jet

    engine, the rumble of distant traffic, and the hiss of escaping steam.
    The energy of random sound is distributed continuously over a range of
    frequencies instead of being concentrated at discrete values, so that
    its frequency spectrum may be depicted as a curve of energy density
    plotted against frequency.

        Frequency is related, but not identical, to the subjective pitch.
    Any periodic sound has a tonal character that can be ascribed a
    particular musical note. The note is basically defined by the
    fundamental frequency of the sound. For example, the note A above
    middle C on the piano has a fundamental frequency of 440 Hz. On the
    other hand, random sound has no distinct pitch, being characterized as
    a nondescript rumbling, rushing, or hissing noise, or low and high
    frequency noises depending upon the range of frequencies present.

        Human hearing is sensitive to frequencies in the range of about
    16-20 000 Hz (the "audiofrequency range"). The audible frequency range
    is covered by 10 octave bands. An octave is the frequency interval the
    upper limit of which is twice the lower limit. The so-called
    "preferred frequencies" at the centres of the standardized octave
    bands are spaced at octave intervals from 16 to 16 000 Hz (ISO,
    1975a). It should be noted that the limits of the octave bands are
    f/square root 2 and f square root 2, where  f is the centre
    frequency. The octave band level at a particular centre frequency is
    the level of the sound measured when all acoustic energy outside this
    band is excluded. One-third octave band filters, widely used for
    noise assessment purposes, subdivide each octave interval into three
    parts and provide a more complete description of the sound spectrum.

        In order to measure sound pressure level, the mean square
    pressure must be averaged over a certain period of time. For steady
    sounds, the choice of averaging time is immaterial providing that it
    is long compared with the time period of sound pressure fluctuations.
    Standard sound level meters normally incorporate "fast" and "slow"
    response settings corresponding to averaging times of approximately
    0.1 and 1.0 second respectively (IEC, 1973a) (section 2.2.4).

        Impulsive noise consists of one or more bursts of sound energy,
    each of a duration of less than about one second (ISO, 1973a). Sources
    of impulsive noise include impacts of all kinds, e.g., hammerblows,
    explosions, and sonic booms. These may be heard singly or, as in the
    case of a stamping press, repetitively. To characterize such sounds
    acoustically, it is necessary to estimate the peak sound pressures
    together with the duration, rise time, repetition rate, and the number
    of pulses. The mean square pressure of such sounds may change so
    rapidly that it cannot be measured with a conventional sound level
    meter, even using the "fast response" (0.1 sec) setting. For more
    accurate measurements, a 35-millisecond averaging time is specified
    for standard "impulse" sound level meters (IEC, 1973b). The averaging
    time of the inner ear is very short (about 30 microseconds) and some
    new impulse sound level meters have "peak hold" settings with an
    averaging time of 20 microseconds.

    2.2  Sound Perception and its Measurement

    2.2.1  Loudness and loudness level

        The physical magnitude of a sound is given by its intensity and
    its subjective or perceived magnitude is called its loudness. Loudness
    depends on both intensity and frequency and the average quantitative
    relationship between these factors has been deduced by experiment (see
    for example Fletcher & Munson, 1933; Stevens, 1955).

        The basic unit of loudness is the sone which is defined as the
    loudness of a 1000 Hz pure tone heard at an SPL of 40 dB re: 20 µPa
    under specified listening conditions (ISO, 1959). Two sones equal
    twice the loudness of one sone and so on. For sound at a particular
    frequency, at least over a significant fraction of the practical
    intensity range, loudness is proportional to some power of the sound
    intensity. This is the power law of loudness which is in general
    accordance with the Weber-Fechner law (Stevens, 1957b). In the mid
    audiofrequency range, the exponent in the power law is such that a
    twofold change in loudness corresponds to a tenfold change in
    intensity, i.e., a 10 dB change in level (Stevens, 1957a). At low
    frequencies, loudness changes more rapidly with changes in level. This
    is demonstrated in Fig. 1, which shows a standard set of equal
    loudness contours for pure tones (Robinson & Dadson, 1956; ISO, 1961),
    each line showing how the SPL of the tone must be varied to maintain a
    constant loudness. Each curve, in fact, corresponds to a particular
    loudness in phons. The loudness of a sound, in phons, is, by
    definition, equal to the SPL of that 1000 Hz tone which is equally
    loud -- again under specified listening conditions (ISO, 1959). For
    practical purposes, the relationship between the phon and sone scales
    may be expressed as:

    phon = 40 + log2 (sone)

    2.2.2  Calculation and measurement of loudness level

        Ideally, sound measurement meters should give a reading equal to
    loudness in phons bu it is difficult to achieve this objective,
    because the human perception processes are complex. Nevertheless,
    procedures have been developed and adopted as international standards
    (ISO, 1975b) but, as they are too complex to be incorporated into a
    simple measurement meter, they are rarely used in practice, except
    where the highest possible precision is required.

        For most practical purposes, a much simpler approach is used. A
    filter is used to weight sound pressure level measurements as a
    function of frequency, approximately in accordance with the frequency
    response characteristics of the human ear, i.e., energy at low and
    high frequencies is de-emphasised in relation to energy in the mid-
    frequency range. Most precision sound level meters incorporate three
    selectable filters labelled A, B, and C (IEC, 1973a) and sometimes 

    FIGURE 1

    D-filter (see section 3.7.2) (IEC, 1973b), the characteristics of
    which are illustrated in Fig. 2. The A, B and C filters are intended
    to match the ear-response curves at low, moderate, and high loudness
    respectively. However, extensive experience has shown that the
    A-filter usually provides the highest correlation between physical
    measurements and subjective evaluations of the loudness of noise.
    Levels on the A-scale are also measured in decibel units and are
    commonly expressed as dB(A), a convention that is used throughout this

        The A-weighting is used for sound measurements in a variety of
    situations, as it is widely accepted that the A-weighted sound
    pressure level, Lp(A), is a reasonably reliable and readily measured
    estimate of loudness (Botsford, 1969; Young & Peterson, 1969). It must
    be emphasized that this in only true for broadband sounds with no
    spectral concentrations of energy, in which case Lp(A) is typically
    some 10 decibel units lower than loudness in phons. For narrow
    frequency range sounds, considerable care must be exercised in the
    interpretation of A-weighted sound pressure level readings, since they
    may not accurately reflect the loudness of the sound. It should be
    noted that the A-scale has been adopted so generally that sound levels
    frequently quoted in the literature simply in dB are in fact
    A-weighted levels. Furthermore, many general purpose sound level
    meters are restricted solely to A-weighted measurements (IEC, 1961).

    2.2.3  Sound level and noise level

        The phrase "noise level' is widely used by laymen to describe the
    severity of an environmental noise. In acoustics, the word "level"
    should be reserved for all quantities expressed on a decibel scale. In
    this document, as is now common practice in many countries, the
    phrases "sound level" and "noise level" refer to decibel scales that
    account for human hearing characteristics (the A-weighted SPL scale
    being the most widely used). Care should be exercised to distinguish
    between sound pressure level, sound power level, sound intensity
    level, and sound or noise level.

    2.2.4  The time factor

        Sounds can appear to be steady to the human ear because the
    auditory averaging time is inherently long, much longer than the
    acoustic cycle times. Similarly, sound level measurements can be made
    to appear steady by selecting a suitably long averaging time. On
    precision sound level meters the "slow" value is appreciably longer
    than the auditory averaging time and is used to obtain a steady
    reading, when the signal level audibly fluctuates at a rapid rate. The
    "fast" response time is of the same order as that of the ear.

    FIGURE 2

        Sound level fluctuations, which can be smoothed out by the use of
    the slow response setting, are usually ignored for noise assessment
    purposes. However, difficulties arise when "slow response" readings
    vary significantly with time, as they do in many environments. Often,
    such level fluctuations are small but in some situations, for example,
    near to roads and airports, the fluctuations can be measured in tens
    of dB; the rate of fluctuation can also vary widely.

    2.2.5  Noise exposure scales

        In many noise indices that are well correlated with the
    subjective effects of interest, various underlying acoustic and
    nonacoustic factors have been combined in different ways. These
    composite indices are discussed in section 3.7 and the present section
    is restricted to the question of the physical measurement of noise.

        The basic objective of measurement is to quantify overall noise
    exposure in the simplest possible terms. The physical characteristics
    of a noise which, on the basis of intuition and laboratory experiment,
    might be expected to influence its subjective effects include the
    following: loudness level (recognizing average and peak values
    together with impulsive characteristics where appropriate); total
    noise "dose"; level fluctuation amplitudes; and rates of fluctuation.
    Clearly, the acoustic variables alone have many dimensions; the
    following two procedures are commonly used to measure some of them.

    2.2.6  Equivalent continuous sound pressure level

        To measure an average sound level the meter averaging time is
    extended to equal the period of interest  T, which may be an
    interval of seconds, minutes, or hours. This gives the equivalent
    continuous sound pressure level (Leq) derived from the mathematical

                    1    T     Lp(A)(t)
    Leq = 10 log10      integral           db(A)
                   T   o      1010dt

    Because the integral is a measure of the total sound energy during the
    period  T, this process is often called "energy averaging". For
    similar reasons, the integral term representing the total sound energy
    may be interpreted as a measure of the total noise dose. Thus, Leq
    is the level of that steady sound which, over the same interval of
    time, contains the same total energy (or dose) as the fluctuating

        Equivalent continuous sound level is gaining widespread
    acceptance as a scale for the measurement of long-term noise exposure.
    For example, it has been adopted by the International Organization for
    Standardization for the measurement of both community noise exposure
    (ISO, 1971) and hearing damage risk (ISO, 1975c). It also provides a

    basis for more elaborate composite noise indices discussed in
    subsequent sections including the day-night sound level (Ldn)

        Following the introduction of jet aircraft into commercial
    service, it was suggested that the then existing loudness scales were
    inadequate for aircraft noise rating purposes. An alternative scale of
    perceived noise level (PNL) was developed, with units dB(PN) (Kryter,
    1959). This was derived from the loudness level procedure of Stevens
    (1956) on the grounds that the attribute of perceived noisiness
    defined as the "unwantedness" of the sound was different and more
    relevant to aircraft noise than loudness. In fact, the only difference
    between the calculations involved was the use of different frequency
    response curves. As research progressed towards legislation for
    aircraft noise emission control (US Federal Aviation Regulations,
    1969; ICAO, 1971), the perceived noise level scale was modified to
    include special weightings for "discrete frequency components", i.e.,
    irregularities in the spectrum caused by the noticeable periodic
    components of engine fan and compressor noise, and the duration of the
    sound (Kryter & Pearsons, 1963). This modified quantity, known as
    effective perceived noise level, is expressed in dB(EPN).

        Because PNL could not be measured with a simple meter, a parallel
    development was the D-weighting filter, with characteristics based on
    an equal noisiness (rather than an equal loudness) frequency response
    curve (IEC, 1976). This filter is available on some sound level meters
    and is intended for aircraft noise monitoring purposes.

    2.2.7  Level distribution

        A widely used method of recording the variations in sound level
    is that of level distribution analysis, sometimes called statistical
    distribution analysis. This yields a graph of the percentage of the
    total time  (T) for which any given sound level is exceeded; such
    information can be summarized by reading specific levels from this
    graph. For example L10, L50, and L90, the levels exceeded for
    10%, 50%, and 90% of the time, are frequently used as measures of
    typical peak, average, and background levels, respectively.

    2.3  Sources of Noise

    2.3.1  Industry

        Mechanized industry creates the most serious of all large scale
    noise problems, subjecting a significant fraction of the working
    population to potentially hazardous noise levels. This noise is due to
    machinery of all kinds and often increases with the power of the
    machines. The characteristics of industrial noise vary considerably,
    depending on specific equipment. Rotating and reciprocating machines
    generate sound that is dominated by periodic components; air moving
    equipment tends to generate broad-band random sounds. The highest
    noise levels are usually caused by components or gas flows that move
    at high speed [e.g., fans, steam pressure relief valves) or by

    operations involving impacts (e.g., stamping, riveting, road
    breaking). In industrial areas, the noise usually stems from a wide
    variety of sources, many of which are of a complex nature.

        Machinery noise generation mechanisms are reasonably well
    understood and the technical requirements for low noise output in new
    machinery can usually be specified. The difficulty of reducing the
    noisiness of existing equipment is a serious obstacle to the
    improvement of working environments.

    2.3.2  Road traffic

        The noise of road vehicles is mainly generated from the engine
    and from frictional contact between the vehicle and the ground and
    air. In general, road contact noise exceeds engine noise at speeds
    higher than 60 km/h. The level of noise from traffic is correlated
    with the traffic flow rate, the speed of the vehicles, and the
    proportion of heavy vehicles, which, together with motorcycles, tend
    to be about twice as loud as motor cars.

        Special problems arise in areas where the traffic movements
    involve a change in engine speed and power, such as at traffic lights,
    hills, and intersecting roads.

    2.3.3  Rail traffic

        Trains generate a relatively low frequency noise but variations
    are present depending upon the type of engine, wagons, and rails.
    Impact noises are generated in stations and marshalling yards because
    of shunting operations. The introduction of high speed trains has
    created special noise patterns, especially when such trains pass over
    bridges or other structures that cause amplification of the noise. At
    speeds of around 200 km/h, the proportion of high frequency sound
    energy increases and the sound is perceived to be similar to that of
    overflying jet aircraft. Furthermore, with increasing speed the onset
    of the noise is more sudden than with conventional trains. Thus,
    severe noise problems have been created in countries where high speed
    trains operate, notably in Japan.

    2.3.4  Air traffic

        Aircraft operations have caused severe community noise problems.
    Introduction of the early turbojet transport aircraft led to a surge
    of community reactions against commercial airports, and more research
    has been devoted to aircraft noise than to any other environmental
    noise. The noise generation is related to air velocity, which is an
    important feature for aircraft and aircraft engines. Fast moving
    bodies such as propellers and compressor blades, as well as jet
    exhaust gases are very efficient sources of noise.

        Aircraft noise is characterized by a wide frequency range with
    the periodic components of rotating machinery noise (fans, propellers,
    and rotors) superimposed on a general broadband background noise. For
    jet aircraft, the periodic components tend to be more dominant on
    landing than on take-off when the broadband exhaust noise
    predominates. For aircraft with quiet engines, noise from the hull may
    become dominant when landing.

        Aircraft noise control depends critically on the reduction of
    engine component and gas velocities. The high by-pass ratio turbo-fan
    engines of newer aircraft with components operating at significantly
    lower speeds have resulted in a reduction in aircraft noise levels,
    and offer considerable promise of less noisy airports, as they
    gradually replace older equipment.

    2.3.5  Sonic booms

        The sonic boom is a shock wave system generated by an aircraft,
    when it flies at a speed slightly greater than the local speed of
    sound. The shock wave extends from an aircraft throughout supersonic
    flight in a roughly conical shape. At a given point, the passage of
    the shock wave causes an initial sudden rise in atmospheric pressure
    followed by a gradual fall to below the normal pressure and then a
    sudden rise back to normal. These pressure fluctuations, when
    recorded, appear in their typical form as so-called N-waves. When they
    occur with a separation greater than about 100 milliseconds, the sonic
    boom has a characteristic double sound. Rise times from less than 0.1
    to 15 milliseconds and durations up to 500 milliseconds have been
    recorded for typical sonic booms generated by military or civilian

        Low intensity sonic booms with longer rise times are perceived as
    a noise similar to distant thunder. As the rise time increases, the
    noise becomes progressively sharper and attains a "dry cracking"
    character. An aircraft in supersonic flight trails a sonic boom that
    can be heard over more than 50 km on either side of its ground track
    depending upon the flight altitude and the size of the aircraft
    (Warren, 1972).

    2.3.6  Construction and public works

        Building construction and earth works are activities that cause
    considerable noise emissions. A variety of sounds is present from
    cranes, cement mixers, welding, hammering, boring, and other work
    processes. Construction equipment is often poorly silenced and
    maintained, and building operations are frequently carried out without
    considering the environmental noise consequences.

    2.3.7  Indoor sources

        Indoor noise originates from a variety of sources such as air
    conditioners, waste disposal units, and furnaces. Noises from outdoor
    sources also penetrate through windows and weaknesses in building
    structures, although with some attenuation. Within a building, noise
    is transmitted from room to room through ventilation ducts and through
    the building structure itself. Of particular interest is the low
    frequency sound emitted by ventilation or air conditioning equipment.
    This noise, which often has discreet frequencies, can be generated by
    fans, vibrations in conducting ducts, or at air outlets.

