AMRL-TR-73-90
EPA-550/9-73-001-A
A BASIS FOR LIMITING NOISE EXPOSURE
FOR HEARING CONSERVATION
COMPILED BY
J. C. GUIGNARD
UNIVERSITY OF DA YTON RESEARCH INSTITUTE
DA YTON, OHIO 45469
JULY 1973
JOINT EPA/USAF STUDY
Approved for public release; distribution unlimited.
PREPARED FOR
ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
AEROSPACE MEDICAL RESEARCH LABORATORY
AEROSPACE MEDICAL DIVISION
AIR FORCE SYSTEMS COMMAND
WRIGHT-PATTERSON AIR FORCE BASE, OHIO
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FOR THE COMMANDER
HENNINGVEy von GIERKE, Dr. Ing.
Director, Biodynaraics and Bionics Division
Aerospace Medical Research Laboratory
AIR FORCE/56780/14 August 1973 2500
ALVIN F. MEYER, JR.
Deputy Assistant Administrator
for Noise Control Program
United States Environmental
Protection Agency
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I. REPORT TITLE
A BASIS FOR LIMITING NOISE EXPOSURE FOR HEARING CONSERVATION
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Final technical report
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Compiled by: J. C. Guignard
«. REPORT DATE
July 1973
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ta. ORIGINATOR'S REPORT NUMBER(S)
UDRI-TR-73-29
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thia tipott)
AMRL-TR-73-90 and EPA-550/9-73-001-A
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Approved for public release; distribution unlimited
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Prepared in support of United States
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12. SPONSORING MILITARY ACTIVITY
Aerospace Medical Research Laboratory,
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13. ABSTRACT
A compilation of data is provided, with references to published work, which repre-
sents the present state of knowledge concerning the effects of continuous and
impulsive noise on hearing. The danger to the ear of both occupational and non-
occupational human exposure to noise is considered. Data are Included or cited
which enable quantitative predictions to be made of the risk to hearing in the
American population due to noise exposure in any working or living context.
Recommendations are made concerning the need to obtain more definitive data.
Relevant aspects of noise measurement, the physiology of hearing, and theories
explaining the effects of noise on the ear are discussed in appendices to the main
report. This report deals solely with the effects of noise on hearing; other
physiological or psychological effects of noise are not considered in the present
document.
DD
FORM
,1473
Security Classification
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PREFACE
The Biodynamics and Bionics Division of the Aerospace
Medical Research Laboratory was given the responsibility,
under an Interagency Agreement with the Environmental
Protection Agency, to develop a document which would serve
as a basis for limiting noise for hearing conservation.
The preparation of this document was accomplished by the
University of Dayton Research Institute (UDRI) under
Contract F33615-72-C-1402, P00003. The Aerospace Medical
Research Laboratory efforts in support of this project
were included under Project 7231-03-16, "Auditory Responses
to Acoustical Energy Experienced in Air Force Activities".
Dr. J. C. Guignard and staff of UDRI compiled the main
document, Appendices 1 through 11 and the Bibliography.
The following acted as consultants, contributors,
advisors or reviewers in the course of preparation of the
document:
Dr. W. L. Baughn
Guide Lamp Division
General Motors Corporation
Anderson, Indiana
.Dr. Alexander Cohen
Department of Health, Education and Welfare
Public Health Service/NIOSH
Cincinnati, Ohio
Dr. John L. Fletcher
Department of Psychology
Memphis State University
Memphis, Tennessee
Dr. Aram Glorig
Callier Hearing and Speech Center
Dallas, Texas
Dr. H. E. von Gierke
6570th Aerospace Medical Research Laboratory
Biodynamics and Bionics Division
Wright-Patterson Air Force Base, Ohio
Dr. Terry Henderson
Department of Health, Education and Welfare
Public Health Service/NIOSH
Cincinnati, Ohio
Dr. Karl D. Kryter
Stanford Research Institute
Menlo Park, California
iii
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Mr. Barry Lempert
Department of Health, Education and Welfare
Public Health Service/NIOSH
Cincinnati, Ohio
Dr. J. H. Mills
Central Institute for the Deaf
St. Louis, Missouri
Dr. Charles W. Nixon
6570th Aerospace Medical Research Laboratory
Biological Acoustics Branch
Wright-Patterson Air Force Base, Ohio
Dr. W. Dixon Ward
University of Minnesota
Minneapolis, Minnesota
Major Daniel L. Johnson
6570th Aerospace Medical Research Laboratory
Biological Acoustics Branch
Wright-Patterson Air Force Base, Ohio
Various material was provided by the consultants for
incorporation into the document. Dr. J. H. Mills prepared
the material for Appendix 12. Dr. Karl D. Kryter provided
a paper, "Impairment to Hearing from Exposure to Noise",
that has since been published in the May 1973 issue of the
Journal of the Acoustical Society of America. For this
reason Dr. Kryter's material has not been republished as
part of this document. Dr. Daniel L. Johnson prepared sup-
porting material, "Prediction of NIPTS Due to Continuous
Noise Exposure", which will be published as a separate
technical report identified as AMRL-TR-73-91 and EPA-550/9-
73-001-B. Material was initially provided by the National
Institute of Occupational Safety and Health (NIOSH). How-
ever, that material has not yet been released by NIOSH for
publication. It is to be published in 1973 as NIOSH
Technical Report TR 86, "NIOSH Survey of Occupational Noise
and Hearing: 1968 to 1972" by Mr. Barry L. Lempert and
Dr. Terry L. Henderson. In addition, Dr. William L. Baughn
provided his work, "Relation Between Daily Noise Exposure
and Hearing Loss Based on the Evaluation of 6,835 Industrial
Noise Exposure Cases". This is available as AMRL-TR-73-53
or EPA-550/9-73-001-C.
At the University of Dayton, Miss Barbara McKenna
assisted in the initial literature search and typed most of
the preliminary draft. Miss Barbara Hartung assisted in
completing the bibliography and typed the major part of the
final report and its appendices.
This technical report is also identified as UDRI-TR-
73-29.
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CONTENTS
page
Sections
I. Introduction 1
II. Definitions 3
III. Effects of noise on hearing 7
A. Continuous noise 7
B. Impulsive noise 15
IV. Factors influencing incidence of NIPTS 22
V. Conclusions and recommendations 24
Appendices
1. Existing hearing damage risk criteria and
procedures for evaluating NIPTS Al-1
2. Hearing in the American population A2-1
3. Presbyacusis A3-1
4. Glossary A4-1
5. Physical measurement and description of
noise hazardous to the ear A5-1
6. Audiometry A6-1,
7. Noise-induced hearing loss: industrial
experience and predictive methods A7-1
8. Equivalent continuous sound level A8-1
9. Physiological factors affecting the noise
susceptibility of the ear A9-1
10. Infrasound, vibration and ultrasound A10-1
11. Non-occupational noise exposure: particular
situations All-1
12. Further data on the pattern of threshold
shift produced by prolonged noise in animals
and man (J H Mills) A12-1
Bibliography B-l
v
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BACKGROUND
Good hearing is one of the most important of human senses,
When it is lost or seriously impaired, one is not only disad-
vantaged socially but may be robbed of much potential as a
wage-earner: handicap due to hearing loss is for many people
as tragic as a disability resulting from major injury or
serious illness, reducing the person's capacity to enjoy life
to the full and to follow unfettered his or her chosen trade
or profession.
Rarely, the hearing is deficient from birth or following
a childhood illness; or it may be damaged during adult life
by head injury or by disease affecting the ear or its connec-
tions with the brain. Moreover, some loss of sensitivity of
hearing, particularly for high tones, is experienced by all
adults as they grow older. This phenomenon (presbyacusis),
the onset and progress of which shows considerable individual
and demographic variation, is generally accepted to be a
normal consequence of the human aging process, and to take
place whether or not the ear is affected by disease, injury
or noise.
Nevertheless, the most prevalent and avoidable cause of
hearing loss is excessive noise exposure. Observations in
animals as well as in man show that noise of sufficient in-
tensity reaching the inner ear injures the hearing organ (the
organ of Corti). The principal site of injury appears to be
the hair cells of that organ. As the intensity of the noise
and the time for which the ear is exposed to it are increased,
a greater proportion of the hair cells are damaged or eventu-
ally destroyed. Because the function of the hair cells is to
transduce the mechanical energy reaching the ear into electri-
cal signals, which are then carried by the auditory nerves to
the brain, progressive loss of hair cells is usually accompa-
nied by progressive loss of hearing.
There is a great deal of individual variation in suscep-
tibility to noise damage. However, any man, woman, or child
whose unprotected ears are exposed to noise of sufficient
intensity is in the long run likely to suffer some degree of
permanent noise-induced hearing loss for which there is no
foreseeable cure.
What constitutes a harmful level of noise depends on the
effective duration of the noise exposure and the circumstances,
Generally speaking, the greater the sound pressure level of
the noise, the greater the danger to the unprotected ear, and
the shorter the safe noise exposure time. In. extreme cases,
a damaging exposure to impulsive noise may be as brief as a
few milliseconds: a few unfortunate young people are reputed
VI
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to have suffered irreparable damage to their hearing from
the sound of a few rounds of small-arms fire during their
first day at the ranges. At the other end of the scale,
prolonged exposures to comparatively moderate levels of
noise, of the kind to be heard, for example, in factories,
in vehicles, or in busy city streets, can cause hearing loss
developing over the years in substantial numbers of suscep-
tible people.
It remains an open question whether there is any abso-
lute level of noise below which continuous exposure may be
considered completely harmless for all ears. In this con-
nection, it is important to bear in mind the fact that
neither the loudness of a noise, nor the extent to which
the noise causes discomfort, annoyance, or interference
with human activity, are reliable indicators of its poten-
tial danger to the hearing mechanism. A noise loud enough
to prevent verbal communication may be presumed to be
hazardous to the ear, but the converse is not necessarily
true.
Hearing damage can cause social problems, depending upon
the severity of the loss. Such problems include difficulties
with everyday communication and social integration, the pur-
suit of preferred employment, and for young people with
hearing damage, education (Switzer and Williams, 1967).
The prevalence of hearing loss among workers in noisy
industries has been recognized since ancient times; and a
popular description of excessively loud noise is "deafening".
Yet it is still not adequately appreciated by the general
public that there is a causal link between noise exposure and
hearing loss. If the hazard is understood, it is perhaps
regarded by many people as a remote contingency, or as one
having little consequence for those afflicted. It is possible,
too, that while people exposed to intense noise frequently
experience a substantial noise-induced temporary threshold
shift (NITTS), sometimes accompanied by tinnitus (ringing in
the ears), the fact that very often such symptoms of excessive
noise exposure largely disappear within a few hours, if not
minutes, may mislead the hearer into believing that no perma-
nent damage is done by noise.
Clinical observations of noise-induced hearing loss have
been reported over more than a century. However, the problem
has received intensive study only during the past three or
four decades. Since the second world war, substantial infor-
mation has been gathered on the effects of noise (particularly
industrial noise) upon the ear. Based variously upon the
ava? l-b't.e clata, numerous noise exposure limits have been es-
tablished for *:hc purpose of hearing conservation. Some of
these have received national or international acceptance or
vii
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standardization and some have been embodied in state legis-
lation. An important present difficulty for the legislator,
administrator or noise control engineer concerned with pro-
tecting human hearing against noise, however, is the fact
that confusing and sometimes conflicting guidance is offered
by the multiplicity of official or semiofficial standards,
regulations or guidelines now in existence. In a recent
review, Acton (1967) reported that at least 35 different
hearing damage risk criteria, noise exposure limits or guide-
lines had been adopted or proposed by various authors.
Clearly, there is an urgent need for unification, which
this document attempts to meet. It presents a current con-
sensus of the majority of scientific opinion as to the
incidence of noise-induced permanent threshold shift (NIPTS)
to be expected in selected fractions of the American popu-
lation. The conclusions reached in this report apply to
both occupational and non-occupational exposure at work, in
the home, in transportation, in recreation, or at large in
the street and other public places.
The arrangement of this report is as follows: Section I
(introduction) briefly sets out its main purposes and defines
its scope. It is important to note in this connection that
the present document deals only with the effects of noise on
hearing: it is not concerned with annoyance or other adverse
effects of noise. Section II presents some definitions
crucial to the interpretation of the data given in the main
sections which follow. Section III provides specific guidance
to the selection of noise exposure limits for the purposes of
hearing conservation. Section IV reviews certain factors
influencing the incidence of NIPTS due to noise of a given
type and exposure level. Section V contains some recommenda-
tions concerning the need for further research and field
observation on noise-induced hearing loss, including the
monitoring of the noise exposure and the hearing of the public;
and concerning ways and means of increasing public awareness
of the noise hazard and of promoting a wider acceptance and
implementation of hearing conservation programs.
vnx
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I. INTRODUCTION
I.I Purposes of document
The goal of this document is threefold: (1) it attempts
to arrive at a consensus (summarized in Section III) regar-
ding the effects of noise exposure upon human hearing; (2) it
evaluates the principal factors affecting the incidence of
noise-induced hearing loss in various populations (Section IV);
and (3) it makes recommendations (Section V) concerning noise
exposure levels for the purpose of hearing conservation.
Detailed treatments of relevant topics are given in the
Appendices.
1.2 Scope of document
1.2.1 Population. While the principal findings here reported
apply mainly to healthy adult American men and women of work-
ing age, having normal ears and hearing, some information is
incidentally provided about noise exposure and hearing in
children, in the aged, and in persons with abnormalities of
the ear. "Normal hearing" is discussed in Appendix 2.
1.2.2 Varieties of noise. Data on their effects on hearing
are presented for two main types of noise, namely, continuous
and impulsive noise. These varieties of noise are further
distinguished and defined in Sections II and III. "Noise"
means airborne sound contained within the frequency range
16 Hz to 20,000 Hz (20 kHz). Oscillations outside that range
(ultrasonics; infrasonics; vibration) are also considered
briefly, inasmuch as they may affect the ear.
1.2.3 Ultrasonics; infrasonics; vibration. Appendix 10
provides information on the relatively minor auditory effects
of ultrasound, infrasound and vibration.
1.2.4 Quantification of noise exposure. Although some other
noise-measurement units are alluded to, this document in
general adopts A-weighted sound level (in dBA) for the
specification of continuous noise exposure levels; and peak
sound pressure level (SPL) in decibels (dB) relative to the
international standard reference zero of 0.00002 N/m2 for
impulse noises. When A-weighted sound levels are given in
dBA, the use of international standard measurement techniques,
instrumentation and weighting characteristics is assumed.
Relevant standards are listed at the end of the Bibliography.
Procedures for calculating equivalent continuous sound level
(Lecf) in dBA, in the cases of untypical, interrupted or
intensity-modulated steady-state noise exposure (see
Section III.A) are given in Appendix 8. Appendix 8a may
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also be used to determine exposures in dBA from octave-band
sound levels measured in decibels relative to 0.00002 N/m2.
1.2.5 Measurement of hearing. When referred to in this
document, hearing level or hearing threshold level (see defi-
nitions in Section II) is generally presumed to be, or to
have been, determined by pure-tone audiometry using standar-
dized instrumentation and procedures (see Appendix 6). Noise-
induced permanent threshold shift (NIPTS) is defined as that
part of hearing level ascribable to noise exposure, as opposed
to other factors, such as aging, which also cause an elevation
of the threshold. It should be borne in mind that hearing or
hearing threshold level in decibels is customarily an average
or a median value for a designated population or group of indi-
viduals and that, accordingly, a variance is implicitly associ-
ated with it. Sources of audiometric variance are discussed in
Appendix 6. ;/
1.2.6 Reference to noise-induced temporary threshold shift
(NITTS) and tinnitus.The temporary effects of noise on
hearing are frequently mentioned in some parts of this
document and are discussed in some detail in Appendices 7 and
12 and by Johnson (1973) and Kryter (1973). The relationship
between noise-induced temporary (NITTS) and permanent (NIPTS)
threshold shift is still not entirely clear; however, it is
generally agreed upon by most workers in this field that NITTS
is a useful predictor of NIPTS in many circumstances of noise
exposure. A presumptive relationship between them underlies
many existing schemes for predicting the effects of exposure
to noise, both continuous and impulsive.