    2.3.8  Miscellaneous sources

        Apart from the major categories of noise already identified,
    which affect a large number of people in the community, many other
    sources of noise can be important in individual cases. Firing ranges,
    sports fields, and pleasure grounds are examples of fixed sources,
    while noises from garbage collection and power-operated lawn-mowers
    are other examples of machine-produced noise that can interfere with
    man's comfort and rest. Neighbourhood noise also includes noise from
    domestic animals, farm equipment, boats, and the sirens of emergency


    3.1  Noise-induced Hearing Loss

    3.1.1  Hearing impairment

        Normal hearing is regarded as the ability to detect sounds in the
    audiofrequency range (16-20 000 Hz) according to established
    standards. However, individual hearing ability in man varies. Some of
    these variations may be attributed to the effects of different
    environmental influences (Roberts & Bayliss, 1967); in industrialized
    countries, women generally have better hearing than men (Kylin, 1960;
    Dieroff, 1961; Gallo & Glorig, 1964).

        As a rule, hearing sensitivity diminishes with age, a condition
    known as presbyacusis (Glorig & Nixon, 1962). Consequently,
    corrections for aging should be considered when examining data on
    hearing loss caused by noise exposure. However, the literature
    reflects controversy concerning the degree to which cumulative effects
    of noise exposure in everyday life may contribute to eventual hearing
    loss (socioacusis), thus obscuring the effect due to aging alone.
    Moreover, there is considerable variation between individuals in both
    the amount and rate of hearing loss due to aging. The general pattern
    of progression of presbyacusis has been quite well-established, and
    data are available in numerous reference sources (US National
    Institute for Occupational Safety and Health, 1972; US Environmental
    Protection Agency, 1973a, 1974). Loss of hearing sensitivity due to
    aging occurs mainly at the higher audiometric frequencies and is
    almost invariably bilateral (i.e., in both ears).  Hearing level, noise-induced threshold shift, and hearing

        In order to discuss the effects of noise on hearing, it is
    necessary to differentiate between hearing level, noise-induced
    threshold shift (NITS), and hearing impairment.

        Hearing level refers to the audiometric threshold level of an
    individual or group in relation to an accepted audiometric standard
    (ISO, 1975d) and is sometimes termed "hearing loss". Noise-induced
    threshold shift is the quantity of hearing loss attributable to noise
    alone, after values for presbyacusis (including socioacusis) have been
    subtracted. These values may differ slightly according to where and
    how the presbyacusis data were collected (see for example Hinchcliffe,
    1959; Gallo & Glorig, 1964; Spoor, 1967; US National Centre for Health
    Statistics, 1975).

        Hearing impairment is generally referred to as the hearing level
    at which individuals begin to experience difficulty in leading a
    normal life, usually in relation to understanding speech. Hearing
    impairment has been defined in the USA as an arithmetic average of

    26 dB or more hearing loss at the frequencies, 0.5, 1, and 2 kHz (the
    definition is currently being revised); in Poland, it is defined as
    30 dB or more at 1, 2, and 4 kHz (after age correction), and in the
    United Kingdom, it is 30 dB or more at 1, 2, and 3 kHz. It should be
    noted that a damage risk criterion of 30 dB at 1, 2, and 4 kHz may be
    more protective than a criterion of 26 dB at 0.5, 1, and 2 kHz,
    because hearing loss at high frequencies is usually greater than the
    loss at 500 Hz.  Noise-induced temporary threshold shifta

        A person entering a very noisy area may experience a measurable
    loss in hearing sensitivity but recover some time after returning to a
    quiet environment. This phenomenon can be measured as a shift in
    audiometric thresholds, and is called noise-induced temporary
    threshold shift (NITTS).

        Recovery from NITTS depends on the severity of the hearing shift,
    individual susceptibility, and the type of exposure. If recovery is
    not complete before the next noise exposure, there is a possibility
    that some of the loss will become permanent. Information on NITTS has
    been used for two purposes: first, to predict noise levels that could
    be permanently damaging to the ear, and second, to attempt to predict
    individual susceptibility to hearing loss caused by excessive noise.
    Measurements of NITTS are made by comparing pre- and post-exposure
    audiograms. The extent of NITTS, for the same exposure, varies
    considerably between individuals. Recovery can take hours, days, or
    even weeks after exposure. It should be noted that NITTS can be
    experienced by individuals who already suffer from permanent noise-
    induced hearing losses. Thus, when assessing permanent damage,
    sufficient recovery time in the quiet should be allowed before

        It would appear from recent investigations that the relationship
    between NITTS and the noise-induced permanent threshold shift (NIPTS)
    is very uncertain and that damage-risk criteria should be based on
    epidemiological rather than on NITTS data.  Noise-induced permanent threshold shift

        The typical pattern of NIPTS usually involves a maximum loss at
    around 4000 Hz. Because the loss is sensorineural, it is seen in both
    air and bone conduction audiograms. Noise-induced hearing loss is not
    an abrupt process but occurs gradually, usually over a period of
    years. The rate and extent of loss depends on the severity and
    duration of the noise exposure, but individual susceptibility also 


    a Sometimes called auditory fatigue.

    seems to have a considerable effect on the rate of progression. Noise-
    induced losses are rather similar to losses due to aging and the two
    types of losses are difficult, if not impossible, to distinguish. 
    Fig. 3 shows the progression of noise-induced hearing loss observed in
    workers with increasing duration of exposure to high noise levels
    (Johansson, 1952).

        The first stages of noise-induced hearing loss are often not
    recognized because they do not impair speech communication ability. As
    the loss becomes greater, difficulty may be encountered particularly
    in noisy locations.

        Hearing of important sounds other than speech, such as door
    bells, telephones, or electronic signals, may also be impaired. With
    further loss in hearing, speech communication may be severely
    affected.  Incidence of noise-induced permanent hearing loss

        The prevalence of hearing loss among workers in noisy industries
    has been recognized since ancient times, and excessively loud noises
    are popularly described as deafening. Clinical observations of noise-
    induced hearing loss have been reported for more than a century, but
    it is only recently that the problem has been studied intensively. It
    has been suggested that even though people exposed to intense noise
    frequently experience a substantial noise-induced temporary threshold
    shift, sometimes accompanied by tinnitus (ringing in the ears), the
    fact that very often such symptoms seem to disappear within a short
    time may lead them to believe that no permanent damage has occurred.
    However, neither the subjective loudness of a noise, nor the extent to
    which the noise causes discomfort, annoyance, or interference with
    human activity, are reliable indicators of its potential danger to the
    hearing mechanism.

        As there is considerable variation among individuals, it is very
    difficult to identify a safe limit of noise exposure that can be
    applied for all ears.

        Most current knowledge of hearing loss due to noise has been
    obtained from industrial surveys. There is also evidence that non-
    industrial exposure to noise can be harmful. Results of several
    studies have confirmed that high levels of "rock and roll" and similar
    music can produce considerable temporary threshold shift and even
    permanent threshold shift. Audiograms of "pop-musicians" typically
    show losses at 400 Hz in both ears (Kowalczuk, 1967). It has also been
    shown that men and women are equally at risk of hearing damage, when
    exposed to over-amplified music (Fletcher, 1972). Other non-
    occupational activities that can contribute to hearing loss include
    shooting and motorcycling.

    FIGURE 3

    3.1.2  Relation between noise exposure and hearing loss

        In the normal auditory process, sound vibrations in the air
    travel through the ear canal and cause the eardrum to vibrate. The
    vibrations are then transmitted by the bones of the middle ear to the
    sensory organ of the inner ear (cochlea). Here they are transduced by
    hair cells into nerve impulses and transmitted to the brain, where
    they are perceived as sound or noise.

        Blasts and other intense or explosive sounds can rupture the
    eardrum or cause immediate damage to the structures of the middle and
    inner ear, while hearing loss due to prolonged noise exposure is
    generally associated with destruction of the hair cells of the inner
    ear. The severity of noise-induced hearing loss depends on both the
    location and the extent of damage in the organ of Corti, which, in
    turn, depend on the intensity and frequency of the sound stimulus. The
    higher the frequency, the nearer the point of maximum displacement of
    the basilar membrane is to the base of the cochlea where the basilar
    membrane is narrowest. This point is shifted towards the apex of the
    cochlea as the stimulus frequency decreases. The maximum stimulation
    of cells occurs at the point of maximum displacement. A large part of
    the upper cochlea is responsive to low frequency stimulation and loss
    of hair cells can be quite extensive without significant loss in low
    frequency sensitivity. On the other hand, much more localized portions
    of the basal region of the cochlea are responsible for high frequency
    sound sensation and loss of hair cells in these lower portions results
    in significant losses of high frequency sensitivity (Miller, 1971a).
    The number of hair cells damaged or destroyed increases with
    increasing intensity and duration of noise and, in general,
    progressive loss of hair cells is accompanied by progressive loss of

        Even though numerous experiments have been performed with
    animals, the mechanisms involved in the destruction of the Corti organ
    are not completely clear, although several explanations have been
    proposed. For example, mechanical stresses could destroy cells,
    repeated circulatory troubles through vascular contractions could
    deprive cells of an appropriate blood supply; an increase in local
    temperature could damage proteins, and repeated stimuli could exhaust
    the metabolic supply of cells. Various theories have been reviewed by
    Ward (1973).

        An important fact is that noise-induced hearing loss is of a
    neural type involving irreversible injury to the inner ear.
    Furthermore, such losses are almost always bilateral.  Laboratory studies

        Laboratory studies on temporary and permanent hearing loss and on
    the anatomy of the noise-damaged inner ear have been carried out on a
    number of animal species. Temporary hearing loss studies on human
    subjects have included a variety of noise exposure patterns, including

    noises of different spectra, interrupted noise patterns, and short-
    duration noise exposures. In extrapolating the results of such studies
    to permanent hearing loss in man, it has always been necessary to
    consider: (a) temporary versus permanent threshold shift in man; (b)
    permanent threshold shift in man versus permanent threshold shift in
    animals; and (c) anatomical damage in animals versus permanent
    threshold shift in man. However, it should be noted that a thorough
    knowledge of such relationships has not been necessary. For example,
    in using animals to study the cumulative effects of noise, it has not
    been necessary to assume that the absolute sensitivity of animals and
    man to noise is the same, but merely that the relative sensitivity of
    animals to alternative noises of specified temporal patterns is
    similar to that of man.

        Experimental studies have resulted in the following general

        (a) There is considerable variability among individuals in
    susceptibility to temporary hearing loss, the rate at which temporary
    hearing loss approaches its asymptotic level, and the rate of

        (b) Temporary hearing losses in man are most pronounced at
    frequencies slightly above the predominant frequency of the noise

        (c) In most cases, the rate of increase of, and subsequent
    recovery from, temporary hearing loss is different for impact noises
    and for steady noise. NITTS from impulse noise increases more slowly
    than NITTS from steady noise (Ward et al., 1961) and recovery is
    slower (Cohen et al., 1966).

        (d) In general, the equal energy rule (section 3.1.3) has been
    found to be compatible with experimental results for uninterrupted
    exposures to steady noise. However, it may not always be the best
    predictor of NITTS with regard to the audiometric frequency since it
    tends to overestimate NITTS below 2000 Hz and underestimate losses
    above 2000 Hz (Yamamoto et al., 1968). Although NITTS from interrupted
    noise may be overestimated (Ward, 1970), it is thought that the rule
    gives a good prediction of NIPTS from interrupted noise (Burns &
    Robinson, 1970).

        (e) Audiograms of persons exhibiting temporary hearing loss in
    laboratory studies tend to be similar to those of persons exposed to
    comparable noise over a period of several years (Nixon & Glorig,
    1961).  Occupational hearing loss

        Several reports have been published on the subject of
    occupational hearing loss (Atherley et al., 1967; Burns & Robinson,
    1970; King, 1971; Robinson, 1971; Stone et al., 1971; Baughn, 1973;
    Burns, 1973; Paschier-Vermeer, 1974; Sulkowski, 1974).

        All these studies were cross-sectional audiometric studies and
    many incorporated surveys of noise exposure. Specific occupational
    groups were usually studied, including workers in heavy industry,
    shipyards, textiles, jet-cell test rooms, foundries, transportation,
    and forestry. Some definition of hearing impairment was generally
    applied in order to define a percentage of people with hearing loss.
    Audiograms were usually compared with so-called "normal" thresholds.
    In this respect, presbyacusis was often accounted for. In many cases,
    efforts were made to screen the data to exclude those persons who had
    previously held noisy jobs, possible nonoccupational noise exposures,
    and otological abnormalities. In some studies, such persons were
    purposely included in order to provide a realistic estimate of hearing
    levels in a typical noise-exposed population.

        Virtually every study revealed that workers exposed to intense
    noise daily, for several years, showed noise-induced hearing loss
    fitting the classic pattern. Considerable hearing loss was rare at
    lower frequencies but frequent at higher frequencies.

        In the studies for which noise exposure levels were known, a
    clear relationship was generally seen between increasing incidence of
    hearing loss and increasing noise level. In groups exhibiting
    considerable noise-induced hearing loss, the variation of audiometric
    thresholds was generally higher than in groups not exposed to noise.
    Cases of sudden deafness occurring after long-term exposure to noise,
    without previous impairment, have been reported in Japan (Kawata &
    Suga, 1967) and may indicate special susceptibility.

        Taking into account duration of exposure and age as well as other
    pathological conditions, Rey (1974) found that the proportion of
    workers with noise-induced deafness (defined as 25 dB average loss at
    0.5, 1, and 2 kHz) was as high as 60% in the metal industry (noise
    levels equal to and above 95 dB(A)). Cohen et al. (1970) compared the
    mean hearing levels of exposed workers with those of a control group
    for several noise intensities and several durations of exposure and
    found that noise levels between 85 and 88 dB(A) could be harmful to
    the ear, and that, even at 75 dB(A), there was some loss of hearing.

        According to two other studies performed in industry, there is a
    definite risk of hearing damage associated with prolonged exposure to
    noise levels between 85 and 90 dB(A) (Roth, 1970; Martin et al.,

        Fig. 4 compares the percentages of workers with hearing
    impairment as a function of age for unexposed groups and for groups
    exposed to occupational noise levels of 85, 90, and 95 dB(A) (NIOSH,
    1973b). In this case, hearing impairment is defined as an average
    hearing loss greater than 25 dB(A), at frequencies of 1, 2, and 3 kHz.  Factors that may influence the incidence of noise-induced
             permanent threshold shift

        Certain people who live in remote and generally quiet areas of
    the world have been found to have unusually acute hearing in
    comparison with members of urban populations in corresponding age
    groups (Rosen et al., 1962). However, it is not clear whether such
    audiometric differences are due to the lack of noise exposure alone.
    Differences in the patterns of hearing found between communities that
    are widely separated geographically and culturally may result from
    cultural, dietary, and genetic factors and differences in general
    environment (Rosen et al., 1962; Rosen & Rosen, 1971).

        Although it has been suggested that older people are more
    susceptible to NIPTS (Kryter, 1960), there is no clear experimental
    evidence that this is so (Kupp, 1966; Nowak & Dahl, 1971). Indeed,
    studies by Schneider et al. (1970) and Davis (1973) indicate that
    there is probably no causal relationship between age and
    susceptibility to NIPTS, at least in people of working age.

        There is some controversy in the literature as to whether
    pathological changes in the middle ear protect the inner ear from
    noise-induced damage, or whether they may instead increase the chance
    of noise-induced hearing loss. Some authors have expressed the view
    that in cases of middle ear damage, bone conduction becomes more
    effective and that the defence action of the middle ear muscles is
    impaired (Mounier-Kuhn et al., 1960; Ward, 1962; Dieroff, 1964; Mills
    & Lilly, 1971). In contrast, others have reported cases where noise-
    induced hearing loss was less in damaged ears than in normal ears
    (Johansson, 1952).

        Variation in individual susceptibility to noise-induced permanent
    hearing loss is illustrated by observations from surveys of
    occupational hearing loss, which indicate that workers from the same
    noisy environment display radically different audiograms, and that
    some workers, even after many years of exposure to noise, show little
    or no sign of noise-induced hearing loss.

    FIGURE 4

        Factors causing such differences in individual susceptibility
    could include fatigue of the acoustic reflex, anatomical differences
    in the structure of the middle and inner ear, the functional status of
    the autonomic system, and latent vitamin B deficiency (Kawata, 1955).

        To some extent, the ear is protected from damage by the middle
    ear reflex or stapedius reflex. The contraction of the stapedius
    muscle changes the movement of stapes which increases the impedance of
    the conductive mechanisms. The amount of sound energy delivered to the
    inner ear is reduced by about 15-20 dB at low and middle frequencies
    (Miller, 1961). The effectiveness of the middle ear reflex as a
    protective device varies with the intensity and the spectrum of the
    sound. In normal ears, the onset of the reflex occurs at sound levels
    of 75-90 dB. In man, the muscle contraction subsides very quickly
    after the onset of the sound for frequencies above 3000 Hz, while for
    lower frequencies, the contraction can last for a considerable time
    (Johansson et al., 1967). Impulsive sounds or sounds with a sudden
    onset can penetrate the ear without stimulating the protective
    mechanism, because of a time lag in the muscle contraction.
    Furthermore, the reflex action weakens with fatigue and thus provides
    little protection against prolonged steady sounds. The fact that its
    effectiveness also varies considerably among individuals may be
    related to variations in individual sensitivity to certain sounds.

        Measurements of NITTS have been used to investigate the
    protection provided by the stapedius reflex. In patients with
    peripheral facial palsy including unilateral stapedius muscle
    paralysis, the NITTS after low frequency noise exposure was
    significantly greater in the affected ear than in the unaffected ear
    (Zakrisson, 1974). However, results of animal studies, in which the
    stapedius muscle was severed, contradict these findings (Steffen et
    al, 1963; Ferris, 1966).  Combined effects of intensity and duration of noise exposure

        Most data concerning the long-term hazard of noise are related to
    occupational exposure. There is a shortage of information about short-
    term exposures, and very little information concerning exposures
    lasting longer than 8 h. In order to predict the effects of long-term
    noise exposure, investigators have been obliged to extrapolate the
    results of field observations and laboratory investigations of NITTS.
    It is difficult to establish limits for safe noise exposure, since
    predictions using different methods of extrapolation conflict with
    each other. The following is a brief review of the bases of some of
    the methods used to integrate the combined effects of intensity and

        The equal temporary effects rule is the hypothesis that the NIPTS
    due to long-term, daily, steady-state noise exposure is equal to the
    average NITTS produced by the same daily noise in healthy young ears
    (Ward et al, 1958, 1959). In a later study, Ward (1960) suggested that
    metabolic insufficiency induced in the hearing organ by noise might

    underlie both the temporary and permanent hearing defects caused by
    excessive noise. NITTS studies also tend to support the observation
    (reflected in industrial studies of NIPTS) that for a given length of
    exposure, frequently interrupted noise is less harmful than continuous
    steady-state noise of the same level (Ward et al, 1959; Miller et al.,

        An extension of this theory is that NIPTS is unlikely, if there
    is complete recovery from the NITTS before the beginning of the next
    day's exposure. An early occupational noise criterion was based on
    this assumption (Kryter et al., 1966).

        The equal energy rule is the theory that the hazard to hearing is
    determined by the total sound energy (the integrated product of sound
    intensity and duration) entering the ear each day. This rule has
    natural appeal, since the exposure dose is quite simple to assess and,
    according to epidemiological data, is reasonably well correlated with
    the accumulated physical damage. The rule allows a 3-db increase in a
    steady sound level for each halving of the duration (Burns & Robinson,
    1970; Ward & Nelson, 1971; US Environmental Protection Agency, 1973b;
    Martin, 1976). However, it should be noted that the range of sound
    duration covered by this rule might be limited by the need for
    protection against possible damage by high level, short duration,
    impulsive sounds (section 3.1.3).

        Various other theories are based, to a certain extent, on the
    equal temporary effect hypothesis. Such criteria are usually
    identified by the change in sound level that is necessary for each
    doubling of the exposure duration, e.g., the "5-dB rule" means that
    the level must be 5 dB less for each doubling of the exposure
    duration. The rules most frequently quoted in the literature are:

        (a) 3 dB rule: equal energy rule incorporated in ISO standard
    1999 (ISO, 1975c);

        (b) 5 dB rule: purported to partially compensate for typical
    interruptions and intermittency and used in the 1969 Walsh-Healey
    Public Contracts Act in the USA (Federal Register, 1969);

        (c) 4 dB rule: purported to be more reliable for protection at
    higher frequencies than the 5 dB rule and used by the United States
    Air Force (US Air Force, 1973); and

        (d) 6 dB equal pressure rule, a more conservative criterion
    suggested by some research workers (US Department of Health, Education
    and Welfare, 1972).

        None of the rules (a) to (d), account for a reordering of the
    noise exposure pattern, i.e., the predicted risk is independent of the
    order in which a sequence of sounds is experienced, even if this
    sequence includes periods of quiet. Thus, there is some conflict
    between these rules and the equal temporary effect hypothesis.

        To simplify different damage risk criteria, noise exposure
    histories are frequently expressed as equivalent 8-h continuous
    levels. For example, using the equal energy (3 dB) rule, an exposure
    of 88 dB for 4 h could be expressed as an equivalent level of 85 dB.  Estimation of hearing impairment risk

        The hearing loss that may result from noise exposure, can be
    expressed in terms of probable NIPTS, or hearing impairment. For
    example, the percentage of people who will suffer an NIPTS of 5 dB
    (the smallest amount measurable) at the most sensitive frequency 
    (4000 Hz) may be defined as a function of an equivalent 8-h level
    (Fig. 5). From this diagram, an 8-h equivalent level of 75 dB(A) can
    be identified as the limit for protection against significant NIPTS
    (ISO, 1975c). Since it is often impractical to reduce occupational 8-h
    equivalent noise levels to 75 dB(A), practical criteria for "safe"
    levels have been based upon less stringent definitions of hearing
    impairment or hearing handicap. For example, "damage-risk" has been
    defined as the percentage of a population with a given amount of
    hearing impairment after corrections have been made for those people
    who would "normally" incur losses from causes other than noise
    exposure. Table 2 shows the percentage risk and the total percentage
    with impaired hearing resulting from various levels of noise and years
    of exposure (ISO, 1975c).  The importance of high-frequency hearing

        It is common practice to assess hearing handicap for compensation
    purposes, and even for prevention purposes, in terms of the ability to
    understand "everyday" speech. According to the ISO definition (ISO,
    1975c), hearing handicap begins with a 25 dB loss averaged for the
    frequencies 500, 1000, and 2000 Hz. However, in most languages, speech
    includes energy at higher frequencies and therefore good high
    frequency hearing is important for speech intelligibility, especially
    when listening conditions are less than optimal (i.e., in background
    noise or when the speech is disorted in some way) (Kryter et al.,
    1962; Harris, 1965; Niemeyer, 1987; Acton, 1970; Kuzniarz, 1974;
    Antansson, 1975). Under good listening conditions, impaired hearing
    may not diminish speech intelligibility because of the redundancy
    (multiplicity of cues) of speech (section 3.2.1). This redundancy is
    reduced in noisy conditions or when the speech is muffled, the accent
    or the message is unfamiliar, or when these constraints occur in

    FIGURE 5

        Table 2.  Percentage of exposed people with impaired hearing as a function of occupational
              noise level (Leq (8-h) dB(A))x after different periods of exposure

                                                           Period of exposure
    Occupational                                                 (years)
    noise level     Cause of impairment                                                        
    Leq 8-h dB(A)                               0    5   10   15   20   25   30   35   40   45

        <80           (a)  All causes y         1    2    3    5    7   10   14   21   33   50
                      (b)  Occupational noise   0    0    0    0    0    0    0    0    0    0

         85           (a)  All causes           1    3    6   10   13   17   22   30   43   57
                      (b)  Occupational noise   0    1    3    5    6    7    8    9   10    7

         90           (a)  All causes           1    6   13   19   23   26   32   41   54   65
                      (b)  Occupational noise   0    4   10   14   16   16   18   20   21   15

         95           (a)  All causes           1    9   20   29   35   39   45   53   62   73
                      (b)  Occupational noise   0    7   17   24   28   29   31   32   29   23

        100           (a)  All causes           1   14   32   42   49   53   58   65   74   83
                      (b)  Occupational noise   0   12   29   37   42   43   44   44   41   33

        105           (a)  All causes           1   20   45   58   65   70   76   82   87   91
                      (b)  Occupational noise   0   18   42   53   58   60   62   61   54   41

    Table 2 (contd)

                                                           Period of exposure
    Occupational                                                 (years)
    noise level     Cause of impairment                                                        
    Leq 8-h dB(A)                               0    5   10   15   20   25   30   35   40   45

        110           (a)  All causes           1   28   58   76   85   88   91   93   95   95
                      (b)  Occupational noise   0   26   55   71   78   78   77   72   62   45

        115           (a)  All causes           1   38   74   88   94   94   95   96   97   97
                      (b)  Occupational noise   0   36   71   83   87   84   81   75   64   47

    x Based on: ISO (1975c).
    y The values in row (a) for Leq < 80 dB(A) are estimates of the percentage of people with
      hearing impairment caused by factors other than occupational noise exposure and should be
      subtracted from row (a) in all cases to obtain row (b) the percentages of people with
      impairment attributable to occupational noise. Impairment is defined as a loss of 25dB or
      more averaged for the frequencies 500, 1000, and 2000 Hz.

    Example: Out of a group of people exposed to an occupational noise level of 95 dB(A) Leq (8-h)
             for 25 years, 39% will exhibit hearing impairment. However, 10% (see c) would have had
             impaired hearing without exposure to occupational noise. Thus the risk of occupational noise
             damage is 29%.

        The use of a simple, unweighted average at 500, 1000, and 2000 Hz
    for assessing noise-induced hearing handicap is restrictive because
    most hearing loss occurs at higher frequencies. Consequently, the
    frequencies 3000 Hz and 4000 Hz are included in damage-risk formulae
    by some countries.

    3.1.3  Effects of impulsive noise

        At present, most know]edge of hearing loss due to impulsive noise
    comes from studies of the effects of gunfire (see for example Coles et
    al., 1968) with some limited data from industrial situations (Dieroff,
    1974; Ceypek & Kuzniarz, 1974). Important properties of impulsive
    noise exposure include the peak SPL, duration, rise and decay times,
    type of wave form, repetition rate, spectrum, and number of impulses.

        The present state of knowledge is that a hazard exists and,
    accordingly, that ear protection should be worn when impulsive noises,
    measured with appropriate instrumentation, exceed an SPL of 140 dB for
    more than 5 milliseconds regardless of rise time, spectrum, or the
    presence of oscillatory transients. Higher peak levels may be
    tolerable for durations of less than 5 milliseconds. Levels in excess
    of 165 dB SPL, even for short durations, are likely to cause cochlear
    damage (Acton, 1967; Burns & Robinson, 1970). It should be noted that
    the response time of the acoustic reflex (section is of the
    order of 100-300 milliseconds, which is too long to give any
    protection against such short duration sound (Coles et al., 1968;
    Coles & Rice, 1970). 

        Although it is not common practice to extend the equivalent 8-h
    sound level criteria down to impulsive durations, the recent studies
    of Rice & Martin (1973) and Martin (11976) suggest that the criteria
    based on the equal energy rule, may be applicable to high-intensity
    impulsive noise (Fig. 6).

    3.1.4  Infrasound and ultrasound

        Frequencies below 16 Hz are referred to as infrasonic
    frequencies. Perception of sound from 100 Hz down to about 2 Hz is a
    mixture of aural and tactile sensations. For example, frequencies
    around 10 Hz, can cause discomfort through a modulation of the vocal
    cords. Reactions caused by extremely high levels of infrasound can
    resemble those of mild stress reaction and may include bizarre
    auditory sensations, describable as pulsation and flutter. High levels
    of infrasound can cause resonance responses in various organs in the
    human body, although the long-term effects of such stimulation are not
    known (Johnson, 1973).

    FIGURE 6

        The effects of high intensity ultrasound (above 20 kHz and 105 dB
    SPL), which will be discussed in a separate document, are reported to
    be similar to those observed during stress. However, these effects may
    be partly due to associated high (but less than ultrasonic) frequency
    sound (Acton, 1967). Although it is usually accepted that levels below
    105 dB SPL have no adverse effects, there is evidence from one
    experiment, that physiological changes can occur at lower levels
    (98-102 dB) (Lisickina, 1968).

    3.2  Interference with Communication

    3.2.1  Masking and intelligibility

        The interference of noise with speech communication is a process
    in which one of two simultaneous sounds renders the other inaudible.
    The ratio of a given desired signal (speech, music) to that of the
    interfering noise will determine whether or not the signal can be
    perceived. The higher the level of the masking noise and the more
    energy it contains at speech frequencies, the greater will be the
    percentage of speech sounds that are inaudible to the listener.

        An important aspect of communication interference in occupational
    situations is that the failure of workers to hear warning signals or
    shouts may lead to injury. Although cases do not appear to have been
    documented in the literature, there is anecdotal evidence of such

        In the last half century, knowledge concerning the masking of
    simple signals such as pure tones, narrow bands of noise, and even
    isolated phonemes of speech has increased considerably. Empirical
    relationships are available that permit accurate prediction of the
    audibility for a normal-hearing listener of a particular speech sound
    in the presence of a specified noise (Webster, 1969, 1974; Kryter,
    1970). However, communication is almost never carried on by means of
    single acoustic signals, but rather by a rapid sequence of different
    speech sounds, the overall intensity and spectral distribution of
    which are constantly shifting; in fact, the same word, when repeated,
    may be quite different acoustically. Furthermore, even when the
    masking noise is judged to be steady, the energy in different
    frequency regions fluctuates from moment to moment.

        Most of the sentences of ordinary discourse can be understood
    fairly well, even when a large number of individual speech sounds are
    masked, because of the redundancy of speech. Even when a particular
    sound is masked or even omitted, the word or sentence in which it
    occurs may be correctly perceived because the remaining sounds are
    sufficient to convey the meaning. However, the interpretation required
    to compensate for the masking effect is an additional strain on the

        Other characteristics of the communication process may affect the
    effectiveness of communication, when additional sounds are present.

    Examples of such factors are the familiarity of the listener with the
    dialect or accent of the speaker, the presence of reverberation, the
    importance and familiarity of the message, distance from speaker to
    listener, the motivation of the listener, and any hearing loss that
    may produce a degradation in the perceived sound. Thus, the
    relationship between the spectrum, level, and temporal characteristics
    of a masking noise and the "intelligibility" of ordinary speech, i.e.,
    the proportion of speech correctly understood is very complex. Much
    research has involved the measurement of intelligibility of nonsense
    syllables and of isolated words in phonetically-balanced lists. Based
    upon work with real sentences, conversion charts have been constructed
    to transform scores involving only words to approximate expected
    scores for sentences of ordinary speech. For example, when 75% of the
    items on a list of isolated words are correctly perceived, about 95%
    of the key words in a sentence of ordinary discourse will be correctly
    heard (Kryter, 1970). Sentence intelligibility refers to the
    percentage of key words that are perceived correctly in a series of

    3.2.2  Speech interference indices

        Many attempts have been made to develop a single index based on
    the characteristics of the masking noise that directly indicates the
    degree of interference with speech perception. Naturally, such indices
    involve considerable degrees of approximation. The three most common
    indices are: the articulation index (AI), speech interference level
    (SIL), and the A-weighted sound pressure level (Lp(A)).  Articulation index

        The AI (French & Steinberg, 1947; Kryter, 1962) is the most
    complicated of these indices, since it takes into account the fact
    that some frequencies are more effective than others in masking
    speech. Frequencies below 250 Hz and above 7000 Hz are not included,
    as they are not considered to contribute to the intelligibility of
    speech. The frequency range from 250 to 7000 Hz is divided into 20
    bands, each of which contributes 5% to the total intelligibility. In
    order to determine the AI for a particular noise, the difference in dB
    between the average speech level and the average noise level in each
    of these 20 bands is calculated, and the resultant numbers are
    combined to give a single index. Essentially, this process predicts
    how much masking of individual speech sounds will occur and then
    integrates this information.