1.2.7 Non-auditory physiological, psychological, and nuisance
effects of noisedThis document deals solely with the effects
of noise on hearing. It does not provide data nor does it
make recommendations concerning the limitation of noise
exposure according to criteria other than those of hearing
conservation.
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II. DEFINITIONS
II.1 Definitions and Glossary
This section contains some definitions of important
terms used later in the document. A more comprehensive
Glossary of acoustical terms germane to the present work
is given in Appendix 4.
II.2 Audible range (frequency range of human hearing)
The frequency domain of human hearing by air conduction
may be taken as extending from 16 Hz to 20,000 Hz and to be
contained between a lower boundary, namely, the threshold of
audibility (see Glossary), and the upper boundary, namely,
the threshold of pain in the ear. The threshold of audibility
depends heavily upon frequency and has received international
standardization (ISO, 1964).
II.3 Continuous noise
On-going noise which lasts for at least 500 ms, and which
may last indefinitely, of which the level either does not vary
by more than ±5 dB (constant or "steady-state" noise) or does
not vary more rapidly than 40 dB in 500 ms in the case of
level variations exceeding ±5 dB (fluctuating noise). Contin-
uous noise which is interrupted by periods of subjective
silence, or by periods of noise at levels below 55 dBA, may be
described as intermittent noise (see Section III.A). Some
examples of different kinds of continuous noise exposure are
given in Table II.I.
II.4 Hearing level
In this document, hearing level means the difference in
decibels between an individual or a group threshold of hearing
(measured with a puretone audiometer at a specified test fre-
quency or frequencies) and a specified standard reference zero,
which is a function of audiometric frequency.* Positive values
mean (as is usually the case) an elevation of threshold (poorer
hearing). When hearing level is defined for a group the median
and related centile values are customarily specified.
Strictly speaking, "hearing level" (HL) relates to ASA 1951,
whereas a level related to ISO:1964 or ANSI:1969 should
properly be called "hearing threshold level" (HTL).
"Hearing level" is however an acceptable usage provided
that the reference zero is clearly specified.
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Table II-I. Varieties of continuous noise exposure.
Variety of
Exposure
Description
Typical Examples
Steady-state
Single continuous daily
exposure (typically 8
hours but may be shorter
or longer) at a level
constant within ±5.dBA.
Steady plant noise
in factories. Steady
urban noise. Sound of
a waterfall. Shipboard
noise. Noise inside
vehicles or in aircraft
in flight.
Fluctuating
Noise
Noise is continuous but
level rises and falls
(rapidly or gradually)
more than ±5 dBA during
exposure.
Many kinds of processing
or manufacturing noise.
Traffic noise. Airport
noise. Many kinds of
recreational noise (eg,
vehicle-racing; powered
lawn-mowing; radio and
TV) .
Intermittent
Noise is discontinuous:
ie, the level falls to
immeasurable low or to
non-hazardous levels
between periods of
noise exposure of which
more than one affects
the ear during the day.
Note;this can be re-
garded as a special case
of fluctuating noise
(see Section III.A.3).
Many kinds of industrial
noise (especially in
construction work, ship-
building, forestry, air-
craft maintenance, etc.).
Many kinds of recreational
noise (eg, drag-racing;
rock concerts; chain-
sawing) . Light traffic
noise. Occasional air-
craft flyover noise.
Many kinds of domestice
noise (eg, use of electric
appliances in the home).
School noise.
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II.5 Hearing risk
According to Robinson (1971) hearing risk is "that per-
centage of the population whose hearing level, as a result of
a given influence (e.g., noise; age; pathological conditions
of the ear), exceeds a specified value, minus that percentage
whose hearing level would have exceeded the specified value
in the absence of that influence, other factors remaining the
same". Procedures for estimating risk due to noise exposure
are given in Section III.
II.6 Impulsive noise
Impulsive noise in this document means intense transient
noise of short duration (less than 500 ms), rapid growth (more
than 40 dB in 500 ms) and often rapidly changing spectral
composition (cf. steady-state noise). Two main types (A and B)
are described (see Section III.B and Appendix 5). Typical
sources of impulse noises are the discharge of firearms, hard
impacts in industrial processes, shot-firing in mining or
quarrying, and sonic boom.
II.7 Noise-induced permanent threshold shift (NIPTS)
The amount in decibels by which a hearing or hearing
threshold level is permanently elevated as a result of noise
exposure. It is assumed to be an addition to the increase in
hearing level ascribed to aging or, in some individuals,
otological causes. (The term noise-induced hearing loss is
sometimes used loosely as a synonym.) See the definition of
permanent threshold shift below.
II.8 Permanent threshold shift (PTS)
A permanent (irreversible) change in hearing level. It
is measured in decibels at specified audiometric test fre-
quencies or groups of frequencies.
II.9 Presbyacusis*
Permanent threshold shift (elevation), chiefly involving
the higher audiometric frequencies above 2000 Hz but ultimately
involving lower frequencies also, ascribed solely to advancing
age (see Appendix 3). Note, it is presumed ultimately to
affect all ears, regardless of otological health or noise exposure,
* An alternate spelling, presbycusis, while etymologically
incorrect, is in common use.
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In otologically normal individuals or groups of specified
age, standardized values (ISO, 1964) may be subtracted from
measured or predicted hearing level in order to determine
or estimate NIPTS. The term may also be applied to other
manifestations of age-related loss of auditory efficiency,
such as impaired discrimination of speech in noise.
11.10 Sociacusis
Permanent threshold shift ascribed to frequently un-
avoidable acoustic influences in the environment (e.g.,
urban noise) and not due either to aging or to occupational
noise exposure. Loosely, non-occupational NIPTS.
11.11 Speech _frequencies
Those audiometric test frequencies at which good hearing
is held to be essential to spoken communication. Opinion is
divided as to which frequencies should be so designated.
Commonly, in clinical audiology (and notably in industrial
practice in the United States), the determining frequencies,
originally recommended by the American Medical Association,
are 500, 1000 and 2000 Hz (see Appendix 1). In some foreign
countries and in the state of California, hearing at 3 kHz is
included in the assessment. Recent opinion argues strongly
for the recognition of 4 kHz also as crucial to full speech
reception, and especially to speech discrimination in a noisy
background. When using the term "speech frequencies", there-
fore, it is important to specify tham numerically.
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III. EFFECTS OF NOISE ON HEARING
III.A Continuous noise
III.A.I Definition and varieties of on-going "continuous"
noise
To most people "noise" means unwanted sound of a notice-
able if not objectionable intensity which goes on for an
appreciable time (seconds to hours), such as factory or office
noise, or noise which is continuously or regularly present in
the living environment such as traffic noise. When the
quality and the intensity of the noise is practically constant
(varying less than ±5 dBA) over an appreciable time (seconds
or longer), it is often called "steady-state" noise. The use
of that term implies that certain parameters or statistical
properties of the noise do not change significantly over the
interval of measurement or description: and such an assumption
underlies the use of measurement techniques (including the use
of sound level meters) based upon time-averaging the sound
pressure at the measurement point. Continuous noise must be
distinguished from the other main type of noise affecting man
and the ear, namely, impulsive noise, which is defined and
discussed in Section III.B below.
III.A.1.1 Temporal patterns of continuous noise. For
descriptive and analytical purposes, continuous noise is, as
a first approximation, assumed to be strictly steady-state,
that is, to continue at a constant level (expressed in dBA),
without interruption, for a designated daily exposure time.
It is convenient to consider a typical industrial daily
exposure of 8 hours, because such an exposure is suffered
daily by millions of ears in the processing and manufacturing
industries. Most of our knowledge of the effect of contin-
uous noise upon the human ear comes from industrial audio-
logical experience, as is brought out below, in Appendix 7
and in Johnson (1973). More ears are at risk from quasi-
steady-state noise exposures of about 8 hours a day, 5 days
a week for a working lifetime than from any other variety of
noise exposure.
III.A.1.2 Non-steady-state (fluctuating or intermittent)
continuous noise.Many ears are exposed at work, in trans-
portation, or in supposedly resting or recreational periods
to a variety of noises which are not continuous or which
vary in intensity. Some varieties of non-steady-state
exposure were summarized, with examples, in Table II.I in
Section II. For purposes of evaluating the hazard of such
noise to the ear, it is in many circumstances appropriate to
determine an equivalent continuous sound level for fluctu-
ating or intermittent noise. A computational procedure for
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carrying out this "noise-averaging" technique is referred to
in Section III.A.3 below and a method set out in Appendix 8.
III.A.1.3 Relationship between on-going noise and hearing
loss. There is a plethora of published information about the
effects of long-term noise exposure upon the hearing of workers
in the manufacturing and construction industries, as well as
that of aviators and others in noisy occupations: several
recent monographs and surveys have been published on this
topic (Burns, 1968; Burns and Robinson, 1970; King, 1972;
Kryter, 1970; Robinson, 1971). A recent survey by the
National Institute of Occupational Seafety and Health (1972)
contains a descriptive summary of some of the more important
audiometric surveys carried our in the United States and
abroad during the preceding decade. Certain important work,
providing the basis of the conclusions reached below in the
present document, is discussed in some detail in Appendix 7;
cognate work is presented in Johnson (1973), Kryter (1973)
and Mills (Appendix 12).
Temporary hearing loss (or noise-induced threshold shift,
NITTS), lasting from a few seconds to several days or weeks
can result from brief exposure to high sound levels or from
day-long exposure to more moderate levels of continuous noise.
Regular (day-by-day) exposure to such levels over a long period
(days to years) can result in damage to the inner ear, associ-
ated with a sensorineural hearing loss (NIPTS) which is perma-
nent and, so far as is presently known, incurable. It can only
be prevented by protecting the ear from excessive noise exposure.
NIPTS is usually preceded by, and may at any time be accom-
panied by, a NITTS attributable to fatigue of the hearing organ.
The typical pattern seen in the audiogram is a maximum loss in
the range 4000 to 6000 Hz, with a somewhat smaller loss
(initially) at the higher test frequencies. Because the loss
is sensorineural, it is seen in both air- and bone-conduction
audiograms.
III.A.1.4 Tinnitus associated with occupational NIPTS.
Tinnitus (ringing in the ears) may be, at first, the only
symptom in many cases of occupational hearing loss; and it is
fairly frequently associated with the condition. Chadwick
(1970) has reported an incidence of 30% in one industrial
survey in Britain. Tinnitus is not necessarily diagnostic of
noise-induced damage to the ear, however, for the symptom is
also associated with other disorders of the ear or auditory
nervous system, unrelated to noise exposure.
Chadwick (1970) has also commented that many patients with
occupational NIPTS notice new symptoms only upon changing from
one noisy job to another; or from a noisy job to a quiet one,
possibly because they have adapted to or learned to cope with
any handicaps due to the noise in a familiar situation.
-------
III.A.1.5 Relationship Between daily occupational (8-hour)
exposure to steady-state noise and NIPTS. The following
paragraphs refer to procedures for predicting the risk or
amount of hearing loss to be expected from occupational-type
noise exposure. The predictive data summarized here in
Section III.A and used by Johnson (1973) are derived from an
international triad of studies in the field of industrial
noise-induced hearing loss, namely, the work of Baughn (1966,
1973), Passchier-Vermeer (1968, 1971) and Robinson (1968, 1971).
These methods may be used to predict the effect upon hearing
at selected centiles of the adult population of daily 8-hour
exposure to steady-state distributed noise at levels in the
range 75 to 90 dBA, sustained over periods up to 50 years.
Further details of industrial audiological experience and
related research germane to this section are given, with
references, in Appendix 7 and in Johnson (1973). The three
predictive methods summarized in the following paragraphs
have been selected because (1) they permit calculation of
NIPTS for designated percentiles of the adult population;
(2) although they are based mainly upon the audiometric test
frequencies 500, 1000 and 2000 Hz ("speech frequencies")
currently accepted as essential to the evaluation of hearing
impairment by most otologists in the United States, they also
include data permitting the inclusion of 4000 in the assess-
ment; and (3) they show fair agreement with one another.
These aspects are considered in detail by Johnson (1973).
III.A.1.6 Method and data of Passchier-Vermeer (1968; 1971).
Passchier-Vermeer (1968; 1971) has analyzed the audiometric
data from several surveys of industrial hearing loss. Making
allowances for presbyacusis, she has published (1968) proce-
dures with graphs for determining the noise-induced part of
hearing level evaluated as a function of daily noise expsoure
for the 25th, 50th and 75th centiles of a working population.
In 1969 and 1971, she published some additional data including
10th and 90th centile estimates. Her methods and results,
which are applicable to daily 8-hour exposures to industrial-
type noise up to 100 dBA, are summarized in Appendix 7 and
discussed in Johnson (1973). Appendix 7 also contains an
explanation of certain approximations (such as those used to
obtain 10th and 90th centile values) and extrapolations intro-
duced in that Appendix to the present document.
III.A.I.7 Method of Robinson (1968; 1971; Robinson and Cook,
1968). Robinson has devised an idealized method for predicting
hearing loss resulting from noise exposure. His method is
based on a unique mathematical relationship (the hyperbolic
tangent) between noise exposure and NIPTS, which is adjusted
parametrically for population centile and audiometric frequency.
The method applies to otologically normal adults exposed to
industrial noise for 8 hours per day over a period ranging
from 1 month to 50 years. It yields estimates of the percen-
tages of the exposed population who may develop NIPTS as a
-------
function of noise exposure (noise "immission"). Robinson's
method can be c iticised on the grounds that it is based upon
a single altho^rh substav tial study of otologically screened
British indust^i^ -. workers*; and that the mathematical nice-
ness of the predictive theory may not be entirely justified
by the realities of industrial audiometric data and their
sources of variance. Robinson's (1971) specific definition
of "risk" (to hearing) was defined in Section II above. His
predictive method and certain modifications of it are descri-
bed in more detail in Appendix 7 and Johnson (1973).
III.A.1.8 Method of Baughn. Baughn (1966, 1973) has amassed
data from extensive industrial audiometric studies in the
United States. His work provides insight into how NIPTS
develops at various centile points as a result of typical
industrial noise exposure in the range 78 to 92 dBA. The pre-
diction of NIPTS may in some respects be too high, however,
owing to a probable contamination of the data by residual TTS
and masking in the circumstances in which the audiometry was
conducted**.
III.A.1.9 Averaging NIPTS predictions over the three "indus-
trial" methods.A summary chart of certain predictions which
can be made concerning NIPTS and risk by combining the pre-
dictions of Baughn, Passchier-Vermeer and Robinson (see
Johnson, 1973) is presented in Table III.A-I. More detailed
tables in that report show NIPTS predicted using the three
methods for continuous noise levels in the range 75 to 90 dBA.
Johnson (1973) also provides a further explanation of the
rationale and method adopted to yield these averages. A brief
explanation of the terras used in Table III.A-I is given below.
III.A.1.10 Calculation of speech impairment risk (SIR).
Kryter (1973) has proposed an alternate method for evaluating
noise exposure in terms of speech impairment risk. The method
leads to a generally more pessimistic view of noise hazard
than the predictive methods outlined above. Critical comment
on Kryter*s proposal has received simultaneous publication
(Cohen, 1973; Davis, 1973; Lempert, 1973; Ward, 1973). See
also Johnson (1973).
III.A.1.11 Evaluative parameters of NIPTS.
(1) Maximum NIPTS (90th percentile). This is defined as the
maximum value of NIPTS reached during 40 years of noise
exposure after the age of 20 for the centile designated,
namely, the 90th (i.e., 90% of the population do not
exceed the stated value of NIPTS).
* Hinchcliffe's (1959) data are used for the presbyacusis
correction.
** In some measurements, only 20 minutes' recovery from the
industrial noise was allowed before testing.
10
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(2) NIPTS (90th percentile) at 1-0 years. The expected
NIPTS after ten years of exposure during adult life
is not exceeded by 90% of the population.