        Although the AI is an accurate index for the prediction of the
    effects of noise on speech intelligibility, it is complicated to use
    and difficult for the layman to interpret. Thus, simplified procedures
    for estimating the AI from weighted measurements of octave-band levels
    have been developed (Kryter, 1962).  Speech interference level

        The SIL was designed as a simplified substitute for the AI
    (Beranek 1947). Contributions to intelligibility by the lowest and
    highest frequencies have been omitted to a greater extent than for the
    AI. A modern version of the SIL is the arithmetic average of the sound
    pressure levels in the three octave bands centred at the preferred
    frequencies 500, 1000, and 2000 Hz (abbreviated SIL 0.5, 1, and 2).
    Many variations of SIL in terms of the specific octave bands to be
    averaged have been suggested. For example, SIL (0.25, 0.5, 1, 2)
    includes the 250 Hz band. At the present time, the US National
    Standards Institute recommends SIL (0.5, 1, 2, 4) as providing the
    best estimate of the masking ability of a noise.  A-weighted sound pressure level

        The simple A-weighted SPL is also a useful index of speech
    interference. The A-weighting process emphasizes the middle
    frequencies, as do the AI and SIL, but does not omit the lowest and
    highest frequencies completely.

        Experiments have shown that the AI is more accurate than any of
    the SILs or the A-weighted SPL in predicting the speech-masking
    ability of a large variety of noises. For noises of practical
    importance however, A-weighted SPL and SIL continue to be used, as the
    advantage of accuracy in the AI does not outweigh the ease of
    measurement of the first two indices. Comparisons of SILs and
    A-weighted SPLs show that, on average, the SIL is about 10 decibels
    lower than the A-weighted SPL for the same degree of interference
    (Klump & Webster, 1963; Kryter, 1970), although for unusual noises the
    average difference could vary substantially.

    3.2.3  Perception of speech out-of-doors

        Measurements indicate that, during relaxed conversation in the
    home, the speech level is approximately 55 dB(A) (Kryter, 1970;
    Pearsons et al., 1976), and that as the noise levels increase, people
    tend to raise their voices to overcome the masking effect. The
    so-called "normal effort" voice resembles a "stage" voice, and is used
    when people are given a prepared text to read (Korn, 1954), or when
    they wish to project their voices. Since everyday speech is spoken at
    a reasonably predictable level, it is possible to express many of the
    empirical relationships between background noise level and speech
    intelligibility in a single graph, as in Fig. 7 (US Environmental
    Protection Agency, 1974).

    FIGURE 7

        This figure, which is applicable to outdoor conditions, is based
    on the assumptions and empirical observations that:

        (a) at a distance of 1 m from the speaker, relaxed conversation
    occurs at a voice level of approximately 56 dB(A) and normal and
    raised voices at levels of approximately 66 dB(A) and 72 dB(A),
    respectively; and

        (b) for 100% sentence intelligibility the speech level should
    exceed the noise level by 10 dB(A). When the speech level is 10 dB(A)
    lower than the noise level, intelligibility falls to 95%. Because of
    the redundancy of speech, 95% intelligibility usually permits reliable
    although not necessarily comfortable conversation. The location of the
    curves in Fig. 7 may shift in certain circumstances, although it is
    difficult to predict to what extent spatial factors may facilitate or
    impair speech communication in noise. Lower noise levels may be
    required, if the speaker does not enunciate clearly or if the speaker
    and the listener use different dialects. People with hearing
    impairment may need more favourable speech-to-noise ratios depending
    on the variation of speech-to-noise ratio with frequency.

        Adequate communication in higher noise levels than those
    indicated in Fig. 7 can occur, if the messages are restricted, e.g.,
    when only numbers are being transmitted. Lipreading or observing
    facial or manual gestures may also improve communication. If the noise
    source is clearly localized at a position different from that of the
    speaker, speech communication may be possible in higher noise levels
    than those indicated in Fig. 7.

        Intermittent and impulsive noises as well as noises fluctuating
    in level will provide various degrees of masking. Again, the
    redundancy of speech means that an isolated short burst of noise is
    unlikely to produce much disruption in the communication process;
    however, the likelihood of disruption increases with increasing
    duration and frequency of occurrence of the noise bursts.

        The detailed characteristics of noises are also important. While
    the A-weighted SPL is an adequate index of the speech-interfering
    quality of many noises, others may require a more detailed analysis.
    This is true of noises that are dominated by either low or high
    frequencies, e.g., the rumble of distant traffic or the hiss of
    compressed air. For unusual noises, the AI should be calculated for a
    reliable prediction of speech intelligibility.

    3.2.4  Indoor speech communication

        The relationships shown in Fig. 7 apply only to outdoor (free
    field) communications, as they depend on the applicability of the
    inverse square law. Relationships indoors are different because of
    reverberations caused by reflections from the walls, floor, ceiling,
    and objects in a room. Instead of decreasing 6 dB for each doubling of
    distance, the sound level of the speech or the noise may drop by only
    1 or 2 dB. There is no simple formula that will predict speech
    interference indoors. Instead, it is usual to set standards on the
    basis of the average noise levels that have been judged in the past to
    be acceptable in similar settings.

        For example, Fig. 8 (US Environmental Protection Agency, 1974)
    shows the estimated sentence intelligibility, at speaker-listener
    distances greater than 1 m, as a function of A-weighted SPL in the
    reverberant conditions found in a typical living room. This shows that
    for 100% intelligibility, which is considered desirable for indoor
    listening conditions, a background noise level of less than 45 dB(A)
    is required.

    3.3  Pain

        Aural pain is induced, when the tympanic membrane tissue is
    stretched by large amplitude sound pressures. Under extreme
    conditions, the membrane can rupture (Hirsch, 1968).

        Although there is a fairly wide range of individual variability
    especially for high frequency stimuli (von Gierke et al., 1953), the
    threshold of pain for normal ears is in the region of 110-130 dB. The
    threshold for physical discomfort is in the region of 80 dB (Spreng,

        In abnormal ears, for example in cases of inflammation, pain may
    be caused in the eardrum or middle ear by sound levels of about 80-90
    dB SPL. By comparison, people without eardrums may feel no sensation
    of pain at sound levels of up to 170 dB SPL.

        A second type of aural symptom occurs as a result of abnormal
    function in the cochlea. Certain sensorineural disorders, and most
    frequently noise-induced hearing losses, are accompanied by a
    condition called auditory recruitment. Recruitment is defined as an
    abnormal increase in loudness perception. The phenomenon of
    recruitment is commonly used for the diagnosis of noise-induced
    hearing loss (audiometric suprathreshold tests). In some cases of
    sensorineural hearing disorders, such as Ménière's disease, another
    symptom appears in addition to recruitment called syscusis, which is a
    lowering of the threshold of aural discomfort and pain.

        An important consideration with regard to aural pain is the
    effect of noise on hearing-aid users. Discomfort associated with
    exposure to sudden loud noises, loud music, and even raised voices is
    a common complaint of people who wear hearing aids. Hearing aids that
    automatically limit output to 100-120 dB SPL or less, provide
    protection for sensitive ears, provided they are properly selected and
    fitted (Gabrielsson et al., 1974).

    FIGURE 8

    3.4  Sleep

    3.4.1  Nature of sleep disturbance

        Many people experience sleep disturbance due to noise and the
    problem has been reviewed by several authors (see for example,
    Griefahn et al., 1976). Social survey data indicate that sleep
    disturbance is considered to be a major environmental noise effect
    (Alexandre, 1974). However, in what proportion noise contributes to
    regularly occurring sleep disturbances or awakenings in the general
    population is not clear. Noise exposure can cause difficulty in
    falling asleep, disrupt sleep patterns, and awaken people who are

        Detailed laboratory studies of the problem have been made by
    monitoring electroencephalograph (EEG) responses and changes in
    neurovegatative reactions during sleep. Many of these experiments have
    only involved small numbers of test subjects over limited time periods
    and under laboratory conditions. Care must therefore be exercised in
    extrapolating conclusions to the population at large.

        Several stages of sleep can be identified from EEG responses. On
    relaxing, prior to sleep, the EEG pattern changes from rapid,
    irregular waves to a regular pattern; the alpha rhythm. This is
    followed by sleep stage 1, characterized by prolonged reductions in 
    wave amplitude and frequency. Later, in sleep stage 2, the pattern 
    changes to one of bursts of waves (spindle waves) mixed with single, 
    slow waves of relatively large amplitude (K-complexes). About 30-45 
    minutes later, periods of slow, high amplitude waves (delta waves) 
    appear in the EEG (stage 3). When the delta waves occur for about 50%
    of the recording period, the deepest sleep, stage 4, is reached. About
    an hour and a half later, the EEG pattern resembles that found in
    stage 1, but electrodes placed near the eye reveal rapid eye movement
    (REM); this is the stage during which most dreaming occurs. Some
    research workers have been able to elicit relatively complex motor
    responses to verbal instructions in the REM stage of sleep (Evans et
    al., 1966).

        During normal sleep, a person progresses through sleep stages 1-4
    with occasional reversals, the time spent in deep sleep and in the
    lighter stages of sleep depending upon age. With increasing age, a
    greater proportion of time is spent in the lighter sleep stages; from
    the age of 60 years onwards, sleep stage 4 is almost totally absent.
    It is considered that all stages of sleep are necessary for good
    physiological and mental health.

        Stimulation by noise causes changes in the EEG pattern lasting
    for a few seconds or more. These may appear as K-complexes (increases
    of wave frequency) that are only detectable by close inspection of the
    EEG recording, or changes of sleep stage. It has been reported that
    the effects of noise are related to the stage of sleep. Results from
    some studies suggest that thresholds for awakening are lower in the
    REM sleep stage, for nonimpulsive as well as impulsive noises (Berry &

    Thiessen, 1970). EEG pattern changes are least likely to occur in the
    REM stage (Thiessen, 1972).

        The effects of noise upon sleep depend upon the characteristics
    of the noise stimulus, the age and sex of the sleeper, the history of
    previous sleep, adaptation, and motivation.

    3.4.2  Influence of noise characteristics

        In studies of the effects of noise upon sleep, a variety of
    stimuli have been used including synthetic sounds as well as the
    sounds of aircraft (flyover noise and sonic booms) and road traffic.

        The effects of noise on sleep appear to increase as the ambient
    noise levels exceed about 35 dB(A) Leq (Beland et al., 1972). In one
    study, the probability of subjects being awakened by a peak sound
    level of 40 dB(A) was 5%, increasing to 30% at 70 dB(A). When changes
    in sleep stage were taken as an indication of disturbance, the
    proportion of subjects affected was 10% at 40 dB(A) and 60% at 
    70 dB(A) (Thiessen, 1969). It was also observed that subjects who
    slept well (based on psychomotor activity data) at a noise level
    (Leq) of 35 dB(A) complained about sleep disturbance and had
    difficulty in falling asleep at an Leq of 40 dB(A). At the higher
    level of noise, subjects took over an hour to fall asleep initially,
    and awakened frequently during the sleep period (Karagodina et al.,

        Exposure to noise levels of 48-62 dB(A) resulted in changes in
    sleep EEG patterns, manifested especially as an initial depression or
    interruption of alpha rhythm (Wilson & Zung, 1966). For sound stimuli
    of 70 dB(A), the most likely reaction was to awaken, followed by
    shifts in sleep stages (Thiessen, 1970). At 50 dB(A), 50% of subjects
    showed one of the following reactions: (a) slight changes in EEG
    pattern lasting for a few seconds; (b) pattern changes lasting up to a
    minute; (c) change of sleep stage; (d) awakening.

        It has been reported that brief acoustic stimuli are the most
    effective in eliciting EEG-K-complex in stage 2 of sleep (Vetter &
    Horvath, 1962). When the sleep disturbance effects of impulsive tone
    bursts, simulated sonic booms, and truck noise ranging from 85-105 dB
    were compared, it was observed that the frequency of awakening was
    lower for the impulsive noise and independent of the noise level.
    Increases in the level of truck noise and aircraft flyover noise
    increased the frequency of awakenings and shifts in sleep stages
    (Berry & Thiessen, 1970).

        The rate of occurrence of stimuli and/or fluctuation in the sound
    level were also found to influence sleep. The noise of low density
    traffic disrupted sleep more than that of high density traffic (Mery
    et al., 1971). Similarly, steady white noise of 40 dB(A) was not found
    to affect sleep, although fluctuating road traffic or factory noise

    with the same median level caused sleep disturbance (Osada et al.,
    1968). Short duration sounds of passing aircraft and trains with peak
    levels up to 60 dB(A) caused a similar degree of disturbance as steady
    noise at 40 dB(A), even though their total duration was less than 30
    minutes per night (Osada et al., 1969, 1972b, 1974). Hord et al.
    (1966) reported that a 3-second, 30 dB, 1000 Hz signal during sleep
    caused an increase in the heart rate of 5 subjects over a short period
    and that the response was most marked during REM sleep.

        The increase in eosinophils and basophils normally occurring
    during sleep was inhibited by continuous noise, such as traffic or
    factory noise, at levels of 40 dB(A) or more and by intermittent
    noise, such as aircraft or train noise (Osada et al., 1968, 1969,
    1972a, 1974).

        The number of field studies on sleep disturbance after noise
    exposure is very limited. In a study made during a 3-month period
    (Rylander et al., 1972a), civilian and military subjects were exposed
    during the night to sonic booms with peak over-pressures in the range
    of 6-64 Pa. It was observed that at about 60 Pa, 15% of military
    personnel had an increased rate of awakening and 56% of civilians
    reported sleep interference and difficulties in getting back to sleep.

    3.4.3  Influence of age and sex

        A number of studies have indicated that the sleep of children and
    young persons is less affected by noise than that of middle-aged or
    older persons (Dobbs, 1972; Nixon & von Gierke, 1972).

        On the other hand, children of 4-6 years of age seem to be
    particularly disturbed by sudden arousal from sleep stage 4 (Miller,
    1971b). It has also been reported that babies, who have had
    gestational difficulties or have suffered brain injury, are
    particularly sensitive to noise (Murphy, 1969).

        Certain data indicate that women are more sensitive to noise
    during sleep than men (Steinicke, 1957; Wilson & Zung, 1966; Lukas,
    1972b) and that middle-aged women are particularly sensitive to
    subsonic jet aircraft flyovers and simulated sonic booms (Lukas &
    Dobbs, 1972).

        Ando & Hattori (1970) found that about 50% of the women who had
    moved to Itami City, near Osaka Airport in Japan, during the first 5
    months of pregnancy said that, after birth, their infants slept
    soundly through the aircraft noise. However, this was true for less
    than 15% of the infants whose mothers had moved in during the last 5
    months of pregnancy. Because of limitations in the methods used in
    this study, these results should be considered with caution.

    3.4.4  Influence of previous sleep deprivation, adaptation,
           and motivation

        The amount of accumulated sleep time affects the probability of
    awakening. Arousal is more likely to occur after long periods of
    sleep, irrespective of the stage of sleep (Dement & Kleitman, 1957;
    Lukas & Kryter, 1970). Adaptation to noise during sleep is present if
    repeated exposure to sound stimuli during sleep results in
    progressively less interference with normal sleep.

        LeVere et al., (1972) studied the EEG response and task
    performance of six 20-24-year-old males. The experiment lasted 14
    nights, 7 of which involved exposure to 80 dB(A) jet aircraft noise
    for 20 seconds, 9 times each night. No adaptation in EEG noise
    response was observed. In studies on the effects of simulated sonic
    booms on sleep, Lukas & Dobbs (1972) concluded that some adaptation
    occurred. Thiessen (1972) reported that although the awakening
    response seemed to diminish with time, there was no adaptation of the
    EEG response to aircraft and traffic noise.

        Results of studies of simulated sonic booms with indoor intensity
    levels of 80-89 dB(A), applied alternatively 2 and 4 times each night
    for 2 months, did not reveal any adaptation in EEG pattern and
    vegetative function during, and shortly after stimulation. In the
    first quarter of the night, there was a significant reduction of the
    total time spent in the deepest stage of sleep but during the
    remainder of the night (with 4 booms) the duration of deep sleep was
    comparable with the nightly total before and after the noise test
    series (Jansen & Grifahn, 1974).

        Motivation and instructions given to subjects before sleep may
    influence the effects of noise on sleep. An ability of sleeping
    subjects to discriminate among various types of stimuli has been
    observed in experiments where the discrimination was learned when the
    subject was awake (Wilson & Zung, 1966). Research workers employing
    simulated sonic booms to investigate the effects on sleep behaviour,
    moods, and performance instructed their subjects to "ignore
    disturbances and attempt to get the best night's sleep possible". They
    found that the number of responses to booms were lower than those in
    similar studies where instructions had not been given (Collins &
    Iampiatro, 1974).