(3) Average NIPTS. The gross average value of NIPTS ob-
tained by averaging over a 40-year exposure duration
and also over all the population percentiles. Note;
this figure differs by only a couple of decibels from
the median NIPTS value after 20 years of exposure.
(4) Maximum Hearing Risk. Hearing risk is defined as the
difference between the percentage of people with a
specified hearing handicap in a non-noise exposed (but
otherwise equivalent) group. The hearing risk varies
with exposure duration and the Maximum Hearing Risk is
defined as the peak value (largest difference) that
occurs during the 40 years of exposure. Normally, but
not always, this peak value occurs after 40 years of
exposure.
III.A.2 Duration of exposure longer or shorter than 8 hours
III.A.2.1 Exposures to continuous noise exceeding 8 hours.
An equivalent continuous sound level (Leq)in dBA may be
calculated for varying exposure times, based upon a nominal
daily exposure of 8 hours (see Appendix 8). For that duration
only, the equivalent continuous sound level, Leq, is numeri-
cally equal to the average measured sound level in dBA. As in
the case of unbroken steady-state exposures lasting less than
8 hours (see below) the nomogram in Appendix 8 may be used to
find Leg for uninterrupted steady-state exposures of more than
8 hours. Thus, for a continuous 24-hour exposure, Leq is
4.8 dB greater than for an 8-hour exposure to the same noise
(this can be approximated to 5 dB) . Expressed another way,
the hazard to hearing from a continuous 85 dBA noise lasting
24 .hours is similar to the hazard of an 8-hour exposure to
9D -cUJA,, provided of course that the noise is steady-state,
broadly -distributed in frequency, fairly uniform in spectrum
without substantial discrete tonal components, and free from
airy significant addition of impulse sounds (see Section III.B).
fan exposure exceeding 24 hours may be treated as an
indefinite exposure. Allowances for level fluctuations in
continuous noise, for intermittency (interruptions) , and for
the significant presence of simultaneous tonal components or
impulses during prolonged exposure may be considered to obey
rules similar- to those which govern these allowances in the
case of exposures shorter than 8 hours (see below) .
III.A.2.2 Exposures to continuous noise for periods less
than 8 hours. The risk to hearing in the case of daily
exposures to on-going noise for periods (minutes to hours)
less than 8 hours can also be evaluated by calculating an
11
-------
Table ITT.A-I Summary of effects predicted for continuous
noise exposure at selected values of A-weighted*
sound level. Figures in parentheses show the
audiometric test frequencies (in kilohertz)
included in the evaluation.
Speech (.5,1,2) Speech (.5,1,2,4) 4 kHz
75 dBA for 8 hours
Max NIPTS (90%-ile) 1 dB 2 dB 6 dB
NIPTS at 10 yrs (90%-ile) 015
Average NIPTS 001
Max Hearing Risk** N/A N/A N/A
80 dBA for 8 hours
Max NIPTS (90%-ile) 1 dB 4 dB 11 dB
NIPTS at 10 yrs (90%-ile) 139
Average NIPTS 0 14
Max Hearing Risk** 5% N/A N/A
85 dBA for 8 hours
Max NIPTS (90%-ile) 4 dB 7 dB 19 dB
NIPTS at 10 yrs (90%-ile) 2 6 16
Average NIPTS 1 39
Max Hearing Risk** 12% N/A N/A
90 dBA foir 8 hours
Max NIPTS (90%-ile) 7 dB 12 dB 28 dB
NIPTS at 10 yrs (90%-ile) 4 9 24
Average NIPTS 3 6 15
Max Hearing Risk** 22.3% N/A N/A
* Values given are arithmetic averages obtained from predictions
using the methods of Baughn, Passchier-Vermeer and Robinson
(Johnson, 1973).
** 25 dB ISO Fence or Hearing Handicap (re: ISO: 1964). Averaged
from the methods of Baughn and Robinson (see Johnson, 1973).
12
-------
equivalent continuous sound level, Leq' as explained in
Appendix 8, provided that the noise is approximately steady-
state. The calculation of LeCj normalizes the daily exposure
to a duration of 8 hours for Che purposes of entering the
tables given in Section III.A.I.
III.A.3 Allowances for level fluctuation or interruption
of noise
III.A.3.1 Fluctuating level in on-going noise. The Inter-
national Organization for Standardization(ISO, 1971) in its
current Draft Recommendation (ISO/DR 1999) for assessing
noise exposure at work recommends a method which embodies
the A-weighted "equal-energy" rule, namely, the computation
of an equivalent continuous sound level, Leg (see Appendix 8
and the nomogram therein) , in dBA. This method is probably
the best available method of predicting the effect of noise
on hearing in the case of continuous noise of which the level
fluctuates slowly (seconds to hours)* during the working day;
and it may, with circumspection, be extrapolated to cover
distributed noise of fluctuating level which goes on for
longer than the typical working exposure of 8 hours.
The arbitrary ISO (1971) protective weighting of 10 dBA
for impulsiveness in the noise may be more questionable.
Recent work by Passchier-Vermeer (1971) has indicated that
this figure may not be realistic in the case of distributed
industrial noise with impulsive components, although her work
does in general confirm the validity of the equivalent level
method based on "equal-energy" in the case of on-going noise
with slow but not impulsive fluctuations. Findings by Cohen,
Kylin and LaBenz (1966) in an occupational setting have
indicated that, in some circumstances, combined exposure to
continuous and impulsive noise might be less noxious than
exposure to similar continuous noise alone. Such a paradoxi-
cal effect might be attributable to a difference in the
protective action of the acoustic reflex (see Appendix 9),
although this is at present speculative. Impulsive noises
must be evaluated separately and specifically using appropri-
ate measurement techniques, for reasons given in Section III.B,
III.A.3.2 Intermittent noise. It is reasonable to treat
intermittent exposure to steady-state non-impulsive noise as
a special case of fluctuating level. For by "intermittent"
noise is generally meant merely a substantial change in level
from some potentially hazardous level, if not to silence, at
least to a very low level (below 55 dBA).
The fluctuation in level must be non-impulsive, i.e., slow
enough to be followed by a standard sound level meter on
the "slow" setting.
13
-------
Such intermissions are known to be protective/ probably
by allowing recovery of normal physiological function in the
auditory system. Because there is no evidence for a threshold
of noxiousness of noise so far as the hearing organ is con-
cerned, it is desirable that the noise during any period of
relative quiet should be measured, and included in the compu-
tation of Leg. "Intermittent" noise may thus be treated in
the same way as noise of varying level, and equated analyti-
cally with continuous noise for the purpose of predicting
hazard or risk.
14
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III.B Impulsive noise
III.B.I Definition and varieties of impulsive noise.
Elaborating the definition given in Section II, impulsive
noise is one or more transient acoustical events, each of
which lasts less than 500 ms and has a magnitude (change in
sound pressure level) of at least 40 dB within that time.
A single impulse may be heard as a discrete event occurring
in otherwise quiet conditions; or it may be superimposed
upon a background of continuous noise. Impulsive noise may
be characterized by the following basic parameters:
1. Peak sound pressure level (in dB re 0.00002 N/m2)*
2. Duration of the event (in milliseconds or microseconds)
3. Rise and decay time
4. Type of waveform (time-course)
5. Spectrum (in the case of oscillatory events, Type B)
6. Total energy of the event (impulse)
7. Number of impulses in a cumulative exposure
8. Intervals or average interval between impulses
III.B.1.1 Types of waveform. Coles and others (Coles et al,
1968; Coles & Rice, 1971) have distinguished two main varieties
of acoustic impulse affecting the ear. These are described in
Appendix 5. In many instances, however, impulsive noises are
not readily classifiable into one of these simple categories:
considerable caution must be exercised in evaluating the
hazard according to the tentative criteria presently recom-
mended. It is important to appreciate that impulse noises
can only be described, distinguished as to type, and properly
measured by means of oscillographic techniques. It cannot be
done using a conventional sound level meter, even on the "fast
response" setting of the instrument. In the "Type A" impulse,
For reasons connected with measurement practice in the
English-speaking countries, the over-pressures associated
with sonic booms in aerospace operations are customarily
expressed in pounds/ft2 (psf) relative to atmospheric
pressure. .Although deprecated, this convention is adhered
to in the present document when citing data expressed in
psf by other authors.
15
-------
there is a rapid rise to a peak SPL followed by a decay to
a negligible magnitude. In the classical "Friedlander"
type of event, a subsequent negative pressure wave occurs,
of much smaller magnitude. In evaluating the hazard due
to this type of impulse only the duration of the positive
part of the event is counted as the duration of the impulse.
In the single "Type B" (oscillatory) event, the duration is
taken as being the time for the envelope to decay to a
value 20 dB below the peak. The acoustic effect of an
impulse can be greatly modified by the circumstances of
exposure (e.g., the presence of reverberant surroundings).
III.B.2 Effect of peak sound pressure level and duration
of impulses.Most of our knowledge of the aural hazard due
to impulse noise, and practically all the data systematically
relating exposure parameters to threshold shift, comes from
studies of the effects of gunfire on the ear, with some
supporting evidence from industrial data. The evidence for
the conclusions here presented is reviewed in Appendix 1.
III.B.2.1 Incidence^of NIPTS as a function of peak SPL. An
estimate of hearing damage risk following daily exposure to
a nominal 100 rounds of gunfire (rifle) noise at 5-second
intervals is shown in Table III.B-I (Kryter & Garinther,
1965) . An important assumption implicit in these data is
that a given TTS2 (TTS measured at 2 minutes) will eventually
lead to an equal amount of NIPTS.
Referring to the CHABA criterion, Kryter and Garinther
(1965) agreed that a tolerable exposure for 90% of the people
using military type rifles would be 100 rounds/day at peak
SPL not exceeding 150 dB at the ear; or 160 dB if the crite-
rion of protection was to be 75% of the people. Commenting
on the data of Coles et al (1968), Kryter (1970a) has
presented a unifying table in which, using certain empirical
rules of conversion, TTS data of Coles and other sources are
corrected to a common set of nominal exposure conditions
similar to those cited above. Kryter's (1970a) table, inclu-
ding corrections to be used to predict probable damage due to
gun noise in 25% of people, is reproduced below (Table III.fi-
ll).
III.B.2.2 Effect of duration of impulses. The present state
of knowledge indicates that a hazard exists, and accordingly
that ear protection should be mandatory, when isolated Type A
noise exposures (e.g., gunshots) exceed a peak sound pressure
level of 150 dB* at the ear for more than 5 ms regardless of
rise time, spectrum, or the presence of oscillatory transients,
A maximum permissible peak of 140 dB is prescribed by the
Occupational Safety and Health Act, 1970. The validity of
a limit Specified in terms of level alone has been
questioned by McRobert and Ward (1973).
16
-------
Table III.B-I.
Estimated expected permanent hearing level
(in dB re ASA:1951) in selected percentiles
of the most sensitive ears following nominal
daily exposure to rifle noise (during typical
military service), namely/ 100 rounds at about
5 second intervals (Kryter & Garinther, 1965).
Percentile
exceeding HL
170
165
160
150
140
Audiometric test frequency (Hz)
1000 2000 3000 4000 6000
90
70
50
67
52
47
60
45
25
50
40
20
45
30
10
10
25
50
10
25
50
10
25
50
10
25
50
10
25
50
25
15
0
16
9
0
15
7
0
10
3
0
0
0
0
35
25
10
20
10
0
16
8
0
15
4
0
5
2
0
70
55
35
62
32
12
25
18
0
15
8
0
10
2
0
85
65
45
60
45
25
45
35
15
35
25
10
30
18
5
at the ear, grazing incidence.
Substantially lower values must be regarded as limiting in the
case of repetitive Type A or Type B impulse noise exposures
of the kind sustained in industry (McRobert & Ward, 1973).
As the duration decreases below 5 ms, higher peak values may
be tolerable; but an absolute maximum of 165 dB SPL for iso-
lated impulses of short duration (below 3 ms) has been
suggested as a limiting level exceeding which is likely to
lead to cochlear damage in at least 50% of ears (see Acton,
1967; Coles et al, 1968; Kryter & Garinther, 1965; Rice & Coles,
1965).
The figure of 165 dB SPL absolute maximum for single
impulses is considered overconservative by some authorities in
relation to extremely brief exposures. The work of Loeb and
Fletcher (1968), for example, has shown that substantially
higher peaks may be tolerable by a majority of ears provided
that the duration of each impulse is very brief (less than
17
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Table III.B-II. Threshold Shift From Gun Noise* (Adapted from Kryter, 1970a)
TTS2 or HL2 (Av 1, 2 & 3 Kilohertz)
Equaled or Exceeded by 25% of People
00
Peak
SPL
Grazing
Study**
Coles & Rice
Elwood et al
Acton et al
Murray & Reid
Smith & Goldstone
Kryter & Garinther,
weapon D
Kryter & Garinther,
weapon B
Kryter 6 Garinther,
weapon A
to ear
160
160
159
159
161
173
138
138
159
176
181
158
159
168
173
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
Corrected to
Listening
Condition
Open field
Reverberant
Open field
Reverberant
Open field
Open field
Open field
Reverberant
Open field
Open field
Open field
Open field
Open field
Open field
Open field
No. of.
Rounds
10-50
10-50
20-50
20-50
20
1
100
100
100
10
25
100
100
100
Measured
7
10
7
19
7
7
0
5
5
17
5
10
22
55
dB
dB
dB1
dB1
dB»
dB1
dB
dB1
t
dB
dB
dB
dB
dB
dB
100 Rounds
20 log 10
19
22
19
31
20
47
0
5
20
27
48
17
10
22
55
by Adding
No . Rounds
dB
dB
dB
dB
dB
dB
dB
dB.
dB4
dB8
dB
dB
dB
dB
dB
No. of
Subjects.
20
20
20
20
12
12
19
19
?
1
1
30
30
36
8
* TTS2 or HL2 average of 1, 2 and 3 kHz as found in or estimated from various studies of threshold
shift from gun noise, one to ten seconds or so between impulses.
References in Kryter (1970a).
TTS data were not given at 1 kHz; average for 1, 2 and 3 kHz was taken to be the TTS at 2 Hz.
Data were given as TTS average of 2 to 6 Hz; this multiplied by 0.3 was taken as TTS average
2 and 3 kHz'.
Authors stated that "noise approached an auditory hazard for about 5% of people."
4 Estimated from authors' statement that 159 peak SPL "commonly" (taken to mean in 50% of people)
caused 40 dB peak TTS - after 100 rounds or more.
* Average 5i2 to 8,192 Hz TTS^s corrected to TTS2 by adding 10 dB and then to average TTS2 at
1, 2 and 3 Hz by multiplying result by 0.5.
**
i
a
1,
t
-------
100 microseconds). Moreover, Coles and Rice (1970, 1971)
have suggested allowing 172 dB SPL for single impulses of
100 microseconds' duration; and over 180 dB for impulses of
less than half that duration (irrespective of pulse shape).
This may be overlenient. There is no evidence that such
levels are safe for a majority of ears, no matter how brief
the exposure. A simplified statement of the limits of
safety recommended by Coles and Rice (1971) is given in the
following Table III.B-III. (The diverging DRC for "A" type
noises is included for comparison but is not recommended here.)
Table III.B-III. Damage risk criterion for impulse noise
(nominal exposure: 100 impulses) as a
function of duration of the impulse
(adapted from Coles & Rice, 1971).
Duration (seconds) 10"4 10~3 10~2 10'1 1.0
Max. safe SPL for 90% of ears
(dB re 0.00002 N/m2) 172 163 157 150 144
Cole's limit for "A" type
spikes 172 163 162 162 162
III.B.3 Allowance for repeated impulses
CHABA Working Group 57 has recently arrived at an empi-
rical weighting factor for reducing permissible levels of
exposure when multiple impulse noises are heard. Essentially,
the current recommendation of that Working Group is to add or
subtract 2 decibels from permissible values for each halving
or doubling, respectively, of the number of impulses (or 5 dB
for every tenfold change in the total number in a series of
impulses). A related rule which, although less linear, may be
more realistic in form, is that of Coles and Rice (1971) , shown
in Table III.B-IV. It is probable, however, that the factors
of -20 dB and more for 1000 impulses and upwards given in this
table may be overconservative (McRobert & Ward, 1973).
Table III.B-IV. Suggested correction factors for number of
similar impulse noises, relative to zero for
a nominal exposure of 100 impulses (approxi-
mated from Coles & Rice, 1971).
Number of impulses 1 10 100 1000 104 105
Correction factor (dB) +15 +10 0 -20 -30 -35
19
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III.B.4 The question of high-frequency hearing losses due
to impulse noise
Coles (1971) and Loeb and Fletcher (1968) have drawn
attention to the fact that, although hearing loss due to
many kinds of intense short-lived or impulsive noise appear
audiometrically identical with loss due to continuous noise
(showing the characteristic audiometric notch at 4000 Hz
and progressive upward spread), certain kinds of impulsive
noise, such as gunfire, are not uncommonly associated with
a substantial immediate TTS and potential permanent loss at
higher frequencies (6 to 8 Hz and upward). This may be
associated with particular parameters of the noise exposure,
such as extremely rapid rise and high peak level (Coles,
1971).
Such high-frequency loss is not predicted, or is not
treated as significant, by many of the existing damage risk
criteria or methods of hazardous noise exposure evaluation,
which are narrowly restricted to "speech frequencies" below
4000 Hz. High-frequency sensitivity can however be impor-
tant for several purposes in life, such as the reception of
speech heard against a background of noise (Hirsh, 1973);
the localization and identification of faint, high-pitched
sounds in a variety of occupational (including military)
and social situations; and the appreciation of many human
and environmental sounds (e.g., music; birdsong; orienting
sounds and so on). High-frequency hearing loss associated
with impulse noise exposure (Hodge & McCommons, 1966; Loeb,
Fletcher & Benson, 1965) should accordingly, in our view,
be prevented whenever practicable (Loeb & Fletcher, 1968).
III.B.5 Factors influencing hazard due to impulse noise
There is no unequivocal evidence that a practical
distinction need be made between the sexes or between age
groups when predicting hearing damage risk due to impulse
noise as it is here defined. Nor does any definitive
evidence exist for a significantly different degree of
susceptibility to impulse-noise-induced PTS in the case of
children or persons with otological abnormality. Factors
generally influencing susceptibility to NIPTS due to noise
in general are considered further in Section IV and the
appendices referred to in that section.
III.B.6 Combined exposure to on-going noise with added
impulsive noise; allowance for impulsivenes"s"
When impulsive noise exposure takes place at the same
time as continuous noise, the hazard of each element to the
hearing mechanism should be evaluated separately (so far as
it is possible to distinguish them in the measurement)
against its respective criterion. A conservative approach
20
-------
is then to treat combined hazards as simply additive. For
example, if at a given centile of the population at risk,
a continuous noise exposure were predicted to cause NIPTS
of 10 dB; and a concurrent impulse noise exposure were
predicted to produce 5 dB of NIPTS, then the combination
may be predicted to produce 15 dB of NIPTS at that centile,
Alternately, some authorities might argue in favor of a
logarithmic rule which would be somewhat less conservative,
21
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IV. FACTORS INFLUENCING INCIDENCE OF NIPTS
IV.1 Varied action of different factors
Factors either influencing (+) the incidence of NIPTS
or known to be essentially unrelated to it are listed in
Table IV-I. Table IV-I shows that some factors appear to
increase the risk of NIPTS while others decrease it; and
that some, while they may be significant factors determining
group hearing levels measured in population surveys, show
no clear evidence of being related causally to NIPTS.
Supporting evidence for the effects summarized in Table IV-I
is referred to in Appendices 1-3, 7 and 9-11, and in Johnson
(1973) .
Table IV-I. Effect of various factors on incidence of NIPTS.
The symbol (?) means present evidence is
equivocal or lacking.
NO SIGNIFICANT
FACTOR INCREASES DECREASES EFFECT
Age ? ?
Sex +
Nationality +
Race +
Physiological state:
i. General health ? ?
ii. Activity +
iii. Defensive mechanisms* +
Prolonged exposure +
Interrupted or modulated
exposure +
Ear protection +
Adverse environments:
i. Vibration + noise ? ?
ii. Hypoxic states ? ?
iii. Ototoxic drugs ? ?
"Public awareness" +
* Principally the acoustic reflex: effect depends greatly on
the individual and the noise exposure.
IV.2 Factors increasing the risk of NIPTS
The only factor known to increase the likelihood of a
person developing NIPTS is increased exposure to hazardous
noise. The influence of duration of exposure has been
summarized in Section III. Although it is possible that
the older ear may be more susceptible than the younger ear
(see Appendix 3), such a phenomenon is difficult to distin-
guish epidemiologically and the question of age-enhanced
22
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susceptibility to NIPTS remains open. It is a tenable but
unproven hypothesis that certain defects or diseases of the
ear, or a poor general state of health, might increase pre-
disposition to NIPTS. This is mentioned again in Appendices
7 and 9. There is some evidence (Misrahy e_t alf 1961; Dayal
et_ aJL, 1971; Falk, 1972) that certain ototoxic drugs may act
synergistically with noise to damage the hearing organ.
IV.3 Factors mitigating risk
Physiologically, the acoustic reflex is known to protect
the hearing against noise. The degree of activation of the
reflex, however, and the amount of protection which it affords
varies with individuals and the character of the noise exposure.
The variability of the acoustic reflex makes it difficult to
assess its protective value in practice or to make allowance
for it in predictive models. The action of the acoustic
reflex is discussed in Appendix 9.
The use of artificial ear protection (earplugs, earmuffs
and kindred devices) decreases the risk of NIPTS substantially
but this again is a difficult factor to allow for in predic-
tive formulae, because the use of ear protection (especially
in non-occupational noise exposure situations) is neither
universal nor uniform. In this connection, however, it is
reasonable to presume that, as the population at large is made
increasingly aware of the hearing hazard from noise, the
public response (e.g., use of ear protectors; noise-avoidance)
will be reflected in a decreasing incidence of NIPTS attribu-
table to environmental noise.
IV.4 Factors not directly affecting susceptibility to NIPTS
Certain intrinsic factors have been observed to influence
group hearing levels measured in public health and industrial
surveys. Notable among these factors is sex (women frequently
having been found to have better hearing, age for age, than
men) , with smaller correlations showing up in relation to
other demographic factors such as race and social and economic
status (Glorig et al, 1954; Glorig & Roberts, 1965; Roberts &
Cohrssen, 1968). fKere is no conclusive evidence, however,
that differences in sex, race or national origin are associated
with any inherent predisposition to noise-induced hearing loss
(see Appendix 2) . These factors may accordingly be disregarded
when formulating hearing damage risk criteria or noise exposure
limits.
23
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V. CONCLUSIONS AND RECOMMENDATIONS
V.I Noise exposure hazardous to the hearing
Sufficient basic data now exist to enable the risk to hear-
ing from specified noise exposures to be predicted on a statist-
ical basis. There is however a need to reevaluate the question
of what constitutes a real or "significant" noise-induced hearing
loss. Hitherto, in the case of continuous, spectrally distributed
noise (the commonest variety in urban and industrial settings),
this question has been considered only in the context of occupat-
ionally related hearing loss. Now it has to be considered in the
wider social context of possible damage to the hearing from en-
vironmental noise to which the general population may be exposed,
either voluntarily or unwittingly, in the course of day to day
living.
Such an extension of the preventive concepts worked out over
many years of industrial hearing conservation raises new admin-
istrative questions concerning the social and ethical criteria by
which hearing conservation standards (i.e., statements recommend-
ing noise exposure limits in various living and working contexts;
and as to the percentage of the population to be protected against
a specified amount of NIPTS) should be set. It must be recogniz-
ed that decisions of this kind necessarily become even more arbit-
rary than they have been hitherto in the area of hearing conserv-
ation at the "speech frequencies" when the hearing is daily threat-
ened by regular occupational exposure to noise. It is, for in-
stance, inherently more difficult to define acceptable margins of
safety governing the selection of exposure limits to protect gen-
eral populations against non-occupational exposure; for such pop-
ulations differ much more widely than, say, a relatively homogene-
ous group of industrial workers (presumed all to be adults below
the age of retirement) in regard to their health, range of sus-
ceptibility to NIPTS, life-style (determining circumstances of
noise exposure) and the personal and social significance of any
hearing loss which may be sustained.
Such difficulties notwithstanding, however, the present re-
port provides basic information from which predictions of hazard
to the hearing from a variety of noise (both occupational and non-
occupational) can be made. The principal data were summarized in
Section III: justifications for the use made of the data are to
be found elsewhere in this report (expecially in Appendix 7) and
in Johnson (1973), the companion report. The main conclusions to
be drawn are set out below. Kryter (1973) has presented an altern-
ate approach to the prediction of risk to hearing ("Speech Im-
pairment Risk").
24
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V.I.I Conti-nuoiis noise ex-posure. The present consensus is that,
in the case of daxly exposure to continuous noise without strong
tonal components, a level of 75 dBA sustained for 8 hours (or 70
dBA for 24 hours) per day is the threshold for detectable noise-
induced permanent threshold shift (NIPTS): exceeding that thres-
hold may cause NIPTS exceeding 5 dB in up to 10% of the people
after a cumulative noise exposure of 10 years. This hearing
change is predicted for the most sensitive audiometric frequency,
namely,-4000 Hz. At the conventional speech frequencies (0.5, 1
and 2 kHz), the threshold may be 10 dB higher, that is, 85 dBA
for a daily exposure of 8 hours. In the case of daily noise ex-
posure for cumulative durations other than 8 or 24 hours, an
equivalent continuous sound level (i.e., an effective level norm-
alized to an 8-hour exposure) may be calculated, using the method
set out in Appendix 8, in order to evaluate the risk.
V.I.2 Impulsive noise. Detectable effects upon hearing may be
expected to result in a minority of people (<10%) from exposure
to impulses exceeding a peak sound pressure level as low as 125
dB (grazing incidence at the ear) for more than 3 milliseconds;
and a level of 150 dB SPL exceeded for more than 3 milliseconds
may be taken to be the threshold of hazard for many purposes, de-
pending upon the duration, number, rate and pattern of repetit-
ion, character and spectrum of the impulses (see Section III.B).
With certain provisos, isolated exposures up to 165 dB peak SPL
may be permissible in some circumstances when the impulse durat-
ion is very brief (less than 1 millisecond). Note: impulsive
noises must be identified and measured using an oscillographic
analytical technique in order to evaluate the hazard properly: an
ordinary sound level meter is not suitable to the purpose.
V. 2 Need for further research into the relationship between
noise exposure and NIPTS
The following areas are considered to be in urgent need of
investigation.
(i) Patterns of NITTS and NIPTS and their interrelationships in
man associated with continuous noise exposures extending be-
yond the conventional 8-hour working duration of daily oc-
cupational exposure (ethical investigations of the effect
of continuous exposure of up to 24 hours and longer in man
should be included).
(ii) The significance of high-tone (above 2000 Hz) losses of
hearing in relation to speech discrimination.
(iii) The effect of interruption and fluctuation in level of con-
tinuous noise exposure upon the growth of NITTS and NIPTS
in man.
25
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(iv) Susceptibility to NITTS and NIPTS as a function of age (in-
cluding childhood).
(v) Hazard due to impulseive noise as a function of character,
number, rate and pattern of repetition and spectrum of im-
pulses. Here, particularly, NIPTS and cochlear damage pat-
tern studies in animals are necessary to supplement impulse-
NITTS observations in human subjects.
(vi) Appraisal of the hearing hazard of non-occupational exposures
to noise, both continuous and impulsive.
V.3 Need for public noise monitoring and audiometric supervision
V.3.1 Noise monitoring of the environment. Public exposure to
noise at work, in transportation, in the community, in recreation
areas, and in the home can be estimated from proper measurements
of the noise at representative sites and should be redetermined
at appropriate intervals. Detailed and reliable information about
exposure levels and the temporal pattern of exposure is crucial
to the evaluation of environmental noise hazards. Existing tech-
niques of noise monitoring in the vicinity of aerospace operations,
highways, and industrial sources of noise serve as models in this
connection.
V.3.2 Noise-exposure monitoring of the population. Properly
selected and instructed voluntary random samples of designated
sections of the population, otologically screened, should be pro-
vided with personal noise dosimeters (see Appendix 5), in order
to evaluate their personal noise-exposure histories at appropri-
ate intervals.
V.3.3 Monitoring audiometry and otological supervision. The
same subjects should be tested (in properly controlled conditions)
at intervals not exceeding 6 months, in order to follow up any
hearing changes associated with their measured or estimated noise
exposure and advancing age. The audiometric test frequencies us-
ed should span at least the range 500 to 8000 Hz. Children from
the age of 7 years should be included in the survey.
V.4 Information and education
V.4.1 Education and protection of the public. There is a real
need to raise public awareness (and indeed that of many physic-
ians, engineers and administrators) regarding the hazard of en-
vironmental noise to the hearing mechanism. Programs designed
to educate the public about noise hazards can be envisaged, per-
haps along the lines of present television programs and advert-
isements concerning such matters as domestic and highway safety,
the prevention of disease and of alcohol and drug abuse, and the
26
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care of the natural environment. There is much to be said for
encouraging the public to regard a periodic hearing test as a de-
sirable and routine part of health care throughout life. Such
hearing tests are desirable irrespective of whether or not the
person is knowingly exposed to hazardous noise at work or during
his or her recreational pursuits. The tests might usefully be
introduced routinely at school with a view to their continuation
on a voluntary basis throughout adult life.
V.4.2 Warning notices. Graphic posters and warning notices can
be used to reinforce public awareness in specific situations (e.g.,
at work or in.certain recreational settings such as shooting
ranges) where the noise is hazardous to the hearing. It is es-
sential that such posters or notices be forcefully drawn and that
they convery a clear instruction when protection of the ears is
required. A British warning notice for use in industry (Depart-
ment of Employment, 1972), reproduced in Figure V-l, is one of the
most effective that we have seen in this connection. It is in-
tended to be displayed at the entrances to noise-hazardous areas;
or, with modified wording, to be affixed to dangerously noisy
machines. For more general use in public areas, the wording "Pro-
tect Your Ears" might be more suitable.
USE EAR PROTECTORS
Figure V-l.
Warning notice against hazardous noise.
Color: black inscription on a yellow field,
(Department of Employment, 1972)
27
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Appendix 1
EXISTING HEARING DAMAGE RISK CRITERIA
AND PROCEDURES FOR EVALUATING NIPTS
Al.l The origin of the "AAOO" rule for evaluating hearing
handicap
A widely used formula for determining the amount of
occupationally related hearing impairment to be deemed sig-
nificant for purposes of compensation was devised by a
committee of the American Academy of Ophthalmology and
Otolaryngology in 1959. The basis of the formula was the
contention that the arithmetical average hearing threshold
level for pure tones at 500, 1000 and 2000 Hz gives the best
estimate of hearing for everyday speech. It was implicitly
assumed that, because hearing for speech is particularly
important, hearing above 2000 Hz being of less social value
need not be included in the assessment. According to the
formula, impairment is nil until an AHL of 25 dB (the "low
fence") is exceeded; and is total above a "high fence" of
92 dB AHL (ISO, 1964). Between the fences, a simple linear
relationship between loss and impairment is assumed, so that
1.5% impairment is calculated for every decibel increase in
AHL (giving 100% or total impairment at the high fence) .
In the case of loss mainly affecting one ear, impairment is
based arbitrarily upon a 5:1 weighting ratio between the
good and bad ears (a person with one normal and one totally
deaf ear being deemed for purposes of compensation to be
approximately 15% impaired).