        It has been observed that effects of motivation on sleep
    disturbance depend to a certain extent upon the stage of sleep
    (Miller, 1971b). Instructions and financial incentives produced an
    increase in the frequency of stage shifts and awakening following
    exposure to moderate sound stimuli of different kinds (Wilson & Zung,

    3.4.5  Long-term effects of sleep disturbance by noise

        The long-term physiological and psychological effects of noise-
    induced sleep disturbance are practically unknown (Lukas, 1972b). Some
    insight into possible consequences may be obtained from experiments
    studying behaviour and performance after noise-induced sleep
    deprivation. A review of the influence of noise exposure on task
    performance is given in section 3.8.

        Some experiments have demonstrated that intense noise may improve
    performance in persons who have been without sleep and are tired, even
    when they are performing a task that would be highly affected by
    noise, if sleep had been normal (Corcoran, 1962; Wilkinson, 1963). On
    the other hand, LeVere et al. (1972) found decreased performance in a
    task involving a memory component after nightly exposure to 80 dB(A)
    aircraft noise.

        Tasks involving monitoring, mental arithmetic, and pattern
    discrimination were not influenced following nightly exposure of 24
    male subjects to 8 simulated sonic booms (100 Pa at 1-h intervals for
    12 nights) (Chiles & West, 1972). Cantrell (1974) exposed 20 men to
    80, 85, and 90 dB(A) tonal pulses with a 22-second interval throughout
    24 h for 10 days. EEG recordings showed evoked response activity
    during sleep but clearcut effects on various task performance tests
    were not observed. Exposure of 6 male subjects to a 15-second,
    80 dB(A) noise, 24 times per night resulted in a significant
    deterioration in the performance of a choice reaction/memory time test
    (LeVere et al., 1975).

        The results of studies reported so far suggest that the type of
    noise occurring during sleep as well as the type of performance test
    applied determine whether effects can be found or not. No observations
    have been reported concerning possible effects after repeated
    disturbance over a prolonged period of time or on the effects on
    populations exposed under real-life conditions.

    3.5  Nonspecific Effects

    3.5.1  The stress response

        Exposure to noise may evoke several kinds of reflex responses,
    particularly when the noises are of an unknown character or
    unexpected. These reflex responses are mediated through the vegetative
    nervous system and represent a part of the reaction pattern that has
    commonly been named the stress reaction. This response generally
    reflects primitive defence responses of the body and may also develop
    after exposure to other stimuli.

        If the exposure is temporary, the system usually returns to a
    normal or pre-exposure state within minutes. If the noise stimulation
    is sustained or consistently repeated, it has been postulated that
    persistent changes may develop in the neurosensory, circulatory,

    endocrine, sensory, and digestive systems. However, most available
    information on such effects has been obtained from animal experiments
    in which high levels of noise were used.

        Neurophysiologically, noise is a potent stimulus for the
    establishment of a reflex are incorporated in the syndrome of general
    adaptation to chronically maintained stress (Selye, 1955, 1956). The
    reticular and hypothalamic portions of the brain represent the centre
    of the reflex arc, the acoustic pathways represent the afferent
    branches and the ascending/descending nervous projections represent
    the efferent branches. Target organs include the visceral organs
    (heart, blood vessels, intestines, endocrine glands etc.) which are
    innervated by the autonomic nervous system and the hypothalamo-
    diencephalic centres that regulate the alternating rhythms of sleep-
    arousal, endocrine secretion, and other functions (Bergamini et al.,
    1976). The action of noise on the reticular formation depends not only
    upon its level and duration, but also upon its temporal
    characteristics. While impulse noise produced a stable and prolonged
    excitation of the reticular formation of the midbrain and of the
    temporal cortex in rabbits, results of one study showed that similar
    effects due to continuous noise exposure became insignificant after
    one hour (Suvorov, 1971).

        The reflex reactions also include changes in the functioning of
    the adrenal glands. In studies by Henkin & Knigge (1963), exposure of
    rats to continuous, high intensity sound (130 dB, 220 Hz) resulted in
    an initial high rate of hormone secretion followed by a depression of
    corticosterone output and a return to normal or high levels. In
    another experiment, an increased urinary excretion of epinephrine was
    found in 9 normal rats as an after-response to repeated 2-second
    exposures to high frequency sound (20 kHz) at 100 dB (Ogle & Lockett,
    1968). Temporary eosinopenia and temporary changes in the adrenal
    gland occurred in mice exposed daily to a single, 15 or 45-min period
    or intermittant periods (alternating 100-min periods) of noise at a
    level of 110 dB, 10-20 kHz (Anthony & Ackermann, 1955). However, in
    studies by Osintseva (1969), pathological changes could not be
    demonstrated in the adrenal glands of rats, one month after exposure
    to a noise level of 80 dB for periods ranging from 18 to 26 days.
    Horio et al. (1972) suggested that discrepancies in the reported
    results might be due to differences in the intensity and duration of
    noise exposure. As an example, they reported a study on 4 groups of
    rats (number pre group not stated) that were exposed for 8 h to noise
    of 60, 80, and 100 phons. Compared with control animals, the blood
    concentration of adrenal 11-hydroxy corticosteroid rose rapidly at the
    beginning of exposure reaching a maximum level within 15 min that was
    directly proportional to the intensity of the noise. Levels fell to
    those of the control group within 1-4 h. The results of a study by
    Anthony et al. (1959) showed that exposure to white noise (150-
    4800 Hz, 140 dB SPL) produced different acute effects in the mouse,
    rat, and guineapig. The authors concluded that the noise exposure was
    not harmful to the animal except in terms of hearing. Exposure was for
    15 min per day over a 4-week period. There was a reduction in activity

    (exploratory), which was most obvious in the guineapig. Some of the
    mice and rats exhibited a freezing reaction. There were no apparent
    changes in the weight of the adrenals, but the width of the
    fosciculate zone in rats and mice was greater in exposed animals. This
    is a sign of increased adrenocortical activity. No changes were seen
    in serum ions or blood sugar. Thus, the authors concluded that short-
    term noise exposure did not give rise to excessive adrenocortical

        In a study by Rosecrans et al. (1966), groups of 12 rats were
    exposed to variable stress (sound, flashing lights, and cage
    oscillation) for 3, 5, or 7, four-hour periods per week, for 16 weeks.
    The noises were 100 dB compressed air blasts, bells, buzzers, and
    tuning fork impulses for periods of 30 seconds at 5 min intervals. All
    the stress programmes produced significant increases in plasma
    corticosterone levels compared with unexposed controls. Furthermore,
    levels were significantly higher in isolated rats than in animals
    housed in pairs, indicating that isolation should also be considered
    as a stress.

        In human studies, increased urinary excretion of epinephrine and
    norepinephrine after exposure to 90 dB (2000 Hz) for 30 min was a
    constant finding in 5 healthy subjects and in 3 groups of 12 patients
    who, (a) had high blood pressure without known cause; (b) were
    recovering from a heart attack; or (c) were psychotic (Arguelles et
    al., 1970). Exposure of 5 healthy male students, twice a day for
    30 min to noise levels of 55, 70, or 85 phons resulted in changes in
    the levels of leukocytes, eosinophils, and basophils, as well as in
    urinary 17-hydroxycorticosteroid, compared with controls exposed to
    levels of 30-45 phons (Tatai et al., 1965, 1967). In another study, 6
    subjects were exposed for 2 or 6 h for several days to noise levels of
    40, 50, and 60 dB(A). Urinary excretions of 17-hydroxycorticosteroids
    and noradrenaline increased significantly during the period of
    exposure (Osada et al., 1973).

    3.5.2  Circulatory system responses

        Vasoconstriction or vasodilation of blood vessels can be induced
    by high levels of noise during acute exposures. Several studies in
    animals have demonstrated that prolonged exposure to high levels of
    noise can cause a persistent increase in blood pressure. In the study
    by Rosecrans et al. (1966), the stress increased the average blood
    pressure of rats by approximately 3.9 kPa (30 mmHg) compared with that
    of control animals. It has also been reported that the absence of
    sound can cause hypertension in rats (Lockett & Marwood, 1973).

        Other animal studies have shown that the cerebral blood supply
    can be influenced by high levels of noise. Alternating spasms and
    dilation of the arterial blood vessels were observed in rats exposed
    to a continuous noise level of 100 dB (Alekseev et al., 1972). At
    levels up to 100 dB, the constriction was proportional to the amount
    by which the overall SPL exceeded 70 dB, reaching values as much as

    40% higher than resting values. As well as creating a condition of
    generalized vasoconstriction, continuous exposure of rats to a noise
    level of 110 dB SPL, for 48 h, resulted in an inadequate supply of
    blood to the cochlear cells (Lawrence, 1966; Lipscomb & Roettger,
    1973). These reports suggest that damage to the cochlear tissue may
    result from an insufficient supply of oxygen and other nutrients
    (section 3.1.2).

        As a result of observations made in animal experiments, the
    relationship between noise exposure and chronic circulatory disease
    has been investigated in man. Ten subjects were exposed to 90 dB white
    noise for 29 min. No effects were observed on cardiac output, cardiac
    rate, cardiac stroke volume, or pulmonary artery pressure (Etholm &
    Egenberg, 1964). Klein & Grübl (1969) found an approximately equal
    distribution of increases and decreases in the pulse rate of the
    internal carotid artery among 40 persons exposed to 92-96 dB noise for
    10 seconds.

        Differences between the sexes have been demonstrated in an
    experiment involving exposure to jet aircraft and to railway and pile-
    driver noise of 70-85 dB(A) (Osada et al., 1972b). Pulse rate
    fluctuations, vascular constriction, and increase in urinary
    noradrenaline levels were greater in female subjects than in males.
    From studies by Jansen (1970) and Lehmann & Tamm (1956), it can be
    concluded that meaningless noise causes an ergotropic reaction in the
    circulatory system with peripheral vasoconstriction and reduction of
    heart stroke value without change of pulse rate and blood pressure.

        Certain authors have found evidence in man of an association
    between continuous noise exposure and constriction of blood vessels
    that is primarily manifested in the peripheral regions of the body
    such as fingers, toes, and earlobes (Lehmann & Tamm, 1956; Grandjean,

        Some workers have reported that vasoconstriction does not
    completely adapt with time, either on a short-time or long-term basis,
    and that effects often persist for a considerable time after cessation
    of the noise. Peripheral vessel constriction has been found to occur
    equally in noise-sensitive and noise-insensitive subjects (Valcic,
    1974). It has been suggested that vasoconstriction, with its
    concomitant effect on the circulatory system in general, will
    eventually lead to heart disease (Jansen, 1969). A higher incidence of
    circulatory problems, peripheral blood flow disturbances, and
    irregularities of heart rate have been reported among steel workers
    exposed to a noise level of 95 dB (Jansen, 1961).

        Significantly increased blood pressure levels compared with those
    of control groups have been reported from studies on machine-shop
    operators (Andriukin, 1961) and weavers (Parvizpoor, 1976). According
    to Jonsson & Hansson (1977), differences in blood pressure levels were
    also found in a noisy factory, between a group of workers with hearing
    losses and another group with no loss of hearing.

        In view of some epidemiological shortcomings in the previous
    studies, particularly with reference to the selection of population
    segments, further studies in the industrial environment are required
    to elucidate the association between exposure to noise and increased
    blood pressure. Community studies are scarce and should be extended,
    since tendencies similar to those found in industrial populations have
    been observed. In a survey involving residents around an airport,
    psychophysiological and medical tests showed that experimental
    exposure to aircraft noise caused constriction of blood vessels, and
    increases in heart rate and electrical muscular activity. However, a
    tendancy for blood pressure to be higher among persons living in the
    noisier areas was not statistically significant (Deutsche
    Forschungsgemeinschaft, 1974).

    3.5.3  The startle reflex and orienting response

        Certain noises, especially those of an impulsive nature, may
    cause a startle reflex, even at low levels. The startle (Molinie,
    1916) occurs primarily in order to prepare for action appropriate to a
    possible dangerous situation signalled by the sound. It consists of
    contraction of the flexor muscles of the limbs and the spine and a
    contraction of the orbital muscles that can be recorded as an eye
    blink. It may be followed by an orienting reflex that causes the head
    and eyes to turn towords the source of a sudden sound in order to
    identify its origin (Thackray, 1972). The startle reflex can sometimes
    be followed by a fright reaction, in which case the effects on the
    circulatory system become more pronounced. Skin conductance is also
    influenced due to alterations in perspiration. A dose-related
    depression of the galvanic skin response was found after exposure to a
    15-second white noise (Klosterkötter, 1974).

        The presence of these reflexes is detected either by noting
    behavioural reactions or by the electrophysiological study of muscle
    tension and activity (Galambos et al., 1953; Davis et al., 1955).
    Although low level sound stimulation may be sufficient in abruptness
    and information to induce a startle reflex, the fact that a person has
    experienced some degree of startle, may often only be recorded

        For meaningless noise of various types, it has been observed that
    orienting reflexes are elicited at the very beginning of a series of
    stimuli; but that habituation occurs. At higher noise levels,
    habituation is less marked.

        Experiments involving sonic booms (outdoor levels ranging from 60
    to 640 Pa and corresponding indoor levels ranging from 20 to 130 Pa)
    demonstrated that startle reactions in 56 female volunteers increased
    with the intensity of the boom. The reactions of the subjects were
    evaluated using two different steadiness tests and a tracking test
    (Rylander et al., 1974b). A tendency to habituation and a masking

    effect of background noise was also found. The possible long-term
    effects on human subjects of sustained repetition of acute startle
    reactions are not known.

    3.5.4  Effects on equilibrium

        A high level of noise may influence equilibrium because of the
    stimulation of the vestibular sense organ. However, available data
    concerning this subject are both inconclusive and inadequate.
    Complaints of nystagmus (rapid involuntary side-to-side eye
    movements), vertigo (dizziness), and balance problems have been
    reported after noise exposure in the laboratory, as well as in field
    situations. However, the levels needed to cause such effects in
    personnel working on jet engines were quite high, typically, 130 dB
    SPL or more (Dickson & Chadwick, 1951). Less intense noise levels
    ranging from 95 to 120 dB SPL also disturb the sense of balance, if
    there is unequal stimulation of the two ears. This was demonstrated in
    laboratory studies in which subjects wearing various combinations of
    ear protectors and balancing on rails of different widths were exposed
    to various noise levels (Nixon et al., 1966; Harris, 1974).

    3.5.5  Fatigue

        Additional strain on the body, induced by noise, may cause the
    development of fatigue either directly, or indirectly through
    interference with sleep. A variety of environmental agents as well as
    conditions within the individual may cause symptoms of fatigue - thus
    the role of noise as a causal factor is difficult to establish.

        In one study, symptoms of extreme fatigue were reported by
    subjects exposed to high levels of infrasound; this was interpreted as
    evidence of a direct link between fatigue and high intensity noise
    (Mohr et al., 1965). In another study, workers from workshops with 5
    different levels of noise intensity ranging from 50 to 125 dB were
    investigated. In this case, no simple relationship was found between
    noise levels and feelings of fatigue. The authors suggested that
    social as well as cultural factors should be taken into account to
    obtain a better understanding of the way exposed persons feel about
    noise (Matsui & Sakamoto, 1971).

        The influence of noise on fatigue can also be related to
    performance. As will be discussed in section 3.8, noise may interfere
    with performance as well as leave it unchanged or even improved. Since
    many studies on performance have not taken fatigue into consideration,
    the question arises as to whether the strain of overcoming noise
    disturbance in order to maintain performance might not lead to

        Questions concerning fatigue are usually included in social
    survey studies on annoyance (section 3.7) but, so far, no extensive
    evaluation of these data in relation to noise exposure levels has been

    3.6  Clinical Health Effects

    3.6.1  Background

        Earlier in the document, it has been shown that exposure to noise
    may result in a variety of biological reflexes and responses. Most of
    the information has been derived from short-term studies on animals
    and human subjects, but it has been postulated that, if provoked
    continuously, such responses would ultimately lead to the development
    of clinically recognizable physical or mental disease in man.

        Numerous clinical symptoms and signs have been attributed to
    noise exposure including nausea, headache, irritability, instability,
    argumentativeness, reduction in sexual drive, anxiety, nervousness,
    insomnia, abnormal somnolence, and loss of appetite (Jirkova &
    Kromarova, 1965).

        From a theoretical point of view, an assessment of the causal
    relationship between noise exposure and such nonspecific health
    effects presents difficulties. Increases in blood pressure level,
    heart disease, gastric ulcers, and other stress-related syndromes have
    a multifactorial origin. It is difficult to exercise sufficient
    control over all relevant risk factors in epidemiological studies,
    particularly as several of the risk factors such as social class,
    personal habits, and personality characteristics are difficult to

        The study of selected population segments exposed to high levels
    of noise in industry has been suggested as an epidemiological model to
    overcome some of these difficulties.

    3.6.2  General health

        In one study, medical records of 969 workers exposed to noise
    levels of 85-115 dB were compared with those of workers in areas where
    levels were 70 dB or less (Jirkova & Kromarova, 1965). In addition to
    a higher incidence of hearing loss, the noise-exposed group was found
    to have a higher prevalence of peptic ulcers and hypertension. In a
    previously cited study (Jansen, 1962) on workers exposed to high
    intensity noise, there was evidence of a higher frequency of
    circulatory problems and a higher incidence of fatique and
    irritability in the exposed group compared with the controls. Cohen
    (1973) studied the medical records of 500 workers working in noisy
    areas (95 dB(A) or more) and those of a group matched for age and
    length of plant experience, working in quieter areas (80 dB(A) or
    less). The noise-exposed workers tended to have more symptomatic
    complaints and more diagnosed medical problems. It is difficult,
    however, to relate these findings to noise only, since noisy work
    places are, presumably, also work places with other health hazards.
    Benko (1959, 1962) examined workers exposed to noise levels of 

    110-124 dB and found a persistent narrowing of the visual field as
    well as a decrease in colour-perception. The second finding could not
    be varified in studies reported by Kitte & Kieroff (1971).

        Methods of studying industrial populations have shortcomings that
    make it difficult to draw conclusions concerning the different
    populations. The group is always selected, i.e., those not able to
    tolerate the exposure and those developing medical symptoms may have
    left. The group usually consists of males in good physical condition
    and older age groups are under-represented.

        Only a few studies of the relationships between general health in
    the population and noise exposure are available. In a study by
    Karazodina et al., (1969), 140 000 patients registered at the
    outpatient departments of different hospitals were divided into those
    living 6-10 km from large airports and those living in quiet areas. A
    2-4 fold increase in hypertension, nervous disorders, gastritis,
    gastric ulcers, and auditory disease was found in the noise-exposed
    group. As an increase was also found in respiratory disease, factors
    other than noise pollution may have been responsible for the
    differences between the two groups.

        In a study on aircraft noise around Munich, Federal Republic of
    Germany, no signs of disease were found in a thoroughly examined
    sample of the population exposed to 82-100 dB(A) aircraft noise
    (Deutsche Forschungsgemeinschaft, 1974).

    3.6.3  Mental health

        An association between exposure to high levels of occupational
    noise and the development of neurosis and irritability and also
    between environmental noise and mental health has been proposed by
    several workers. Herridge (1972) suggested that noise was not a direct
    cause of mental illness but that it might accelerate and intensify the
    development of a latent neurosis.

        Studies of the records of some, 124 000 persons living in a noisy
    area around London Heathrow airport and in a quieter area nearby
    revealed a higher rate of admittance to mental hospitals in the noisy
    area (Abey-Wickrama et al., 1969). However, the design of the
    epidemiological study was questioned by other workers (Chowns, 1970)
    and the finding could not be verified in a later investigation
    (Gattoni & Tarnopolsky, 1973). The relationship between noise
    exposure, the presence of mental disorders, and annoyance was studied
    in a field investigation on 200 persons, half of whom lived near
    London Heathrow airport. No association was found between noise
    exposure and mental morbidity, but symptoms of mental disorders were
    more common among those who reported that they were very annoyed by
    the noise (Tarnopolsky et al., 1978).

        The consumption of tranquilizers and sleeping pills has been
    proposed as an indication of latent disease or mental disturbance in
    noise-exposed communities. Grandjean (1974) reported an increase in
    the consumption of such drugs among persons exposed to aircraft noise.
    Findings to the contrary were reported from a study of subjects living
    in the neighbourhood of Munich airport (Deutsche
    Forschungsgemeinschaft, 1974). A possible explanation for the
    discrepancy between the two studies is the manner in which the
    questions concerning drug consumption were posed and related to
    aircraft noise exposure.

    3.7  Annoyance

    3.7.1  Definition and measurement

        Annoyance may be defined as a feeling of displeasure associated
    with any agent or condition known or believed by an individual or a
    group to be adversely affecting them. While it is often useful or
    necessary from a practical point of view to focus attention on a
    single agent, in this case noise, it should be recognized that, in
    real life, it is only one of a combination of environmental stresses.

        Annoyance is generally related to, the direct effects of noise on
    various activities, such as interference with conversation, mental
    concentration, rest, or recreation. The degree of physical exposure as
    well as intervening psychosocial variables determine the occurrence
    and extent of the annoyance response. All these variables must be
    measured in experimental or epidemiological studies, in order to
    arrive at an appropriate judgement concerning annoyance effects
    (Borsky, 1972).

        Numerous techniques have been devised to measure annoyance
    (section 3.7.4). A subject can classify the degree of annoyance
    verbally (from "not annoyed" to "very annoyed") or with the aid of a
    number scale (e.g., 1-7 or 1-10). The annoyance can then be assessed
    using these responses, or by different scaling techniques based on
    several other questions relating to disturbance and activity
    interference (Kryter, 1970).

        Studies on annoyance have been made in both laboratory and field
    experiments. Different degrees of annoyance can be described with
    relatively high precision, and the results seem to be reproducible
    between different studies, although it has been questioned whether
    there is a consistent relationship between annoyance measurements
    (Berglund et al., 1974).

        Laboratory studies on annoyance involve judgements of individual
    noise events in controlled environments. Such studies have isolated
    some of the acoustic and sociopsychological factors contributing to
    annoyance. Examples of such factors are the level of noise, its

    spectral, temporal, and impulsive characteristics, information
    conveyed by the noise, the sex, age, and occupation of the respondent,
    and attitudes towards the source of the noise.

        A number of surveys have been performed to determine how
    annoyance reactions are affected by, and related to noise (McKennell,
    1961; Cedarlöf et al., 1963, 1967; Auzou & Lamure, 1966; Bruckmayer &
    Lang, 1967; Coblenz et al., 1967; Lamure & Bacelon, 1967; Griffiths &
    Langdon, 1968; TRACOR, 1971; Deutsche Forschungsgemeinschaft, 1974;
    Grandjean, 1974; Rylander et al, 1974a; Nishinomiya, 1976). Methods
    that allow the prediction of annoyance from measurements of the
    physical characteristics of the noise have been suggested. These
    studies have also served as a basis for the development of noise
    criteria and standards. Few studies have included an analysis of the
    incidence of annoyance in relation to the specific health effects
    described previously.

        The following sections describe present knowledge concerning the
    relationships between annoyance and different kinds of noises.

    3.7.2  Instantaneous noise dose

        It is generally assumed that the annoyance effects of short-term
    exposure to noise are a function of loudness, i.e., the louder of two
    sounds will cause the more annoyance. There are many data in the
    literature on the measurement of loudness, defined as the perceived
    magnitude of sound, and numerous techniques exist for estimating
    loudness from the spectral analysis of the sound. The most complex
    (Stevens, 1956; Zwicker, 1959; Kryter & Pearsons, 1963) are based upon
    accepted auditory function theory and give loudness estimations in
    phons. More practical alternatives to these are available based on
    standard sound level meters in the form of A, B, and C frequency
    weighting filters that simply weight the sound energy in accordance
    with various auditory frequency response functions (section 2.2). The
    A-weighted SPL has gained widespread acceptance as a suitable noise
    level scale for general use. Other units have been developed for
    particular noises e.g., the perceived noise level (PNL) for aircraft
    noise (section 2.2.6).

    3.7.3  Long-term noise dose

        Characteristics related to the disturbance and annoyance-inducing
    potential of long-term noise exposure include the manner in which the
    loudness level (instantaneous noise dose) varies with time (e.g., the
    distribution of noise events over a 24-h period). Considerable effort
    has been devoted to the search for an acoustic index of chronic noise
    exposure. The major requirements of such an index are that it should
    be well correlated with human reactions and that it should be
    convenient to measure. Thus, for airport noise, which is characterized
    by infrequent but very intense aircraft sounds superimposed on
    relatively low background levels, indices have emerged that are based
    upon measurements or estimates of the individual aircraft sound

    levels. For road traffic noise, usually involving much greater vehicle
    movement frequencies, it would be quite impractical to record or
    estimate the level of each individual vehicle. In this case, noise
    variables are based on automatically integrated noise analysis. For
    certain industrial noise environments, indices are calculated from
    sound level meter readings of a set of relatively steady levels. Most
    indices include a summation process that accounts for the repetitive
    or continuous nature of the sound.  Aircraft noise

        An early general noise exposure index was the composite noise
    rating (CNR) devised by Rosenblith & Stevens (1953) for assessing
    environmental noise nuisance. Initially, this index was quite
    elaborate, accounting in a semiquantitative way for average noise
    level, discrete frequencies, impulsiveness, repetitiveness, and
    background noise. Some psychosocial factors were also taken into
    account by considering time of day (on the assumption that people are
    more noise-sensitive at night) and the history of the previous noise
    exposure of the community. It was later modified in the light of new
    experience (Stevens et al., 1955) and a special version was developed
    for application to airport noise (Stevens & Pietrasanta, 1957). The
    aircraft noise model was modified to its currently existing form
    (Galloway & Pietrasanta, 1967) largely to simplify it and to
    incorporate the PNL. Essentially, CNR has the form:

    CNR = LPN + 10 log10  N +  C

    where  N is the number of aircraft sounds during a particular time
    interval, LPN is their mean peak PNL and  C is the sum of a
    collection of weighting factors that account for time of day, season
    of the year, and ground engine test runs, to which the community is
    particularly sensitive. The procedure provides guidance on the
    community reaction to be expected as a function of noise level.

        Later developments of the CNR were the noise exposure forecast
    (NEF), (Bishop & Horonjeff, 1967) and the total noise exposure level
    (TNEL) recommended by the International Civil Aviation Organization
    (ICAO, 1971).

        On the basis of a social survey at London Heathrow Airport by
    McKennell, (1961), it was deduced that airport noise exposure should
    be expressed as a noise and number index (NNI) (Wilson, 1973).

    NNI = LPN + 12 log10  N - 80

        The main difference between CNR and NNI is the use of a "number"
    coefficient of 15 rather than 10. Robinson (1969) later remarked that
    this difference really represented an "intermittency" correction in
    the case of NNI, implying that community annoyance grows with the
    frequency of event more rapidly than is indicated by the equal energy
    concept inherent in the CNR formula. Doubts arose concerning the

    validity of the factor 15 following a later survey around London
    Heathrow (MIL Research Limited, 1971) and a Swiss study by Grandjean

        The relative influence of the noise and number terms is still a
    basic issue and a number of subsequent studies (Connor & Patterson,
    1972; Deutsche Forschungsgemeinschaft, 1974; TRACOR, 1971; 1976) have
    not provided any clear answer to the problem.

        A number of variations of the basic formula:

    Noise Index =  L +  K log10  N +  C

    have been adopted for use in various countries, and the effective
    values of K are given for some of these in Table 3. Other suggested
    values of K range up to 24 (McKennell, 1961; Deutsche Forschungs-
    gemeinschaft, 1974). It is evident from the table that, for K, the
    value 10 is commonly in use, probably because of its compatibility
    with the equal energy principle.

        All indices have a great deal in common with each other as well
    as other similar indices not included in the table. All involve
    measurements of average aircraft noise levels expressed in dB(A),
    dB(PN), or dB(EPN). Some take into account the duration of the sound,
    others do not. In most cases, the influence of some psycho-social
    factors is accounted for, directly or indirectly. Basically, the
    differences in various indices for the estimation of mean perceived
    magnitude are small (Botsford, 1969; Young & Peterson, 1969;
    Ollerhead, 1973).

        Other concepts concerning the relationship between aircraft noise
    exposure and consequent annoyance reactions have been suggested which
    contrast with the rather uniform approach to aircraft noise assessment
    just discussed.

        In studies in Scandinavia (Rylander et al., 1972a) and in an
    analysis of earlier studies (Rylander et al., 1974b), the extent of
    annoyance was found to be related to the A-weighted SPL of the
    noisiest type of aircraft. An increasing number of overflights
    increased the extent of annoyance at the same dB(A) level up to a
    certain threshold, beyond which a further increase in the number of
    events did not influence the annoyance. The second finding was also
    present in the second London Heathrow study (MIL Research Limited,
    1971),and a reanalysis of aircraft noise survey data from the USA
    (TRACOR, 1976).

        Table 3.  Examples of aircraft noise exposure Indices

    Country/Organization       Index                    K      References

    France                     Isopsophic Index         10     French Government (1974)

    Germany, Federal           Störindex Q (and Leq)a   13.3   Koppe et el. (1693)
     Republic of

    Japan                      WECPNLa                  10     Japanese Environment Agency (1976)

    Netherlands                "Total Noise Load" B     15     Kosten et at. (1967)

    South Africa               Noisiness Index NI       10     South African Bureau of
                                                               Standards (1973)

    United Kingdom             NNI                      15     Wilson (1973)

    United States of America   CNR/NEF, Ldn             10     Galloway & Pietrasanta (1967),
                                                               Bishop & Horonjeff (1967),
                                                               Von Gierke (1975)

    California                 Community Noise          10     State of California (1970)
                                 Equivalent Level

    ICAO                       TNEL                     10     ICAO (1971)

    ISO                        Aircraft Exposure        10     ISO (1970)
                                 Level LE

    a A special version for aircraft noise.
   Road traffic noise

        The traffic noise index (TNI) was developed from the results of a
    social survey in London (Griffiths & Langdon, 1968). It was based on
    the weighted combination of the sound levels (in dB(A)) exceeded for
    10%, 50%, and 90% of the time according to the formula:

    TNI = L50 + 4 (L10 - L90).

        This index reflects the conclusion that traffic noise annoyance
    depends not only upon the average or typical noise level (L50) but
    also upon the magnitude of the fluctuation (L10-L90). However,
    further investigation revealed that, because of the practical
    difficulties of predicting L90 with an adequate degree of
    confidence, the value of TNI was susceptible to large errors. Thus,
    TNI was subsequently rejected in favour of L10 for traffic noise
    compensation regulations (UK Statutory Instrument, 1975), even though
    its correlation with annoyance was shown to be inferior to that of TNI
    in the original survey.

        Because of a very high correlation between different indices that
    are sensitive to peak levels in the noise-time history, it may safely
    be assumed that any such index will predict traffic noise annoyance
    reactions with equal reliability. Evidence of the importance of peak
    noise levels comes from investigations in England (Langdon, 1976) and
    Sweden (Rylander et al., 1976) in which the extent of annoyance was
    found to be well-correlated with noise levels generated by heavy
    vehicles. The correlation between Leq and annoyance was relatively
    low in the second of these studies.

        A high correlation was found between Leq for urban traffic
    noise and the extent of annoyance in the exposed population in studies
    by Lang (1965).

        A detailed re-evaluation of available data on traffic-noise
    exposure and annoyance has recently been carried out by a working
    group of the International Organization for Standardization. Several
    existing and newly-proposed indices, mostly derived from Leq, were
    correlated with subjective response and though it was recognized that
    insufficient data were available to draw a firm conclusion, it was
    recommended, that, at present, Leq (as described in ISO, 1971)
    should be used for the assessment of road traffic noise.  General environmental noise

        On several occasions, single noise exposure indices that could be
    used to predict the annoyance caused by all kinds of environmental
    noise have been proposed, recognizing that different psychosocial
    influences might alter the dose-response function for different kinds
    of noise.

        In a search for such a general noise index, Robinson (1969)
    modified the traffic noise index to form the noise pollution level
    (NPL) given by

    NPL = Leq + 2.56 delta

    where Leq is the equivalent continuous sound level and delta is
    the standard deviation of the temporal fluctuations of the level. The
    noise pollution level concept has been given considerable attention by
    research workers in various countries. It was rejected by the British
    Noise Advisory Council as a recommended "unified" noise index (Noise
    Advisory Council, 1975), in favour of Leq on the grounds that
    further research into the utility and validity of NPL was desirable.
    Meanwhile, Robinson (1972) and others have considered refinements of
    NPL, effectively making the coefficient of delta a function of level
    fluctuation rate.
         In the USA, after an exhaustive review of available noise impact
    research, an interagency task force concluded that a modified
    equivalent continuous sound level, taken over a 24-h period, with a
    10-dB penalty applied to night-time sound levels, was the noise index
    that combined ease of measurement and high correlation with annoyance,
    complaint behaviour, and overt community reaction caused by noise of
    all kinds (US Environmental Protection Agency, 1973a). This index,
    which was named the day-night average sound level (Ldn), was based
    upon the use of the A-weighted SPL scale (yon Gierke, 1975).