This formula replaced earlier methods for evaluating
hearing within the audiometric range (eg, Fletcher's "Point
Eight" Rule); and has itself been the subject of proposed
modifications. These have included the use of higher audio-
metric frequencies (notably 3000 Hz as is recommended by
British otologists) and the inclusion of some form of speech
audiometry in the evaluation.
Some years ago, when the Committee on Conservation of
Hearing of the American Academy of Ophthalmology and Oto-
laryngology (AAOO) first adopted this relatively simple rule,
It was tailored fairly specifically to the assessment of
hearing handicaps in audiological patients presumed to be
suffering from occupationally-related NIHL. The AAOO rule
was essentially a simplification of the somewhat clumsy
Fowler-Sabine Scale formerly recommended by the AMA in the
Al-1
-------
1940*s. An important (some would say regretable) departure
from the Fowler-Sabine formula was the dropping of 4000 Hz
from the assessment: The AAOO evaluation is based only on
the audiometric frequencies 500, 1000 and 2000 Hz, considered
to be sufficient for the understanding of everyday speech
in quiet conditions. Thus AAOO takes no account of high-
frequency hearing, which may nevertheless be valuable in
the appreciation of nonverbal sounds as well as the discri-
mination of speech in noise. As a compromise, some authori-
ties, including the state of California, include 3000 Hz in
the assessments. The rule retained the Fowler-Sabine
weighting factor of 5:1 in cases of essentially monaural
impairment; and similarly, AAOO made no allowance for any
benefit obtainable from a hearing aid. Controversially,
it also made no allowance for presbyacusis, a factor which
is still a matter of arbitrary ruling which varies from
state to state (Fox, 1972; Van Atta, 1970).
AAOO set the "high fence" of total (100%) handicap at
92 dB hearing level (ref. ISO, 1964) , averaged for the 3
"speech" frequencies (a level of affliction at which the
patient can barely hear very loud speech at a socially
acceptable distance). Beginning handicap was set at a "low
fence" of 25 dB (ref. ISO, 1964) at which level a hearer is
just beginning to have difficulty understanding everyday
speech. The simple AAOO rule presently states that, over
the range of quantifiable handicap, 13$% of handicap is
counted for each decibel rise in hearing threshold level
above the low fence. Some otologists consider the low
fence (an arbitrary "zero" for handicap) to be placed un-
fairly high from the viewpoint of the afflicted.
Al.1.1 The meaning of "handicap". High e_t al^ (1964) have
pointed out that two people with identical hearing impairment,
as determined by pure tone audiometry, do not necessarily
suffer the same degree of handicap as indicated by self-
assessment using a questionnaire. The self-reporting type
of instrument developed by those authors for the self-assess-
ment of hearing handicap illustrates well, in its questionnary
examples of everyday hearing difficulty, what handicaps can
mean to the individual in social contexts.
A1.2 Criteria and limits of noise exposure
That there is a time/intensity "trade-off" for hazardous
steady-state noise is well established; but this has been
embodied in existing criteria in different ways. For the
trade-off is not a simple one. Differing theories underlie
the various criteria for the assessment of hearing risk
Al-2
-------
currently in use. The picture is complicated by inter-
mittent noise exposure, which is frequently the case in
practice. Evidence from TTS experiments generally supports
the view that the effect of intermittent exposures to high
levels of noise separated by relative quiet is less than
the effect of the same total noise exposure received un-
broken (Ward, 1963) . Moreover, the production of a given
TTS by continuous noise requires progressively less time
as the exposure level is increased.
Al.2.1 The "CHABA" criterion. The CHABA damage risk
criterion was based on such observations. Its principal
assumption is that, for a given octave band of noise, all
noise exposures producing the same TTS2 are equally likely
to produce a given PTS (Kryter et al, 1966). This DEC, in
which the trade-off between time and" intensity varies (eg,
between 2 and 7 dB per doubling of time for the 1200-2400 Hz
band) , represented a departure from the simple adoption of
the "equal -energy" rule (3 dB per doubling of time) seen in
earlier criteria (such as AFR 160-3, 1956). The resulting
differences between DRC's are illustrated in Table Al-I
which compares simply the limiting values for continuous
exposure at 1200-2400 Hz in CHABA and AFR 160-3. The latter
is more conservative.
Table Al-I. Comparison of CHABA DRC and AFR 160-3
Exposure time 8h_ 4h_ 2_h 10 min 5 min
CHABA 85 87 105 112 dB
AFR 160-3 85 88 91 105 dB
The 5 dB rule adopted under the Walsh-Healey Act in 1969
appears to have been an expedient compromise: it has some
justification in that it in effect makes an allowance for
intermittency .
Al . 2 . 2 Criteria and exposure limits for steady-state noise.
There is generally firm agreement that, for typical 8-hour
everyday exposures to continuous industrial noises, levels
below 80 dBA for most hearers may be innocuous but that,
as the noise level increases, an increasing number of
people are put at risk and the average magnitude of hearing
loss grows commensurately. This picture is well supported
by a number of substantial audiometric surveys of industrially
exposed people in the United States and elsewhere (Baughn,
1966; Passchier-Vermeer, 1968; Robinson, 1968) (see Section
Al-3
-------
IIIA and Appendix 7). Based on such evidence, a recent
DRC, provided for in 1969 under the Walsh-Healey Act
governing the welfare of workers under public contracts,
was adopted in the United States which allows 90 dBA for
continuous 8-hour exposures. This arbitrary (administra-
tive) limit may result in appreciable NIPTS (more than 15
dB at 4 kHz) in a majority (at least 50%) of workers but
presumably will permit compensable damage to occur in
relatively few cases.
Al.2.3 "AAOO" and cognate rules. It is a basic premise
of these criteria that the chxef (a rigorous interpretation
might say the sole) function of human hearing is to receive
speech signals. Arguing that telephoned speech (band-
limited to some 300 to 3000 Hz) is generally intelligible,
Fletcher (1929) introduced his "point-eight" rule for
evaluating hearing damage in accordance with this philoso-
phy. There was born the practice of averaging hearing
levels at 500, 1000 and 2000 Hz. (Fletcher's original rule
presumed damage to have begun as soon as these levels
reached zeroi.e., there was no "low fence"but not to be
complete until the average had reached 125 dB, an unrealis-
tic "high fence". A reconsideration led eventually to the
modifications embodied in the AAOO rule.)
The AAOO and cognate rules attempt, inter alia, to find
pragmatic answers to the following questions (Ward, 1970):
1) How much hearing loss must occur before
the person affected notices any difficulty?
2) What values of hearing loss constitute
complete loss of hearing?
3) What is the relative importance of different
audiometric frequencies?
4) How important is it to have two working ears?
Al.2.4 The Intersociety Committee (1970) Guidelines, A group
of professional associations* concerned with industrial noise
recently (Intersociety Committee, 1970) revised some previously
published (1967) guidelines intended "...to aid industrial
management and official agencies in establishing effective
hearing conservation programs." The document has also defined
The American Academy of Occupational Medicine; American
Academy of Ophthalmology and Otolaryngology; American Con-
ference of Governmental Industrial Hygienists; Industrial
Hygien6 Association; and Industrial Medical Association.
Al-4
-------
hearing impairment as an average threshold level in excess
of 15 dBf ASA-Z24.5 (1951) (equivalent to 25 dB, ISO: 1964)
at 500, 1000 and 2000 Hz. The guidelines were intended to
prevent that portion of permanent hearing loss due to occu-
pational exposure to continuous noise.
The evaluation of noise in dBA using standard meters
and procedures was recommended by the Committee, as was the
determination or estimation of the total time and temporal
distribution of noise exposure "throughout the working day".
The guidelines, subject to revision, contain numerical data
and procedures for rating the auditory hazard of occupational
noise exposure in terms of risk as a function of age, noise
level and exposure time. The Committee in 1970 deemed 90 dBA
for 8 working hours of steady-state noise experienced day
after day to be a "reasonable objective for hearing conser-
vation", with a permissible increase of 5 dBA* (up to a
permissible maximum of 115 dBA) for each halving of exposure
time. It was pointed out explicitly that the rating pro-
cedure applies only to groups, not to individuals.
The document included some general guidance on methods
of noise control for hearing conservation in industry; and
some recommendations concerning audiometry in industrial
settings. The audiometric frequencies recommended by the
Intersociety Committee for routine testing were 500, 1000,
2000, 3000, 4000 and 6000 Hz. The guidelines are subject
to triennial review and revision.
A1.3 Use of A-weighted decibels
The Intersociety Committee on Guidelines for Noise
Exposure and Control, influenced mainly by the work of
Baughn (1966) in the USA and Robinson (1968) in the United
Kingdom, decided to recommend the use of dBA to yield a
single-number rating of continuous noise hazard (Mercer,1968) .
This unit, as recommended in the present document, has a
number of advantages. These advantages include convenience
of measurement using standard sound level meters; and the
fact that a measurement in dBA can readily be related to< the
ISO standardized NR numbers using the approximate difference
of 5 decibels (dBA = NR + 5) . Measurements on the A-weighting
scale, however, may possibly underestimate hazard to hearing
when the noise contains strong tonal components (Mercer, 1968;
Acton, 1967) or a markedly uneven spectrum. Measurements
using other weighting characteristics (eg, C-weighting) may
have uses in the complete evaluation of the hearing hazard
due to some kinds of noise (see Appendix 5).
* of the "Equal Energy rule,
Al-5
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A1.4 Indices of Cumulative Noise Exposure
Al.4.1 Robinson's "sound-immission" rating. Robinson (1968,
1971; Robxnson & Cook, 1968) has contended that NIHL is
expressible in terms of a composite noise exposure measure
(noise of sound "iiranission") which is proportional to the
total frequency-weighted sound energy received by the ear
over a designated exposure period. Robinson & Cook (1968)
have presented industrial hearing level and noise exposure
data in support of this predictive model valid for 8-hour
daily exposures in the range 1 to 600 months (50 years) to
industrial type noise at levels that range from 75 to 120 dBA.
Based on recent British survey data relating hearing
levels to continuous industrial-type noise exposure, a pre-
dictive method has been presented by Robinson (1968, 1971)
for estimating the magnitude of NIPTS to be expected in
designated fractions of an otologically normal population
due to known or predicted noise exposure. Assuming that
the noise is of the same general type as that found in
manufacturing industries, where the worker's ear is exposed
throughout every shift to a fairly constant (steady-state)
assault, Robinson's formula may be used to determine the
A-weighted noise immission level, EA, as a measure of the
total equivalent exposure.* Noise immission level is de-
fined by Robinson according to the formula:
EA = LA + 10 log (T/TQ),
where LA is the A-weighted sound level of the noise in dBA,
T is the duration of exposure in calendar years (up to 50)
and T0 is the reference duration of 1 year. Robinson main-
tains that this is the measure of total noise exposure that
uniquely determines NIPTS.
Certain important assumptions underlie Robinson's method,
namely:
1) The noise is "reasonably steady" (sic) and continuous
(8h/day) and does not exceed 120 dBA. Its spectrum
may be undefined within the slope limits of ±5dB/
octave (i.e., there are no prominent spectral peaks
or tonal components in the noise).
2) The ear is exposed 5 days per week for a working
life between 1 month and 50 years.
3) The ear is otologically normal but subject to
normal aging (Note; Robinson makes use of
Hinchcliffe's (1959) data to correct for pres-
byacusis).
cf LetJ, the equivalent continuous sound level (Appendix 8)
Al-6
-------
Robinson recommends the "equal-energy" rule for extension
to cover shorter exposures (less than 8h/day and less than
5 days/ week). His method permits entry into the noisy
occupation at.any specified age. Prior experience can be
allowed for if the noise exposure is known to have been
similar to the occupational exposure in question. No
allowance is made for breaks in noisy employment; or for
noise outside work (non-occupational exposure).
Granted the above assumptions, Robinson (1971) uses
the following formula to predict the distribution of
hearing levels (H1(p) for selected centiles) in a working
population whose ears are subject only to occupational
noise exposure of the kind described, and presbyacusis.
Hearing level is predicted for the range 500 Hz to 6000 Hz
+ u(p)
H'(l) = 27.5 ( 1 + tanh ~ + u(p)j + F(N)3,
in which:
E, is A-weighted noise immission level;
X(f) is an audiometric frequency-weight ing parameter;
U(p) is a distributional term given by:
u(p) = 6/2 . erf"1
where p is the centile of population for which H1 equals
or exceeds H*(p) (high centile values being associated
with noise resistant ears); and
F(N) is a further term taking empirically determined values
according to audiometric test frequency and the sample
age in years (N):
;0 when N ^ 20
0
c(f) . (N-20)2 when N >20
(20 years of age is a nominal age of entry into noisy
employment of the young worker without presbyacusis.)
The frequency-dependent values of C and A (in dB) are
given in the following table.
Al-7
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Table Al-II. Values of the parameter A and C
Frequency
(kHz)
0.5
1
2
3
4
6
0.0040 0.0043 0.0060 0.0080 0.0120 0.0140
130.0 126.5 120.0 114.5 112.5 115.5
It may be noted that Robinson (1971) incidentally defines
"risk" as that percentage of population whose hearing level,
because of some specific influence such as noise, age or disease
(or a combination), exceeds a specified value, minus that per-
centage of the population whose hearing would have exceeded the
same value in the absence of the influence in question (assuming
other factors to remain the same).
A1.5 The question of the adequacy of conventional "speech frequen-
cies" assessment
Harris (1965) has contended that the widely adopted convention
of using the average pure-tone auditory sensitivity at 500, 1000
and 2000 Hz to predict a person's ability to understand everyday
speech may not be adequate when, as is often the case, the speech
is of poor quality, interrupted, distorted or noise-masked. From
a study of speech intelligibility among 52 subjects with sensori-
neural hypoacusis, listening to various kinds of degraded speech,
he concluded that a better assessment of hearing disability for
realistic everyday speech is obtained when the audiometric frequen-
cies 1, 2 and 3 kHz are used instead, as is the convention in
British practice. This supports a finding of Kryter, Williams
& Green (1962), who reported that the triad 2, 3 and 4 kHz was the
best predictor of speech reception for phonetically balanced words
(not sentences) in subjects with high-tone hearing losses.
Kryter and his co-workers (1962) showed that some speech tests
and methods of hearing evaluation hitherto adopted introduce a
bias which is apt to lead to underestimation of the importance of
auditory acuity at frequencies above 2 kHz. Some authorities,
notably the state of California, already include 3000 Hz in the
assessment of disability.
A1.6 Impulsive Noise
Impulsive noises have a very high peak level and a short
duration. Examples are the sound of gunfire or explosions,
impacts in industrial processes (eg, drop-forging), and sonic
Al-8
-------
booms. The peak pressure produced at the ear from firing a
self-loading rifle, for instance, can exceed 160 dB for 5 ms.
The noise hazard may be greatly modified and sometimes
enhanced by many factors, including the surroundings in which
the weapon is used. Kryter (1970a) has defined an impulse as
a change of sound pressure of at least 40 dB within half a
second (500 ms) : conversely, he deems steady-state noise to
be present when the overall SPL changes less than 40 dB
between successive 0.5 sec intervals. Kryter has adduced
evidence from his own and other recent work to show that TTS2
at 4000 Hz and, by implication, the risk of NIPTS, can in
many circumstances be predicted with fair accuracy from a
knowledge of the peak overpressure, spectral composition and
number of impulses. For the noise of gunfire, Kryter main-
tains that damage risk to hearing can be evaluated from the
peak overpressure and number of impulses. An important
assumption implicit in these data was that a given TTS, would
eventually lead to an equal NIPTS.
Some procedures proposed by Kryter (1970a) and others
for predicting specifying damage risk to hearing due to gun-
fire and similar noises were summarized in Section IIIB.
The risk to hearing from such noise depends primarily upon
the peak overpressure and the number of impulses experienced;
and to some degree upon the spectral and temporal character-
istics of the noise. Although, in general terms, the pattern
of NIHL produced by impulsive noise is similar to that pro-
duced by steady-state noise, namely, loss beginning and ad-
vancing most rapidly at 4 kHz and above, the different
stimulus parameters call for rather different criteria and
methods for evaluating impulse noise. For this reason, it
should be noted, the current ISO Recommendation (ISO, 1971)
on the assessment of occupational noise-exposure for hearing
conservation purposes states specifically that the method
is not applicable to such noises.