        Over the past few years, there has been a widespread tendency to
    use Leq for general noise assessment purposes because of its
    simplicity. Leq is normally computed for specific portions of the
    24-h day or, alternatively, a weighted average, such as Ldn, is
    computed after emphasizing noise that occurs during noise-sensitive

    3.7.4  Correlation between noise exposure and annoyance

        The direct correlation between long-term noise exposure and
    annoyance has been studied for various kinds of noise exposure. The
    numerous composite noise indices that have emerged from these studies
    have been attempts to improve this correlation, by taking into account
    various factors including: time of day (day, evening, night), noise
    source (e.g., aircraft, road traffic, industrial source) and type of
    neighbourhood (e.g., rural, suburban, commercial). The choice of
    appropriate noise index (Leq, NEF, etc.) normally depended on the
    source whereas the type of neighbourhood was usually considered in the
    interpretation of scale values concerning the likely response (e.g.,
    for land use planning purposes).

        Regardless of how the dose scale was derived, the main technique
    for evaluating its validity was through use of the social survey and
    the annoyance measuring techniques already mentioned. Such surveys
    (e.g., McKennell, 1961; TRACOR, 1971) have shown that the correlation

    coefficient between noise exposure and average response (e.g., the
    average response of all respondents exposed to a given noise) is
    relatively high (> 0.8) implying that the noise scales are useful
    predictors of average reaction. However intersubject variability is
    high, and the correlation coefficient between noise exposure and
    individual annoyance is low (< 0.5). That individuals vary in their
    susceptibility to a particular level of exposure is a biological
    phenomenon common to all environmental influences. For all kinds of
    agents including chemical substances and physical factors, an
    increasing dose will gradually lead to an increasing number of persons
    being affected in any type of population. Thus, for the setting of
    standards, the relationship between the exposure to an environmental
    agent and the reaction has to be based upon the average reaction among
    a group of individuals. This group may be defined as a representative
    sample of the population or a particularly sensitive group. The
    variation between individuals can be attributed to sociopsychological
    factors. In one study of aircraft noise (TRACOR, 1971), the most
    important of the factors were fear of crashes, general noise
    susceptibility, ability to adapt to noise, opinions about the
    importance of the aircraft operations, and belief that the noise could
    be better controlled. The interrelationship between these factors is
    very complex. Even the direction of the causality is not clear: does
    fear of crashes increase noise annoyance or vice versa? The
    multivariate statistical analyses performed in some studies are not
    adequate to resolve such questions and further investigations are

        By comparing results of noise annoyance surveys around major
    airports, it has been found that variation between the reactions of
    individuals is very similar from place to place and from time to time
    (Alexandre, 1970; Ollerhead, 1973; Rylander & Sörensen, 1974).
    Regardless of how the reaction is measured, people express similar
    degrees of annoyance in relation to similar ranges of noise exposure.
    However the total range is considerable. Fig. 9 shows the cumulative
    distribution of annoyed people at London Heathrow airport as a
    function of noise exposure measured in NNI (Ollerhead, 1973). The
    different curves represent different annoyance levels, and each is a
    cumulative normal (Gaussian) distribution with a standard deviation of
    20 NNI. Comparison of these curves with similar data from other
    surveys suggests that they would be valid for any major international
    airport with about 20% of its aircraft movements occurring at night.

        Attempts have been made to combine survey data from various
    sources. Fig. 10 shows two typical results (US Environmental
    Protection Agency, 1973a; Schultz et al., 1976). The differences
    between these two curves reflect different interpretations of the type
    of reaction that constitutes "high" annoyance. The noise exposure
    scale in Fig. 10 is Leq (day time) or Ldn (expressed in dB(A)),
    since these variables tend to be roughly equal for typical 24-h work
    exposure. Interpretation of Fig. 10 for non-typical night-time noise
    exposure would depend upon the night-time weighting selected on the
    basis of local circumstances. In the USA, this is taken to be + 10 dB

    FIGURE 9

    FIGURE 10

    (incorporated in Ldn). Despite the disparity associated with the
    meaning of "highly" annoyed, Fig. 10 indicates that a level of Leq
    (day-time) or Ldn < 55 dB(A) will cause relatively little annoyance
    and may be considered as an ultimate goal for general environmental
    noise exposure.

    3.7.5  Overt reaction

        Complaints and other forms of community overt reaction to noise
    provide important indicators of the existence of a noise problem. On
    the other hand, because of the greater influence of psychosocial
    factors, the number of complaints is very poorly correlated with the
    noise exposure level (McKennell, 1961; TRACOR, 1971).

        Several procedures have been suggested for predicting the
    likelihood of overt reaction to noise exposure taking into account
    some sociopsychological factors. These include the CNR method already
    referred to (Stevens et al., 1955) and the British (BSI, 1967) and ISO
    (ISO, 1971) recommendations. However, in some ways the British and ISO
    practices may be considered as developments of CNR. In the ISO
    procedure, the expected community response is divided into five
    categories ranging from "none" to "very strong" with the descriptions;
    no observed reaction; sporadic complaints; widespread complaints;
    threats of community action; and vigorous community action. The likely
    reaction is specified as a function of the amount by which the rating
    level exceeds: the criterion value.

        Caution must be exercised in the use of such standards, since the
    evidence upon which they are based is fragmentary; indeed the ISO
    recommendation admits to only a "rough connexion" between public
    reaction and noise.

    3.8  Effects on Task Performance

        The effect of noise on the performance of tasks has mainly been
    studied in the laboratory but also to some extent, in work situations.
    Comprehensive reviews of these studies are available (Broadbent, 1957,
    1971; Cohen, 1968; Kryter, 1970; Glass & Singer, 1972; Burns, 1973).
    There have been few detailed studies of noise effects on human
    productivity under normal living conditions.

        In general, when a task involves auditory signals, whether speech
    or nonspeech, noise at any intensity sufficient to mask or interfere
    with the perception of these signals may interfere with the
    performance of the task. When the task does not involve auditory
    signals, the effects of noise on performance are more difficult to
    assess. The literature shows that noise can interfere with or enhance
    performance but that often it does not cause any significant change. A
    possible explanation of this seems to be the different uses of the
    term performance. As already mentioned, the most varied forms of

    reaction (e.g., control activity, rapidity of reaction, learning
    performance, memory training, intelligence tests) are all defined as

        Basically, all performance, whether mental or motor can be
    adversely affected by noise. This effect is likely to be more severe
    as the task becomes more difficult and complex and as the duration of
    the noise exposure increases.

    3.8.1  Noise as a distracting stimulus

        Noise can act as a distracting stimulus, depending on the
    meaningfulness of the stimulus and file psychophysiological state of
    the individual. According to a widely accepted theory in psychology,
    the human sensory system receives more information than can be
    analysed by the higher centres. In order to screen out useless
    information such as noise, the concept of a mental "filter" has been
    developed (Broadbent 1972). This "filter", however, has the following

        (a) it tends to reject or ignore unchanging signals over a period
    of time, even though they may be important, as in vigilance tasks;

        (b) an individual's state of arousal, stress, or fatigue may
    hinder the mental filter's ability to discriminate; and

        (c) the filter can be overridden by irrelevant stimuli that
    demand attention because of novelty, intensity, unpredictability, or
    learned importance.

        Thus a novel event, such as the start of an unfamiliar noise,
    will cause distraction and interfere with many kinds of task. This
    will be equally true, however, of the sudden stopping of a familiar
    noise; and, in each case, the effect will disappear once the novelty
    has worn off. These reaction patterns are well established
    experimentally (Kryter, 1970; Glass & Singer, 1972).

        In 1955, Hebb suggested that changes in stimulation not only
    initiate appropriate cortical responses but also activate or arouse
    areas of the cerebral cortex other than those involved in the
    response. This wider arousal activity originates in the reticular
    formation, a portion of the central nervous system, and affects the
    person's psychological state as well as physiological systems.

        Too low a level of arousal can mean complete absence of activity
    and therefore poor performance. On the other hand, too high a level
    may cause inefficiency through over-reaction to distraction, leading
    to incorrect responses. Thus, loud noise might increase or decrease
    task performance depending on the previous state of arousal.

    3.8.2  Effects on tasks involving motor or monotonous activities

        It appears that steady noise has little, if any, effect upon many
    tasks, once it has become familiar. Such tasks include tracking or
    controlling tasks where noise levels are fairly continuous and where
    average, rather than instantaneous, levels of performance are
    important (Broadbent, 1957; Kryter, 1970). Many mechanical or
    repetitive tasks found in factory work would fall into this category.
    Generally it can be concluded that noise is likely to reduce the
    accuracy rather than the total quantity of work (Broadbent, 1971).

        However, it appears that moderate levels of noise increase
    arousal during monotonous tasks. McGrath (1963) found that various
    auditory stimuli at 72 dB improved visual vigilance performance.

    3.8.3  Effects on tasks involving mental activities

        Studies have occasionally been reported where noise exposure
    produces a mixture of positive and negative effects on task
    performance. Woodhead (1964) showed that noise adversely affected
    tasks involving a combination of memorizing and problem solving.
    However, when noise was introduced into the calculation phase only,
    performance was improved. Other studies by Hockey (1970) showed that,
    sometimes, performance on high-priority aspects of a task could be
    enhanced while performance on low-priority aspects was diminished by
    noise. The author found that by introducing a noise stimulus to a
    visual perception task, centrally-located visual signals were more
    effectively perceived, whereas peripherally-located signals tended to
    be ignored. The theory derived from these studies is that noise can
    increase the tendency to be selectively perceptive. If distraction
    occurs, this may be particularly harmful, but if attention is
    concentrated on the task, it may be helpful.

        Experiments involving complex mental tasks have shown that there
    is an increase in mistakes in the presence of intermittent noise
    stimuli (Glass et al., 1971; Glass & Singer, 1972).

        The effects of noise on performance have been reported to depend
    upon intelligence (Bryan & Colyer, 1973). Under noisy conditions,
    people with high intelligence showed a decrease in the quality of test
    performance whereas people with average intelligence showed constant
    or slightly better performance.

        Tasks that have been described in the literature as being
    particularly affected by noise, even when it has become familiar,
    include tasks of vigilance, information gathering, and analytical
    processes. Vigilance activities are not repetitive, do not allow for
    self-pacing, and demand rapid and accurate decisions. Thus, they are
    more adversely affected by distraction than many other activities.

        There is also some evidence that an individual performing the
    same task becomes less sensitive to noise, if the rate of arrival of
    the signals is low, if motivation is reduced, if the individual tested
    has a low level of anxiety, or if the noise is felt to be under the
    person's own control rather than imposed upon him. Basically these are
    "unarousing" conditions (Broadbent, 1971).

        Because of the effects on vigilance tasks, and on the accuracy of
    continuous serial reaction, it has been suggested that accidents would
    be the most likely indicators of noise effects in industry. Data on
    this subject are scarce; one study showed a higher accident rate in
    noisy places (Raytheon Service Co., 1972), and an earlier study showed
    an increase in errors (Broadbent & Little, 1960).

        Various experiments have demonstrated a disruptive effect of
    noise on learning or information gathering. Wakely (1970) pointed out
    that noise may interfere by competing for the limited number of
    channels available for information input. If the system is already
    overloaded, an individual must take more time to evaluate the
    usefulness of the intruding stimulus or run the risk of making errors.
    When tasks are not self-paced, increased errors will result.

        It has also been found that high levels of noise interfere with
    short-term memory tasks (Jerison, 1954). Noise from sonic booms at
    120 Pa could interfere with the learning of an eye/hand coordination
    skill without impairing the accuracy of the task (Lukas, et al.,

        These findings are important in relation to the specification of
    noise limits for classrooms or offices, where mental work
    predominates. It is important to differentiate between communication
    masking effects on the one hand, and the disturbance of concentration
    caused by noise on the other. In general, students in classrooms
    designed to meet the speech criteria discussed earlier would not have
    problems with interference in learning and other mental work. Although
    it may be tentatively concluded that complex tasks involving mental
    activity such as concentration, perception, or the intake of important
    information are more likely to be affected than those that only
    require predictable motor actions, additional experimental and field
    data are required.

        Noise of short or variable duration and impulsive noise tend to
    produce short residual effects on noise-sensitive tasks. Woodhead
    (1959) found that a one-second noise burst could have residual effects
    on performance of from 15 to 30 seconds. She also found that simulated
    sonic booms of 80-250 Pa produced residual disruptive effects
    (Woodhead 1969). Similar results were reported from an experiment with
    real sonic booms ranging from 40-260 Pa (Rylander et al., 1972b). The
    disruptive, effects seen in these experiments could be the result of a
    startle response (as opposed to the orienting response). These startle
    effects differ from the distraction effect mentioned earlier, by being
    more resistant to habituation.


    4.1  Environmental Noise

        People are exposed to many kinds of environmental noise that can
    be distinguished according to the source of the noise or to its
    physical characteristics such as intensity, frequency spectrum, and
    variations in time. There is wide agreement on both the
    instrumentation requirements and the procedures for the physical
    measurement and description of such noise. International organizations
    have provided standards for measurement, which continue to be revised
    and supplemented as knowledge improves. These standards and up to date
    technical publications can be used as a basis for reliable predictions
    of likely environmental noise in various circumstances.

        Description of noise sources, characterization of noise
    emissions, and understanding of basic noise generation mechanisms are
    also relatively satisfactory.

        Difficulties arise in describing the human noise dose. There are
    two major problems associated with the description of a person's
    cumulative noise exposure over a period of time. During each day, a
    person is exposed to a variety of environmental noises at home, in the
    general environment, and at work. This pattern might change from day
    to day or year to year. The noise exposure pattern and dose change
    with age, lifestyle, occupation, and many other factors. Thus,
    estimates of total noise exposure are always very crude

        From a practical point of view, even if the noise exposure
    history of an individual could be recorded, the data would have to be
    reduced to a few exposure variables that could be correlated with the
    subjective effects caused by that exposure.

        Much noise-related research is focused upon the establishment of
    valid dose description. Because of the importance of correlating the
    various biological effects of noise with the appropriate physical
    characteristics of the environmental noise, many attempts to condense
    the exposure history into single numerical descriptors have been made
    and alternative techniques will continue to be explored. The
    increasing use of personal noise dosimeters in industry might provide
    valuable information on the integrated noise dose experienced by
    people over long periods. However, the problem remains as to which
    variables of the environmental noise are important and can be suitably
    reduced to a single number.

        It is important to keep these basic concepts in mind, when the
    dose-response relationships required for the specification of
    practical exposure guidelines or noise limits are constructed. These
    relationships are complex and in some instances can only be deduced
    from data gathered over a number of decades. Thus, characterizations
    of the exposure variables as well as of the responses, are frequently

    rough approximations. Although it is possible and necessary for the
    solution of specific problems to refine these relationships, the
    consequent complications might hinder the development of a noise
    abatement programme or the achievement of environmental health goals.
    For this reason, the relatively simple and convenient equivalent
    continuous sound level, Leq in dB(A), can be used as a basic, common
    measure of environmental noise, and health criteria should be related
    to this index, whenever possible.

        The period over which Leq is averaged will depend upon specific
    applications. For describing the 24-h general noise environment, a
    weighted average such as the day-night average sound level (Ldn) may
    be used to take account of sensitive periods of the day or night.

        The convenience of combining different acoustic characteristics
    of various noises into a single index is evident. This principle has,
    however, been questioned both for industrial and environmental noises,
    particularly when the number of events is low and there are large
    differences between peak and background noise levels. The individual,
    identifiable influences of different acoustic components in the cause-
    and-effect chain should be recognized, particularly in research, and
    the limitations of the equal energy principle should be borne in mind
    when guidelines are established.

    4.2  Population Affected

        High noise levels are a feature of several work environments and
    extensive efforts are necessary to reduce the incidence of
    occupational deafness. Noise-induced hearing loss occupies a leading
    place among occupational diseases, and, in all nations, industrial
    noise abatement and hearing protection programmes should be a matter
    of priority for bodies that are responsible for the health of the
    working population.

        People who work in less noisy places may run a negligible risk of
    hearing impairment but could suffer from other noise-induced ailments
    derived from stress or chronic fatigue. Noise causes difficulties in
    communication and in work conditions in a wide variety of occupations.

        People are exposed to nonoccupational noise during leisure and
    rest hours. Environmental noise may interfere with, and affect the
    performance of leisure-time activities, causing general annoyance.
    Leisure activities may also introduce a hearing hazard, e.g., rifle
    shooting, loud music in discotheques etc. Nonoccupational noise may
    prevent normal performance at work and may, over a period of time,
    lead to health impairment. For the same reason, people with reduced
    adaptability or reserve capacity such as the sick, the aged, people
    with impaired sleeping functions, or those who are subject to other
    environmental strains may be particularly vulnerable and in need of
    special protection against excessive noise.

    4.3  Specific Health Criteria

    4.3.1  Physical injury

        Exposure to SPLs exceeding 140 dB, even for short periods,
    involves a risk of morphological damage to the ear, usually consisting
    of rupture of the tympanic membrane.

        Aural discomfort is experienced at SPLs above 100-110 dB and
    acute pain begins at SPLs above approximately 130 dB. This must be
    considered as a warning signal of incipient damage and an urgent
    requirement for preventive or protective measures. Painful sound
    intensities are far above those that cause hearing loss, when
    regularly experienced for several hours per day, and even brief
    exposure to such levels should be avoided.

    4.3.2  Hearing loss

        Long-term occupational exposure to high level noise can result in
    a gradual loss of hearing. The time scale of this process varies
    considerably depending on individual susceptibility, noise intensity,
    spectrum, and exposure pattern, and many other factors not yet fully
    understood. In some people, severe damage may be caused in the first
    few months; in others, hearing loss can develop gradually over the
    whole period of a working life. Combined with presbyacusis, it can
    lead to severe handicap and disability that is not amenable to

        In spite of considerable research, no method has yet been found
    to identify individuals who may be particularly susceptible to noise-
    induced hearing loss. For this reason, it is extremely important to
    avoid exposure of workers to noise levels that are known to involve a
    risk of permanent hearing loss. This should be achieved by effective
    noise-control measures. If this is not possible, then workers should
    be protected by a hearing conservation programme following recognized
    occupational health standards. Early detection of incipient hearing
    impairment is most important in the prevention of progressive
    deafness. Since the earliest loss of auditory acuity usually occurs at
    frequencies in the region of 4000 Hz, loss at this frequency is the
    most sensitive indicator of incipient damage. Losses at lower
    frequencies usually indicate progressive damage. NITTS is occasionally
    used to predict NIPTS, but there is little agreement on the validity
    of this practice.

        Recent research and analysis of most of the available data has
    provided a statistical basis for predicting the degree of hearing loss
    likely to be experienced by people exposed to steady noise during an
    8-h working day, for periods up to 40 years. The risk is negligible
    for Leq (8 h) < 75 dB(A). Above this limit, the risk of noise-
    induced permanent hearing loss increases with increase in noise level.
    If the significant noise exposures are concentrated over shorter
    periods during the day, this basic criterion implies that the risk

    would also be negligible with a 4-h exposure to 78 dB(A), a 2-h
    exposure to 81 dB(A), or a 1-h exposure to 84 dB(A). Conversely, if
    additional exposure occurs outside the 8 working hours, for example as
    a result of commuting to work or leisure activities, the limit of safe
    exposure would be more adequately expressed as an Leq of 70 dB(A)
    averaged over a 24-h day.

        Any comparison of noise exposures with recommended exposure
    limits should be based on measurements taken at the worker's ear under
    actual working conditions. Noise levels should be monitored at
    periodic intervals. For fluctuating exposures, the Leq for the total
    workday should be determined. If the noise contains impulsive
    components, the peak pressure, duration, and repetition rate of the
    impulses must be compared with separate limits, in addition to those
    just stated, in order to assure a safe level of noise in an

        Based on available risk tables, legislative provisions or
    recommended practices adopted by several countries specify
    occupational exposure limits in the range of Leq (8-h) = 85 dB(A)
    + 5 dB(A), with an increasing tendency to aim at lower limits. Leq
    (8 h) = 75 dB(A) can probably be considered as the limit below which
    there is little or no risk of permanent hearing damage and no
    necessity for protective measures. Hearing conservation programmes
    should be adopted in the case of routine occupational exposure to
    higher levels.

    4.3.3  Nonspecific health effects

        The nonauditory health effects of noise are complex and not yet
    fully understood. Laboratory and field studies have revealed a variety
    of physiological reactions such as changes in heart rate, blood
    pressure and peripheral resistance, and vestibular reactions. Many of
    these noise-induced reactions are nonspecific and are usually referred
    to as stress reactions.

        Much of the information is based upon animal experiments, many of
    which have been performed on rodents. These animals differ
    considerably from man in their reactions to noise. Thus, it is very
    difficult to assess the significance of such experiments for human
    health and wellbeing.

        The possibility cannot be ignored that short-term, and long-term,
    noise-induced stress, particularly with insufficient time for recovery
    between periods of work, could increase susceptibility to other work-
    related diseases, degenerative diseases, and nonspecific diseases that
    are regarded as consequences of chronic general stress. People
    normally exposed to hazardous stress during work and sensitive groups
    such as the sick, the elderly, pregnant women, and children may be
    particularly at risk. However, although the reported observations are
    considered by many to be indications of potential danger to health and
    have been suspected as predecessors of pathological changes, research

    on this subject has not yielded any positive evidence, so far, that
    disease is caused or aggravated by noise exposure, insufficient to
    cause hearing impairment. More epidemiological and animal studies are
    required to clarify the nature of nonauditory health risks associated
    with noise.

    4.3.4  Interference effects

        Frequent or severe interruption of various human activities by
    noise must affect human health and well-being to various degrees. The
    main interference effects studied have been those associated with
    sleep, communication, and with task performance.

        The probability that sleep will be disturbed by a particular
    noise depends on a number of factors including the interference
    criterion used (e.g., awakening or EEG changes), the stage of sleep,
    the time of night, the noise stimulus, and adaptation to the noise.
    Individual differences in sensitivity are marked. Although
    systematically collected field data on sleep disturbance are limited,
    there is some consensus of opinion that night-time noise levels of 
    35 dB(A) Leq or less will not interfere with the restorative process
    of sleep.

        The masking effect of noise on speech communication is well
    understood and methods are available to calculate word, message, and
    sentence intelligibility as a function of the characteristics of the
    masking noise. These methods are widely used in the design of rooms
    and the specification of background noise from external and internal
    noise sources to satisfy communication requirements. Various acoustic
    engineering reference works give background noise limits for various
    types of rooms such as offices, conference rooms, classrooms, and
    auditoria. However, it has been noted that communication requirements
    in industrial situations frequently do not receive adequate attention,
    particularly with reference to the accident risk. To guarantee
    satisfactory (100%) speech intelligibility in private homes, indoor
    noise levels of less than 45 dB(A) Leq, are generally required.

        Task performance interference is complex and depends to a large
    extent on the nature of the task. It is primarily an occupational
    problem and there is little evidence that it is significant in
    situations where noise does not interfere with communication or does
    not pose a risk of hearing impairment.

        Concentration and mental work of all kinds are often assumed to
    require a quiet environment. However, there are no reliable field data
    to confirm this and it seems likely that the disruptiveness of noise
    depends more upon the information it conveys than upon its level. No
    generalized criteria relating task efficiency and noise level or
    duration can be stated.

    4.4  General Health, Welfare, and Annoyance Criteria

        The health criteria and exposure limits described in section 4.3
    provide guidance for the reduction or avoidance of noise-induced
    effects under specific circumstances. However, they are of limited use
    for decisions concerning the environment of the general population.

        The results of social surveys on the extent of annoyance can be
    used as guidance concerning the relation between different types of
    outdoor noise and the extent of dissatisfaction or annoyance in the
    community. Available data indicate that daytime noise levels of less
    than 50 dB(A) Leq cause little or no serious annoyance in the
    community. With noise at this level, other factors such as transport
    needs, road safety, and the availability of schools are likely to
    cause more concern than occasional noise disturbances. Based on this
    likelihood, daytime noise limits in the region of 55 dB(A) Leq might
    be considered as a general environmental health goal for outdoor noise
    levels in residential areas. However, technological and economic
    limitations may make this goal impracticable, at present, for many
    existing urban areas.


        Noise levels in the environment can be reduced or limited by
    emission control, which should be aimed at noise sources contributing
    most to the effects experienced by man. The relevant sources are not
    always those that contribute most to the total dose from an acoustic
    point of view. Environmental noise control can be implemented by the
    use of environmental noise standards. These standards can be met by
    control at the source, by limiting the number of sources, by the
    physical separation of noise sources and people, and by changes in
    work methods. The technological background and information on dose-
    response relationships for both environmental and industrial noise are
    sufficient to allow appropriate action to be taken and to predict the
    effectiveness of noise abatement programmes.

        The control of environmental noise requires the participation of
    local health authorities and interested organisations. As problems
    caused by environmental noise, such as aircraft and traffic noise, are
    mostly due to mistakes in planning policies, it may be difficult to
    put a sufficiently stringent noise abatement programme into action in
    built-up areas. Care should therefore be taken that planning
    programmes include all long-term noise control measures which may be

        Action concerning specific sources of noise such as cars or
    aircraft, often has to be taken at an international level using long-
    term planning strategy as a background.

    5.1  Noise Control at Source

        The most efficient action against excessive noise is the
    reduction of the noise at source. In industry, noise control
    technology is available for solving many typical noise problems
    arising from the use of machinery. Usually the most effective approach
    is to redesign or replace noisy equipment. If this is not possible,
    significant reductions in noise levels can be achieved by structural
    and mechanical modifications, or the use of mufflers, vibration
    isolators, and noise protection enclosures (Beranek, 1971; Mags,

    5.2  Control of Sound Transmission

        A further reduction in noise can be obtained by increasing the
    distance between people and the noise source. For example, this can be
    achieved in the community by planning the location of transport
    facilities and, in industry, by the careful selection of work sites.
    Sound transmission can also be controlled by the use of partitions or
    barriers, e.g., for traffic noise along streets or, in industry,
    around particularly noisy or disturbing machinery. Reverberent noise
    levels can be reduced by sound-absorbing materials. The techniques for
    the control of sound propagation and transmission are well developed
    (Beranek 1971).

    5.3  Reduction in Length of Exposure

        A reduction in the length of exposure can be used in industry to
    supplement the previous measures, if necessary. This may be
    accomplished by job rotation or by restricting the operation of the
    noise source.

    5.4  Education of Workers

        It is vitally important that persons who face a risk of exposure
    to potentially hazardous noise levels should be educated in: (a) the
    possible consequences of excessive noise exposure; (b) the means of
    protection; and (c) the limitations of these means (e.g. improper use
    of ear-muffs).

    5.5  Ear Protection

        If it is absolutely impossible to reduce noise to a harmless
    level then some form of ear protection, i.e., ear-plugs, ear-muffs,
    and/or helmets, should be used. They should also be used during
    infrequent exposures that may not be part of a worker's normal
    routine. When the use of personal ear protection is necessary,
    attention must be given to: the effectiveness of specific types and
    models of protectors; instruction in their proper use; hygiene,
    discomfort, allergic reactions, and other medical problems that may
    arise through their use; and the means for ensuring proper, diligent,
    and effective use. In this connexion, it is important to provide quiet
    facilities and the opportunity for the temporary removal of ear
    protectors by those working in high noise levels. It should be noted
    that the commonly held view that ear protectors interfere with
    communication is incorrect, at least in continuous, high level noise
    -- indeed, the reverse is often found to be the case.

    5.6  Audiometry

        Pre-employment and follow-up audiometric examinations should be
    included in a hearing conservation programme. They provide
    opportunities for the detection of persons threatened by the
    development of NIPTS in order to take preventive action. Audiometric
    tests are also helpful in monitoring the effectiveness of ear
    protection and of noise abatement programmes. The examinations should
    be performed by qualified technicians under the supervision of
    physicians or health officials. It is usually accepted that the
    measurement of pure-tone air conduction thresholds is sufficient for
    this purpose. However, it should be stressed that periodical checks on
    equipment calibration, background noise levels in testing rooms, and
    audiometric procedures are necessary to minimize measurement errors.
    The frequency of follow-up audiometric tests is, in principle,
    dictated by the type and level of noise exposure. A general rule for
    audiometric testing is to wait at least 16 h after the last noise
    exposure to allow recovery from NITTS.

        Whenever noise exposures are such that an unavoidable risk of
    permanent hearing loss exists, occupational health services should
    provide for a hearing conservation programme. Such programmes, for
    which detailed guidelines exist, contain 3 elements: education
    concerning the hazards of noise; education in the proper use and
    supervision of the wearing of ear protection; and monitoring
    audiometry including periodical medical examination, when necessary.
    Monitoring audiometry, if properly planned and executed, will identify
    workers at risk from incipient hearing impairment, so that they can be
    removed from the noisy workplace before irreversible damage is caused.

        Since present occupational noise standards in most countries
    allow a certain risk of permanent hearing loss, a hearing conservation
    programme is usually highly advisable in addition to the specification
    of maximum exposure levels. Hearing conservation programmes are
    considered desirable when 8-h daily exposures exceed 75 dB(A). Present
    concepts of acceptable risk and economic constraints limit their
    practical application in most countries to levels around 85 dB(A).

        There are data which suggest that exposure to noise during
    leisure time in certain cases may constitute a risk to hearing in some
    segments of the general population. Noise from electronic music,
    discotheques, home power tools, guns, and certain other sports
    equipment might cause hearing impairment. These hearing losses occur
    primarily in young people, frequently prior to their occupational
    exposure. Hazardous noise exposures during leisure time should be
    controlled through consumer product control, noise labelling of
    products, environmental noise limits, and public education. Ear
    protection should be recommended in conjunction with equipment
    producing hazardous noise levels.


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         Regulation 161-35 Attachment 9).

    US Department of Health, Education and Welfare (1972)  Occupational
          exposure to noise. 2nd ed., Washington, US DHEW, Chapter 6,
         p. 17 (HSM 73-11001).

    US Department of Labor (1971)  Occupational noise exposure, (29 CFR,

    US Environmental Protection Agency (1973a)  Public health and welfare
          criteria for noise, Washington, DC, US EPA, 200 pp.

    US Environmental Protection Agency (1973b)  International Congress on
          Noise as a Public Health Problem, Dubrovnik, Jugoslavia,
          May 13-18, 1978, Washington, DC, US EPA, 807 pp. (550/9.73.008).

    US Environmental Protection Agency (1974)  Information on levels of
          environmental noise requisite to protect public health and
          welfare with an adequate margin of safety, Washington, DC, US
         EPA, 175 pp. (550/9.74.004).

    US Federal Aviation Regulations (1969)  Noise Standards: Aircraft Type
          Certification. Washington, DC, Federal Aviation Administration
         (Vol. 3, Part 36).

    US National Center for Health Statistics (1965)  Hearing levels of
          adults by age and sex, United States, 1960-1972. Vital and
          health statistics. Washington, DC, US Department of Health,
         Education and Welfare, 34 pp. (PHS Pub. No. 1000-series 11-No.

    VALCIC, J. (1974) Influence du bruit sur la réaction des vaisseaux
         sanguins périphériques et le rôle des réflexes végétatifs
         (Expériences personnelles).  Actes du Congrès pour la lutte
          contre le bruit de l'association international contre le bruit,
          Bâle, 66: 34-42.

    VETTER, K. & HORVATH, S. M. (1962) Effects of audiometric parameters
         on K-complex of electroencephalogram.  Psychiat. Neurol.,
         144: 103-109.

    WAKELY, H, C. (1970) Noise and human behaviour. In:  Proceedings of
          the Symposium on Environmental Noise, Its Human Economic
          Effects, Chicago, Chicago Hearing Society, pp. 27-34.

    WARD, W. D. (1960) Latent and residual effects in temporary threshold
         shift.  J. Acoust. Soc. Am., 32 (1): 135-137.

    WARD, W. D. (1962) Studies on the aural reflex. II. Reduction of
         temporary threshold shift from intermittent noise by reflex
         activity: Implications for damage-risk criteria.  J. Acoust. Soc.
          Am., 34: 234-241.

    WARD, W. D. (1970) Temporary threshold shift and damage-risk criteria
         for intermittent noise exposures.  J. Acoust. Soc. Am., 48 (2,
         part 2): 561-574.

    See Also:
       Toxicological Abbreviations