Al.6.1 Impulse noise: measurement and evaluation. Coles
(1970; Coles et al, 1968; Coles & Rice, 1971) has distin-
guished three commonly encountered types of impulse noise,
designated as follows:
Type (A) occasional, widely separated, rapidly decaying
impulses (eg, gunfire or intermittent impact or
explosive noise)
Type (B) repetitive but still discrete impulses having a
peak-to-background level of at least 6 dB and
impulse rates of 0.5 to 10/sec. (eg, many indus-
trial processes, such as blanking, manual
hammering, etc.)
Type (C) rapidly repetitive noises (repetition rate
greater than 10/sec) with a ration of peak to
minimum level generally less than 6 dB. This
type of noise is very common in industry (eg,
pneumatic hammering, riveting).
Al-9
-------
Because of instrumental lag and nonlinear!ty, ordinary sound
level meters, whether used on the A-scale or for octave-band
analysis, are generally quite unsuitable for evaluating Type
(A) and (B) noises, although some authorities have recommended
the use of meters set to read dBA with the "slow" response,
corrected by +10 dBA for impulsiveness. Possibly this might
be valid for some varieties of Type (B) noise (Coles, 1970).
Oscillographic measurements (and the use of appropriate DRC's)
are proper to the evaluation of discrete impulsive noises.
By contrast, Coles (1970) maintains that Type (C) noises,
approximating as they do steady-state noise, are validly
measured using the A-weighting scale of a conventional sound
level meter. Coles (1970).has recently reported some proposed
modifications of his DRC for impulsive noise (Coles et al,
1968 ; Coles & Rice, 1969, 1971) including a variable correction
factor spanning some 50 dB for the number of impulses per
exposure in the range 0 to 105 impulses. (See Section IIIB.)
A model procedure for measuring impulsive noise from cap
guns has been published by the Department of Health, Education
and Welfare (Food and Drug Administration) (Federal Register,
36 (134), 13030, July 13, 1971).
Al.6.2 Impulse noise and TTS. In 1962, Ward argued that
damage risk criteria for impulsive noise should best be
expressed in terms of the number of impulses rather than
exposure time per se. The importance of number of impulses
has more recently been brought out by Coles et al (1968;
1970) as noted above. Ward's (1962) argument was based
on his observation that the TTS in the range 500 to 13,000 Hz
(and, by implication, the PTS) produced by impulse noise is
relatively independent of the interval between pulsesat
least for intervals in the range 1 to 9 seconds (a 30-second
interval, however, apparently permitted slight recovery
between stimuli.
Al.6.3 Impulses with an oscillatory component. When the
Impulse contains an oscillatory component ("Type B" of Coles
and Garinther, 1968), the assumptions of Kryter (1970a)
applying to simple, Type A gun noise may require modification;
and spectral information may be needed in the evaluation of
hazard, in addition to a knowledge of the peak pressure, number
and temporal spacing of impulses (Coles et al, 1968; Kryter,
1970a; Ward, 1962; Ward et al, 1968 (CHABA); Ward, Selters and
Glorig, 1961). Oscillatory waveforms can be recorded when guns
are fired in reverberant areas; and from sources of impulsive
noise other than gunfire. It has been argued (Muirhead, 1960)
that even spike impulses must generate an oscillatory component
upon entering the ear, by exciting the resonances of the ear
canal and middle ear structures. This would in part explain the
general similarities between the patterns of threshold shift
produced by both impulsive and distributed steady-state noise.
A single,universal limit of impulse noise exposure, based solely
upon peak pressure, is unlikely to be satisfactory for hearing
conservation purposes (McRobert & Ward, 1973).
Al-10
-------
Appendix 2
HEARING IN THE AMERICAN POPULATION
A2.1 "Normal hearing"
Hearing normally means being able to detect sounds in
the audio-frequency range, namely, 16 Hz to 20,000 Hz
(20 kHz) , at levels which lie at or within 10 decibels of
the normal threshold of hearing and below the threshold of
aural pain in human beings (those boundaries define the
domain of normally audible sounds heard by air conduction) .
Many otologists define normal hearing more narrowly as the
ability to respond appropriately to human speech (the
spectral components of which are contained largely in the
frequency range 250 to 4000 Hz) in average everyday condi-
tions: others dispute so restrictive a definition, however.
There is no evidence that the domain of hearing varies
significantly between normal human populations around the
world. The average or median normal threshold of hearing
for pure tones and the corresponding reference zero for
audiometers have received international standardization
(ISO, 1961, 1964). The upper boundary of normally audible
sound (threshold of aural pain) has not yet received such
definitive recognition but is commonly considered to be
about 135 dB SPL, a value which is largely independent of
frequency (BENOX, 1953).
It is of interest to note that the typical average
level of conversational speech without undue vocal effort,
measured at a customary speaking distance of 1 meter from
the speaker is about 65 dB SPL. Peak intensities of vocal
sounds usually exceed the average level by about 6 dB. A
range of individual variation of some 20 dB about the 65 dB
SPL average is to be expected in the normal speech of differ-
ent speakers.
A2.1.1 Audioloqical uniformity of the population. There is
no inherent difference between the races comprising the pop-
ulation of the United States as regards hearing levels as a
function of either age or noise exposure: human ears are much
the same around the world. Public health surveys may however
reveal demographic differences in hearing levels of adults of
different races or social groups (Roberts and Bayliss, 1967).
Such differences may be attributed to the effect of differing
environmental influences, including non-occupational noise
exposure (sociacusis) .
A2-1
-------
A2.1.2 Hearing in children. Some 5% of school age children
in the USA had deficient hearing according to a survey by
Kodman and Sperrazzo in 1959. A similar incidence has been
reported in the Lebanon by Mikaelian and Barsoumian (1971).
There is no evidence that any substantial fraction of the
hearing loss in American children (Eagles et al, 1963, 1967) is
noise-induced. However, the possiblity exists that the child
population experiences a significant risk from non-occupational
(recreational and domestic) exposure to noise from such sources
as noisy toys, including cap guns, regular firearms (either
as a shooter or a bystander), pop music and recreational
vehicles (see Appendix 11).
A2.1.3 Upper range of hearing in young people. Rosen and
Rosen (1971) have published a comparative survey of the upper
limits of hearing in school-age children and young people
(aged 10 to 19 years) in several countries in Africa, Europe
and North America. That survey suggests that the frequency
range of "normal" hearing in that age group extends to at
least 16 kHz (at which frequency, using a special audiometric
technique, the authors obtained nearly 100% response in some
of the groups); but that the percentage of children responding
(ie, able to detect tones) falls off rapidly at higher frequen-
cies. A response incidence of less than 50% was obtained from
all but one of the nine test groups at 20 kHz. However,
responses in the range 0 to 15% were obtained at 22 kHz; and
responses greater than zero (up to 10% in Maba'an youngsters)
in 4 groups even at 24 kHz. Fewer than 4% of a group of
American (New York) children responded at that frequency.
The prevalence of noisy toys and pursuits among children
and young people in this country (Fletcher, 1972) may well be
reflected audiometrically in those age groups (see Appendix 11).
Rosen and his co-workers have tentatively suggested that
the differences in hearing level of children of different
cultures may be linked with differences in susceptibility to
atherosclerosis and coronary artery disease in later life.
Rosen, Olin & Rosen (1970), citing work in Finland as well as
their own studies, have also contended that a low saturated
fat diet, said to protect against coronary artery disease,
may also protect against sensorineural hearing loss.
A2.1.4 Limits of ultra high frequency hearing. Using a bone-
conducting ultrasonic transducer in selected young adults
(17 to 24 years of age) Corso (1963) also found that some
hearing sensation exists above 20,000 Hz, above which frequency
there is a fairly abrupt decrease in steepness of the threshold
slope (which is steepabout 50 dB/octavebetween 14 and 20
kHz). Corso found that some sensation persisted on bone con-
duction testing at high levels of stimulation at ultrasonic
frequencies up to more than 95 kHz but it is very questionable
whether this can be regarded as part of "hearing". There is
little difference between the sexes in either sensitivity or
range of sensation.
A2-2
-------
A2.2 Effect of "abnormal" ears on average hearing levels
Surveys of hearing levels in general populations can
yield values which are poorer (less acute hearing) than
those obtained from samples, from ostensibly similar pop-
ulations, from whom subjects with certain audiological
abnormalities (sometimes arbitrarily selected) have been
weeded out by a selection procedure. This was observed by
Parnell, Nagel & Cohen (1972) in their survey of the hearing
of residents living near a jet airport, upon comparison of
their subjects' actual hearing with the hearing predicted
for the community from Public Health Service survey (Glorig
and Roberts, 1965).
A2.3 Sources of variation
Apart from the question of changes in hearing with
advancing age, individual and other factors, it is to be
expected that some statistical variation in threshold will
be seen even when a particular ear is retested audiometri-
cally. The variation arises partly from intrinsic sources
(eg, changes in the subject's physiological state) but a
substantial source of variation in practice is imperfection
in the way in which audiometry is conducted (see section on
Audiometry, Appendix 6). Test-retest variance can, however,
be kept to a minimum when serial audiograms are obtained in
accordance with standard procedures, carried out under
properly controlled conditions.
A2.3.1 Individual variation. Hearing surveys are always
subject to possible bias because of the difficulties of
sampling human populations. In voluntary public hearing
surveys, for example, a substantial proportion of people
selected to form a supposedly random sample of the adult
American population may decline to be examined. One cannot
know, in that event, whether or not those who will not be
examined have group hearing levels similar to those who do
participate. If for any reason those refusing do have
different hearing as a group, then the survey cannot truly
reflect the state of hearing of the population sampled.
More reliable data are of course obtainable from "captive"
(eg, industrial or military) populations, of whom every
member can perforce be examined (Riley, 1961); but such
populations do not represent the general population.
A2.3.2 Sex. From early teenage onwards, and particularly
in the age range 25 through 65 years, women in industrial
countries including the United States generally have better
hearing than men. In the elderly, however, above age 75,
the difference tends to become insignificant. Paradoxically,
A2-3
-------
the rate of increase in hearing loss in men over 50 years
of age declines, while increasing in women of the same age.
Female employees have been found to have better hearing
than male employees, even when they work side by side in
noisy industries (Gallo & Glorig, 1964; Flodgren & Kylin,
1960; Dieroff, 1961). Selection processes and circumstan-
tial factors have been postulated to account for this
(eg, that the women were exposed less to non-occupational
sociacoustic influences, such as small-arms noise; show a
higher absentee ratea questionable contention; and are
freer to leave a job in which they find the noise level
objectionable). A more reasonable explanation, however,
may be that, in the industries involved, women benefited
from more liberal and frequent rest periods than were
alotted to men (Ward, Glorig and Sklar, 1961). The decline
in differentiation between the hearing of the two sexes
in old age may be linked with an enhanced aging effect upon
the ear associated with post-menopausal changes in women
(Glorig et al, 1954: Wisconsin State Fair Hearing Survey),
although this" is admittedly speculative.
Ward (1966) investigated various aspects of NITTS in
relation to sex differences, finding that, whereas men were
more susceptible to TTS following low frequency noise ex-
posure (less than 700 Hz), they were less susceptible than
women to high frequency (greater than 2000 Hz) exposures.
Women also appeared to show a greater benefit (in terms of
reduced TTS) from intermittency in the noise exposure.
Ward has suggested another explanation for these findings,
namely, that females have a more efficient acoustic reflex
than males. However, evidence for sex-linked differences in
the fragility of the hearing organ (or fatiguability of the
auditory nerve by noise) proved to be negative in Ward's
investigations.
Generally, it may be concluded that intrinsic differences
between the sexes are of no practical significance in relation
to hearing hazard in noisy environments, or in relation to the
setting of hearing damage risk criteria.
A2-4
-------
Appendix 3
PRESBYACUSIS
A3.1 Hearing changes with age
The threshold of hearing rises, that is, the hearing
becomes less sensitive/ as a natural consequence of aging.
This effect (presbyacusis) involves first and is most
marked at the higher audiometric frequencies, above about
3000 Hz (Hinchcliffe, 1959). At least in urbanized western
populations, presbyacusis appears to be more pronounced,
age for age, in men than in women, but the difference may
be associated with occupational factors and the differences
between the sexes in the pattern of day to day activity
involving noise exposure, rather than with the sex differ-
ence per se_ (Appendix 2). The loss of auditory sensitivity
with advancing age is believed to be due to central neuronal
attrition as well as to peripheral changes in the auditory
system (Konig, 1957; Farrimond, 1962). Aging people are apt
to have increasing difficulty in discriminating auditory
signals and understanding speech heard against a background
of noise. This may be due to an increasing susceptibility
to masking by low frequency {below 500 Hz) noise, as well as
to the loss of auditory acuity in the speech frequency
range. Certain human groups, living in a simple manner in
remote areas of the world where they are not exposed to the
constant din of mechanized civilization, have been found to
have unusually sharp hearing in comparison with urban popu-
lations of corresponding ages: in this connection particular
attention has been given to the Maba'an people of the Sudan.
But it is debatable whether such audiometric differences are
due to the lack of noise exposure alone; for many factors
(including cultural, genetic, dietary and general environ-
mental differences) may underlie differences in the pattern
of hearing found between dissimilar communities who are
videly separated geographically and culturally (Rosen et al,
1968, 1971).
Although it has been suggested that older people are
more susceptible to NIPTS (Kryter, I960), it is debatable
whether individual susceptibility to noise-induced hearing
loss changes appreciably with age (Kup, 1966; Nowak & Dahl,
1971). Some authors have contended that young ears are
more susceptible to noise-damage (more "tender") than older
ones (Schwartz, 1963).
The evidence, however, is inconclusive, the findings
in some studies having been confounded by non-occupational
influences (eg, noise exposure in military service) which
A3-1
-------
were not the same for the age-groups compared. Recent
studies (Hiilse & Partsch, 1970; Schneider et al, 1970)
indicate that there is probably no causal relationship
between age per se and susceptibility to NIPTS, at
least in men of working age. This view is supported
by the work of Loeb and Fletcher (1963).
That the effect of age on hearing is very difficult
to distinguish audiometrically from the influence of
noise exposure and related environmental variables is
evident form data summarized by Burns and Robinson (1970)
and from several studies dealing with or touching on
noise susceptibility as a function of age.
A3.2 Presbyacusis in industrial experience and "presbya-
cus i s correcti ons M
Glorig and Nixon's (1960) contention that aging and
noise exposure alone determine group hearing levels in
otologically healthy members of the general American pop-
ulation has received support from more recent data and
from industrial experience in other Western countries,
notably the United Kingdom (Burns & Robinson, 1970;
Robinson, 1971). The audiogram in presbyacusis (when
supposedly uncontaminated by noise-induced hearing loss)
typically shows a gradual elevation in hearing threshold
level, the effect being greater and positively accelerated
towards the higher audiometric frequencies (Hinchcliffe,
1959; Glorig et al, 1957). Sufficient data now exist from
large surveys of general population to permit average
values ("presbyacusis corrections") to be standardized for
application to group data from noise-exposed populations
(see Table A3-I). Robinson (1968; 1971) contends that
such corrections are beneficial to the data reduction,
enabling the probability with which a portion of loss is
due to aging to be predicted from a knowledge of the
variance data.
Glorig and Nixon (1960) have restricted the definition
of the term "presbyacusis11 to hearing losses caused by
physiological aging, and it is used in this sense in the
present document, although some audiologists use it to
embrace any sensorineural loss occurring in the elderly.
It is important to appreciate the distinction between
presbyacusis and sociacusis (see Appendix 2).
Glorig (1961) estimated a presbyacusis correction
applicable to the three "speech frequencies" (500, 1000 and
2000 Hz) important in the assessment of disability due to
occupational noise-induced hearing loss: his figures are
A3-2
-------
shown below in Table A3-I to illustrate the magnitude of
the effect. Other presbyacusis data, derived from indus-
trial surveys (Passchier-Vermeer, 1968; Schneider ejb al,
1970) are shown in Table A3-II at the end of this Appendix,
For comparison, the British data of Hinchcliffe (1959),
which are used by Robinson (1971) in his predictive method
(see Section IIIA and Appendix 7) are summarized in Table
A3-III.
Table A3-I. Glorig's correction for AHL at 0.5, 1 and 2 kHz
Age (years) 25 30 35 40 45 50 55 60 65 70
Correction
(dB) 0 +1 +1 +2 +2 +2 +3 +5 +7 +13
A3.2.1 Additivity of effect of age and noise. It is
implicit in the use of presbyacusis corrections that the
effects of age and noise on hearing age simply additive.
As Hinchcliffe (1970) has remarked in a recent review,
physiological aging is accompanied by degenerative changes
affecting not merely the organ of Corti but the whole
auditory system, including its central projections. This
may explain some of the features of hearing handicaps
typical of old age, such as loss of discrimination for
normal, distorted and noise-masked speech, which are not
amenable to prediction from pure tone audiometry alone.
Rosen (1969) believes that degenerative arterial disease
in particular is a major factor in the etiology of presbya-
cusis (especially its central component).
A3.2.2 Presbyacusis and audiometric prediction in the
sensitive ear
It has been pointed out by von Schulthess and Huelsen
(1968; von Schulthess, 1969) that, audiologically, the
endogenous and exogenous factors causing the rise in
hearing level with age are not distinguishable. One can
only say that group hearing levels rise naturally with age
(presbyacusis), due probably to both peripheral and central
aging processes (Schuknecht, 1964; Konig, 1957); and that
this effect is enhanced (in a way which for lack of other
evidence is generally presumed to be additive) by noxious
environmental, mostly acoustic influences (Glorig's "socia-
cusis") and specific exposures to excessive noise.
A3-3
-------
TableA3-II. Presbyacusis data. Upper register: Median
age-induced hearing levels (non-noise-expose<
men) , rounded to nearest decibel. From:
Passchier-Vermeer (1968) .
Age
(Years)
25
30
35
40
45
50
55
60
65
70
75
Frequency
250
0
1
1
2
3
4
5
7
9
12
14
Comparative data
corrected
Age
(Years)
25
30
35
40
45
50
55
60
65
to HL
500
0
1
1
2
3
4
6
8
10
13
16
1000
0
1
1
2
3
4
6
8
10
13
17
(Hz)
2000 3000
0
1
2
4
6
8
11
14
18
24
30
0
2
4
6
9
14
18
22
27
33
40
derived from Schneider
= 0
at Age
25
Frequency
250
_
-
_
-
500
0
0
1
1
2
3
4
6
8
1000
0
1
1
2
3
5
7
9
12
2000
0
1
3
4
6
8
12
16
22
(Hz)
3000
0
3
5
8
12
15
20
27
34
4000
0
3
6
9
13
18
23
28
33
40
47
e_t al_
4000
0
3
5
9
14
18
25
32
42
6000
0
4
7
12
16
22
27
33
40
47
55
(1970)
6000
0
4
7
10
14
19-
25
33
42
8000
0
3
16
11
15
22
28
35
43
53
62
/
8000
0
2
5
*
ia
13
25
36
SO
A3-4
-------
Table A3-III. Hearing loss in decibels as a function of age
in clinically normal female ears (random sample
population). Median hearing loss is related to
median threshold at 21.5 years of age (Hinchcliffe,
1959). Note: For the purposes of the present
document, clinically normal female ears may be
equated with non-noise exposed clinically normal
male ears.
AGE FREQUENCY (kHz)
(Years) 0.125 0.25 0.5 1 2 3 4 6 8_ 12
18-24 0 000000000
25-34 0.5 1.0 0.5 1.0 0.5 1.5 3.5 3.5 3.0 5.0
34-44 2.5 2.0 2.0 2.0 2.5 5.5 5.5 6.0 7.0 15
45-54 5.0 3.5 4.0 4.5 5.0 9.5 12 12 23 40
55-64 9.0 6.5 7.0 5.5 8.5 14 20 22 23 63
65-74 10 10 10 12 15 19 22 34 42 70
A3-5
-------
Appendix 4
GLOSSARY*
ACOUSTIC REFLEX. The involuntary contraction of the muscles
(stapedius and/or tensor tympani) of
the middle ear in response to acoustic
or mechanical stimuli.
ACOUSTIC TRAUMA. Damage to the hearing mechanism caused by
a sudden burst of intense noise, or by
blast. Note; The term usually implies
a single traumatic event.
AIR CONDUCTION (AC). The process by which sound is normally
conducted to the inner ear through the
air in the external auditory meatus and
the structures of the middle ear.
AMBIENT NOISE (RESIDUAL NOISE; BACKGROUND NOISE). Noise of
a measureable intensity that is normally
present in the background in a given
environment.
AUDIBLE RANGE (OF FREQUENCY) (AUDIO-FREQUENCY RANGE). The
frequency range 16 Hz to 20,000 Hz
(20 kHz). Note; This is conventionally
taken to be the normal frequency range
of human hearing.
AUDIOGRAM. A chart, table or graph showing hearing threshold
level as a function of frequency.
AUDIOMETER. An instrument for measuring the threshold or
sensitivity of hearing.
AUDIOMETRY. The measurement of hearing.
AUDITORY TRAUMA. Damage to the hearing mechanism resulting
in some degree of permanent or temporary
hearing loss. Note; Auditory trauma may
be caused by agents other than noise, eg,
head injury; burns; sudden or excessive
changes of atmospheric pressure (cf.
acoustic trauma).
* Some abbreviations commonly used in the literature on noise-
induced hearing loss and hearing conservation are explained
on page A4-10.
A4-1
-------
BASELINE AUDIOGRAM. An audiogram obtained on testing after a
prescribed period of quiet (at least 12
hours).
BONE CONDUCTION (BC). The process by which sound is trans-
mitted to the inner ear through the
bones of the skull (cf. air conduction).
BROAD-BAND NOISE. Noise whose energy is distributed over a
broad range of frequency (generally
speaking, more than one octave).
CENTRAL HEARING LOSS. Hearing loss resulting from injury or
disease involving the central auditory
system in brain or from a psychoneurotic
disorder. Note; Central hearing loss
can occur in the absence of any damage
or deficiency in the peripheral hearing
mechanism.
CONDUCTIVE HEARING LOSS (CONDUCTIVE DEAFNESS). Hearing loss
resulting from a lesion in the air-
conduction mechanism of the ear.
CONTINUOUS NOISE. On-going noise whose intensity remains at
a measurable level (which may vary) with-
out interruption over an indefinite
period or a specified period of time.
DAMAGE RISK CRITERION (DRC). A graphical or other expression
of sound levels above which a designated
or a general population incurs a speci-
fied risk of noise-induced hearing loss.
DEAFNESS. 100 percent impairment of hearing associated with
an otological condition. Note: This is
defined for medicological and cognate
purposes in terms of the hearing threshold
level for speech or the average hearing
threshold level for pure tones of 500,
1000 and 2000 Hz in excess of 92 dB*.
DOSIMETER (NOISE DOSIMETER). An instrument which registers
the cumulative occurrence of or exposure
to noise exceeding a predetermined level
at a chosen point in the environment or
on a person. Note; The noise exposure
may be integrated to yield a "dose"
according to a specified rule, such as
the "equal-energy" rule.
ISO, 1964.
A4-2
-------
EAR DEFENDER (EAR PROTECTOR). A device inserted into the
ear canal or the entrance to it, or
placed over the ear, in order to at-
tenuate air-conducted sounds.
EARMUFF. An ear defender which encloses the entire outer
ear (pinna). Note: Earmuffs are
customarily mounted as a pair on a
headband or in a helmet.
EARPLUG. An ear defender, having specified or standard
acoustic characteristics, which upon
insertion occludes the external audi-
tory meatus. Note; Earplugs should be
properly designed, made of suitable
material, and correctly fitted to in-
sure that they are acoustically
effective and do not harm the ear.
FENCE. (Slang.) An arbitrary hearing level or hearing
threshold level, greater than 0 dB,
below which no hearing impairment is
deemed to have occurred ("low fence")
or at which complete (100%) hearing
impairment is deemed to have occurred
("high fence").
FLUCTUATING NOISE. Continuous noise whose level varies
appreciably (more than ±5 dB) with time.
FREE SOUND FIELD (FREE FIELD). In practice, a sound field in
which the effects of spatial boundaries
or obstacles are negligible.
HANDICAP (HEARING HANDICAP). The occupational and social
difficulty experienced by a person who
has a hearing loss.
HARD OF HEARING. Having more than zero but less than 100 per-
cent impairment of hearing for everyday
speech or for pure tones of 500, 1000 and
2000 Hz. Note: This is defined, according
to various standards, in terms of an ele-
vated hearing threshold level of which the
elevation is less than that defining
deafness.
HEARING CONSERVATION (HEARING CONSERVATION PROGRAM). Those
measures which are taken to reduce the
risk of noise-induced hearing loss.
A4-3
-------
HEARING DISABILITY. Hearing handicap prejudicing employment
at full wages.
HEARING IMPAIRMENT. Hearing loss exceeding a designated cri-
terion (commonly 25 dB, averaged from the
threshold levels at 500, 1000 and 2000 Hz).
HEARING LEVEL. The difference in sound pressure level between
the threshold sound for a person ( or the
median value or the average for a group)
and the reference sound pressure level
defining the ASA standard audiometric
threshold (ASA: 1951). Note: The term is
now commonly used to mean hearing threshold
level (qv). Units: decibels.
HEARING LOSS. Impairment of auditory sensitivity: an elevation
of a hearing threshold level.
HEARING THRESHOLD LEVEL. The amount by which the threshold of
hearing for an ear (or the average for a
group) exceeds the standard audiometric
reference zero (ISO, 1964; ANSI, 1969).
Units: decibels.
HEARING THRESHOLD LEVEL FOR SPEECH. An estimate of the amount
of socially significant hearing loss in
decibels. Note: This is measured by
speech audiometry or estimated by averag-
ing the hearing threshold level for pure
tones of 500, 1000 and 2000 Hz.
IMPULSE NOISE (IMPULSIVE NOISE). Noise of short duration
(typically, less than one second) especially
of high intensity, abrupt onset and rapid
decay, and often rapidly changing spectral
composition. Note; Impulse noise is charac-
teristically associated with such sources
as explosions, impacts, the discharge of
firearms, the passage of super-sonic air-
craft (sonic boom) and many industrial
processes.
INDUSTRIAL DEAFNESS. Syn. Occupational hearing loss.
INFRASONIC. Having a frequency below the audible range for man
(customarily deemed to cut off at 16 Hz).
INTERMITTENT NOISE. Fluctuating noise whose level falls once
or more times to very low or unmeasurable
values during an exposure.
A4-4
-------
INTERRUPTED NOISE. Syn. Intermittent noise (deprecated).
MIXED HEARING LOSS. Hearing loss due to a combination of
conductive and sensorineural deficit.
NARROW-BAND NOISE. A relative term describing the pass-band
of a filter or the spectral distribution
of a noise. Note; The term commonly
implies a bandwidth of 1/3 octave or
less (cf. Broad-band noise).
NOISE. Disturbing, harmful or unwanted sound.
NOISE EXPOSURE. The cumulative acoustic stimulation reaching
the ear or the person over a specified
period of time (eg, a work shift, a day,
a working life, or a lifetime).
NOISE HAZARD (HAZARDOUS NOISE). Acoustic stimulation of the
ear which is likely to produce noise-
induced permanent threshold shift in
some of a population.
NOISE-INDUCED HEARING LOSS (NIHL). A sensorineural hearing
loss caused by acoustic stimulation.
NOISE-INDUCED PERMANENT THRESHOLD SHIFT (NlPTS). Permanent
threshold shift caused by noise exposure,
corrected for the effect of aging (presby-
acusis).
NOISE-INDUCED TEMPORARY THRESHOLD SHIFT (NITTS). Temporary
threshold shift caused by noise exposure.
NOISE LEVEL. Syn. Sound level (weighted sound pressure level).
Note: The weighting should be specified.
NOISE LIMIT (NOISE EMISSION STANDARD). A graphical, tabular
or other numerical expression of the
permissible amount of noise which may be
be produced by a practical source (eg, a
vehicle or an appliance) or which may
invade a specified point in a living or
working environment (eg, in a workplace
or residence) in prescribed conditions of
measurement.
NOISE RATING (NR) NUMBERS (CONTOURS). An empirically estab-
lished set of standard values of octave-
band sound pressure level, expressed as
functions of octave-band center frequency,
A4-5
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intended as general noise limits for
the protection of populations from
hazardous noise, speech interference
and community disturbance. Note;
The NR number is numerically equal to
the sound pressure level in decibels
at the intersection of the so-desig-
nated NR contour with the ordinate at
1000 Hz.
NOISE SUSCEPTIBILITY. A measure of the degree of predispo-
sition to noise-induced hearing loss,
particularly of an individual compared
with the average.
NON-ORGANIC HEARING LOSS (NOHL). That portion of a hearing
loss for which no otological or organic
cause can be found. Hearing loss other
than conductive or sensorineural.
NONSTEADY NOISE.
Noise whose level varies substantially or
significantly with time (eg, aircraft
fly-over noise). (Syn: fluctuating
noise.)
NORMAL THRESHOLD OF HEARING. Syn.
threshold.
Standard audiometric
OCCUPATIONAL HEARING LOSS. A permanent hearing loss sustained
in the course of following an occupation
or employment. Note: While noise is
usually presumed to be the cause, other
causes are possible (eg, head injury).
OTOLOGICALLY NORMAL. Enjoying normal health and freedom from
all clinical manifestations and history of
ear disease or injury; and having a patent
(wax-free) external auditory meatus.
PEAK SOUND PRESSURE. The absolute maximum value (magnitude)
of the instantaneous sound pressure
occurring in a specified period of time.
PERCENT HANDICAP. Syn. Percent impairment of hearing.
PERCENT IMPAIRMENT OF HEARING (OVERALL) (PIHO). The estimated
percentage by which a person's hearing is
impaired, based upon audiometirc determi-
nations of the hearing threshold level at
500, 1000 and 2000 Hz (cf. Percent impair-
ment of hearing for speech).
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PERCENT IMPAIRMENT OF HEARING FOR SPEECH (PIHS). An estimate
of the percentage by which a person's
hearing is impaired, particularly at the
frequencies (500, 1000 and 2000 Hz)
deemed important for the perception of
speech. Note; The scale 0 to 100% is
arbitrarily set to correspond linearly
with a standard range of values of
hearing threshold level for speech in
decibels, customarily 25 to 92 dB re ISO:
1964. The percent impairment of hearing
increases by approximately 1.5% for each
decibel of elevation of the estimated
hearing threshold level for speech
(average of 500, 1000 and 2000 Hz).
PERCEPTIVE HEARING LOSS. Syn. Sensorineural hearing loss.(0bs.)
PERMANENT HEARING LOSS. Hearing loss deemed to be irrecoverable.
PERMANENT THRESHOLD SHIFT (PTS). That component of threshold
shift which shows no progressive reduction
with the passage of time when the putative
cause has been removed.
PERSISTENT THRESHOLD SHIFT. Threshold shift remaining at least
48 hours after exposure of the affected
ear to noise.
PRESBYACUSIS (PRESBYCUSIS). Hearing loss, chiefly involving
the higher audio-metric frequencies above
3000 Hz, ascribed to advancing age.
PSYCHOGENIC OVERLAY. Syn. Non-organic hearing loss. (Deprecated.)
RISK. That percentage of a population whose hearing level, as
a result of a given influence, exceeds the
specified value, minus that percentage whose
hearing level would have exceeded the speci-
fied value in the absence of that influence,
other factors remaining the same. Note;
The influence may be noise, age, disease,
or a combination of factors.
SEMI-INSERT EAR DEFENDER. An ear defender which, supported by
a headband, occludes the external auditory
meatus at the entrance to the ear canal.
SENSORINEURAL HEARING LOSS. Hearing loss resulting from a
lesion of the cochlear end-organ (organ
of Corti) or its nerve supply.
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SOCIACUSIS. Elevation of hearing threshold level resulting
from or ascribed to non-occupational
noise exposure associated with the
general social environment and exclusive
of elevation associated with aging.
SOUND PRESSURE LEVEL (SPL). 20 times the logarithm to the
base 10 of the ratio of the sound pressure
in question to the standard reference
pressure of 0.00002 N/m2. Units: decibels
(dB).
SPEECH AUDIOMETRY. A technique in which speech signals are
used to test a person's aural capacity
to perceive speech in prescribed con-
ditions of testing.
SPEECH DISCRIMINATION. The ability to distinguish and under-
stand speech signals.
STANDARD AUDIOMETRIC THRESHOLD. A standardized set of values
of sound pressure level as a function of
frequency serving as the reference zero
for determinations of hearing threshold
level by pure-tone audiometry.
STAPEDIUS REFLEX (STAPEDIAL REFLEX). (Likewise, tensor
tympani reflex.) The reflex response of
the stapedius (likewise, tensor tympani)
muscle to acoustic or mechanical stimula-
tion. Commonly, synonymous with acoustic
reflex.
STEADY NOISE (STEADY-STATE NOISE). Noise whose level varies
negligibly within a given period of time.
TEMPORARY THRESHOLD SHIFT (TTS). That component of threshold
shift which shows a progressive reduction
with the passage of time after the apparent
cause has been removed.
THRESHOLD OF HEARING (AUDIBILITY). The minimum effective sound
pressure level of an acoustic signal
capable of exciting the sensation of hearing
in a specified proporation of trials in
prescribed conditions of listening.
THRESHOLD OF FEELING (TICKLE). The minimum effective sound
pressure level of an auditory signal
capable of exciting a sensation of feeling
or tickle in the ear which is distinct
from the sensation of hearing.
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THRESHOLD OF PAIN (AURAL PAIN). The minimum sound pressure
level of an auditory signal which is
capable of eliciting a sensation of
pain in the ear as distinct from sensa-
tions of feeling, tickle or discomfort.
THRESHOLD SHIFT. An elevation of the threshold of hearing
of an ear at a specified frequency or
average value of frequency. Units:
decibels.
TINNITUS. Ringing in the ear or noise sensed in the head.
Onset may be due to noise exposure
and persist after a causative noise
has ceased, or occur in the absence
of acoustical stimulation (in which
case it may indicate a lesion of the
auditory system).
ULTRASONIC. Having a frequency above the audible range for
man (conventionally deemed to cut off
at 20,000 Hz).
WEIGHTING (FREQUENCY WEIGHTING). The selective modification
of the values of a complex signal or
function for purposes of analysis or
evaluation, in accordance with prescribed
or standardized rules or formulae. Note;
This may be done by computation or by
the use of specified weighting networks
inserted into electronic instrumentation
so as to transform input signals.
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ABBREVIATIONS
AAOO American Academy of Ophthalmology and Otolaryngology
AC Air Conduction
APR Air Force Regulation
AHL Average Hearing Level
AI Articulation Index
AMA American Medical Association
ANSI American National Standards Institute (formerly USASI)
BC Bone Conduction
CHABA Committee on Hearing and Bio-Acoustics
dBA A-weighted decibel (decibels). Also written dB(A).
DRC Damage Risk Criterion
EA Noise immission level
EPA Environmental Protection Agency
HL Hearing Level
HTL Hearing Threshold Level
IEC International Electrotechnical Commission
ISO International Organization for Standardization
LA A-weighted sound level
LA50 A-weighted sound level exceeded 50% of the time
Leq Equivalent continuous sound level
LNP Noise Pollution Level
NIHL Noise-Induced Hearing Loss
NIOSH National Institute for Occupational Safety and Health
NIPTS Noise-Induced Permanent Threshold Shift
NITTS Noise-Induced Temporary Threshold Shift
NPL Noise Pollution Level (also National Physical Laboratory
in England,
NR Noise Rating
OSHA Occupational Safety and Health Act
PB Phonetically Balanced
PTS Permanent Threshold Shift
RMS Root Mean Square
SIL Speech Interference Level
SIR Speech Impairment Risk
SPL Sound Pressure Level
SRT Speech Reception Threshold
TS Threshold Shift
TS4oo TS at asymptote at 4 kHz
TTS Temporary Threshold Shift
TTSo TTS determined 2 minutes after cessation of exposure
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Appendix 5
PHYSICAL MEASUREMENT AND DESCRIPTION OF
NOISE HAZARDOUS TO THE EAR
A multiplicity of acoustical units and measurement
techniques are at present in use in the United States and
around the world. The choice of unit and method of measure-
ment is usually determined mainly by the criterion of
interest (eg, hearing hazard; annoyance) but, even within
the specific province of hearing hazard, several methods of
rating noise are available; and more than one procedure has
achieved national or international standardization.
A5.1 Units and procedures for on-going (continuous) noise
A5.1.1 Principles of measurement. On-going noise in most
situations is nearly always measured with a sound level meter,
an instrument which responds to the pressure oscillations
(sound waves) in the air at the point of measurement in the
sound field. Many kinds of sound level meter exist: measure-
ments should be made using instruments which conform to
current national or international standards prescribing the
characteristics, method of calibration, precision, etc. of
such instruments (see bibliography below). An important
feature of the sound level meter is that it contains elec-
tronic networks which perform a time-averaging integration
of the instantaneous pressure signal, using a time-constant
selected to resemble the integrative function of the human
ear and to permit reading of a reasonably stable averaged
sound level on the meter. Two options, "fast" and "slow"
response, are customarily provided in commercial instruments.
The "slow" response is normally used when evaluating noise
hazardous to the hearing. It is important to appreciate
that the time-averaging feature of the sound level meter
renders it unsuitable for measuring impulsive noises or
bursts of noise lasting less than half a second (see Section
IIIB, and A5.2 below).
Various weighting networks (also now standardized) may
be switched into the measuring circuit in order to discrimi-
nate against certain frequencies in a manner resembling the
human hearing function. When no such network is interposed,
the meter may be used to measure the Sound Pressure Level (SPL)
in decibels (dB), either overall or in octave bands (band
pressure level). When a frequency-weighting network (filter)
is inserted, the meter yields a measurement of weighted Sound
Level. At least 4 networks have been devised, of which 3
A5-1
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("A", "B" and "C") have been standardized internationally
and are customerily provided in commercial equipment. A
fourth, "D", with variations (not yet standardized) is
sometimes included and an "E-weighting" may be introduced
for certain applications in the future. The present con-
sensus is that the most appropriate weighting to use in
the evaluation of noise hazardous to the hearing is "A-
weighting". The measurement of sound level is then
expressed in A-weighted decibels (dBA), which may be
related to sound pressure level in decibels (dB) by the
use of tables or computational procedures (see Appendix 8).
For some purposes (Botsford, 1970), the C-weighting may
also be used to evaluate hearing hazard: the noise is then
measured in dBC.
It is important to remember that the decibel is not
an absolute unit: it is a unit of level or ratio and its
use implies a reference level (zero decibels) to which the
actual measurement relates. In acoustical sound pressure
level measurements, the international standard reference
zero is 0.00002 newtons per metre squared (N/m2). It is
independent of frequency. In audiology, the same unit,
the decibel, is also used to specify the amount of hearing
change or loss with reference to other agreed reference
levels, such as a standard reference zero for audiometry,
which is a function of frequency; or with reference to an
arbitrary otological "fence".
A5.1.2 A-weighted sound level (Unit: dBA) . This is gener-
ally the most convenient and useful measurement to make in
field surveys or on-the-spot investigations of noise.
With certain exceptions, A-weighted sound level is a fairly
reliable predictor of noise hazard in most practical situa-
tions. It can incidentally provide a direct, if approximate,
estimate of loudness level in phons (the original purpose of
the weighting) in the case of tonal or narrow-band random
noise, provided that the noise is not too widely distributed
in frequency or accompanied by significant levels of noise
in other regions of the spectrum. It is useful for making
comparative evaluations of the noise from different sources,
or from the same source before and after noise control
measures have been taken, provided that the noises compared
are broadly similar in spectral composition and that the
source or sources are essentially omnidirectional. The
measurement is made using a sound level meter switched to
the A-weighting network. The conditions of measurement
(including distance from the source) should be reported
along with the level readings.
A5-2
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The A-weighted sound level in dBA* loses power as
a predictor of hazard to hearing, subjective response,
or other frequency-dependent human reactions to noise ,
particularly when comparisons are attempted between
narrow-band or discrete tonal noise on the one hand
and broad-band noise on the other; or when evaluating
noise in bounded spaces whose acoustical characteristics
are strongly frequency-dependent. For such evaluations,
octave band or narrower-band SPL measurements are appro-
priate, from which other predictive indices of the human
response may be calculated as required. Many authorities
now use dBA values in planning for community noise control.
A-weighted sound level is recommended for general use as a
hearing damage risk predictor in the appraisement of noisy
environments. Caution sould be exercised if the noise
contains strong tonal components, however, for dBA measure-
ments may underestimate the hazard.
A5 . 1 . 3 Equivalent continuous sound level, Lpg (Unit: dBA).
This is a notional daily average level of noise (normalized
to a continuous 8-hour exposure) calculated from partial
(less than 24-hour) exposures. It is used to predict
hearing hazard in the event of the daily noise being inter-
mittent, of varying duration, or fluctuating level. The
quantity, Leg, is yielded by the formula:
Leq = i + 90' where
antilog [o.l(L - 90)] , in which
t is the exposure time in hours and L is the sound level
of the exposure in dBA. A procedure for calculating L
is given in Appendix 8.
A5 . 1 . 4 Noise immission level, EA (Unit; numerical, related
to dBA) . This A-weighted index of industrial noise hazard,
devised by Robinson (1971) is given by:
E = L + 10 log (T/T) ,
Some authorities maintain that the unit of measurement
should properly be designated dB(A), which is the usage
in some international standards (ISO, 1971), but dBA
is a commonly used convenience, adopted here.
A5-3
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where:
EA = A-weighted noise immission level;
LA = A-weighted sound level (daily average) in dBA;
T = the duration of working exposure in years; and
TO = the reference duration of 1 year.
Further details of the use of this index are given in
Appendix 7.
A5.1.5 Noise pollution level, NPL (Unit; dBA, dBD or PNdB).
This concept, recently introduced by Robinson (1969), is an
attempt to formulate a unifying criterion embracing all
manner of environmental noise, including ambient noise in
urban communities and intruding noise from traffic, industry
and aircraft operations. The NPL is obtained from weighted
sound level or perceived noise level (PNL) values. It is
defined by the formula:
Ljjp = Leg + 2.56CT,
where Leq is "energy-mean" or equivalent continuous noise
level and tf is the standard deviation of the instantaneous
levels recorded over a specified period of time. It remains
to be seen whether this concept, which is a promising
approach to the unification of noise evaluation procedures
according to the criterion of preserving the peace of
communities, will achieve wide acceptance.
A5.1.6 Noise rating (NR) contours (Unit; none. NR contours
are based numerically on SPL in dB at 1000 Hz). In 1962
Kosten and van Os refined a series of idealized noise
spectra which, with minor modifications, have been accepted
by the ISO for consideration as an international standard.
These curves are a weighted frequency function which
attempts to specify the general decline in the human toler-
ance of noise with rising frequency at any given SPL. The
NR contours are essentially similar to the Noise Criterion
(NC) contours of Beranek (1960) but include a modification
of the frequency scale which takes account of the ISO pre-
ferred frequencies for acoustical measurement (1962). The
change to ISO preferred frequencies introduced small
changes in level.
NR contours are intended as general guidelines according
to a wide range of criteria, including hearing conservation,
speech intelligibility, and the protection of communities
from noise nuisance. There is some doubt as to whether such
A5-4
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a simple figure of merit as an NR number can have such
universal applicability. Nevertheless, NR estimations
appear to provide useful guidance to required acoustical
treatments in offices and other rooms in buildings ex-
posed to interior or intrusive noise. Kosten and van Os
(1962) provided numerous examples of acceptable NR for a
variety of situations (eg, conference rooms, general and
private offices, workshops, hospitals, domestic accommoda-
tion and so on), with correction factors (ranging from
+25 to -5 dB) according to the nature of the noise - for
example, whether it is steady or intermittent; tonal or
distributed - and such factors as the location of the
area in question (eg, in a suburban residential or in-
dustrial area). Passchier-Vermeer (1968) used NR values
calculated for the speech frequency range to specify
averaged occupational noise levels hazardous to the hear-
ing. An approximation acceptable over a fairly wide
range of measurement in practice (and used in the present
report) is given by the relationship:
(NR) = (dBA) - 5 (eg, 80 dBA is equivalent to NR 75).
The NR contours are probably most useful for the
evaluation of interior noises in buildings, where interfer-
ence with verbal communication forms an important part of
the basis of judgment. They are of less certain value, and
have not won universal acceptance, where hearing conserva-
tion is the criterion.
A5.1.7 Sound pressure level, SPL (Unit: dB). For the
overall appraisement of a noisy environment it is often
sufficient to measure simply the sound pressure level (SPL)
of the noise contained within the instrumental frequency
range. This can be obtained as a single reading on a
sound level meter using the "linear" setting. When the
instrument is properly calibrated and used correctly, the
reading may be accepted as the instantaneous value of the
SPL at the particular point of measurement. It is expressed
in decibels above the standard reference pressure (0.00002
N/m2).
Sound level meter circuits, as commonly designed, in-
corporate relatively long time constants (of the order
0.1 to 0.25 sec) which are intended to simulate the
integration time of human audition. These values permit
reasonably stable readings from the meter when, as is
often the case, the instantaneous sound pressure fluctuates.
The reading obtained is thus an estimate (time-average) of
the hypothetical "true" SPL at the instant of measurement.
For this and other reasons, conventional sound level meters
intended for general noise measurements, even when adjusted
A5-5
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for a "fast" response, are not suitable for the measurement
of transient or impulsive noises of rapidly changing level
and frequency content. Filter sets may be used to measure
the octave or third-octave SPL (band pressure level) in
decibels when it is desired to evaluate the spectrum of the
noise (see below).
Many commercially manufactured sound level meters are
of the linear rectifier type (these yield a mean rectified
value of the sound pressure) or have hybrid rectifying
characteristics. The reading given is a sufficient approxi-
mation to the true root mean square (rms) value of sound
pressure to render the instrument adequate for most noise
measurements in the field. For more fundamental determina-
tions it may be appropriate only to use true rms meters but
these are apt to be more complicated and expensive. The
type of meter used, its settings, the conditions in which
readings were taken and other relevant information should
be specified when noise measurements are reported.
As mentioned above, sound level meters customarily
incorporate at least three frequency-weighting networks
which can be switched in selectively when it is desired
to appraise hearing hazard, estimate loudness, or to make
an assessment of the frequency composition of the noise
in question. These weightings are intended to discrimi-
nate against very high and low frequency noise picked up
by the microphone, in a manner simulating the frequency
selectiveness of the human ear. As originally conceived,
the three principal weightings, designated A, B and C,
were intended to give the meter a frequency-response
corresponding with the 40-phon equal loudenss contour for
low intensities of noise (A scale), the 70-phon contour
for moderate intensities (B scale) and an almost flat
response up to about 8000 Hz (but with a slight attenua-
tion at the extremes of the audio-frequency range) for
high-intensity noise (C scale). In recent years these
scales have received national and international standard-
ization, as have the specifications for sound level meters
(IEC, 1965, 1966).
When readings are taken from a sound level meter
using one of the weighting networks, they are properly
called not SPL but sound level readings and are expressed
in decibel units with a suffix designating the scale used.
Thus, a reading of 60 decibels taken with the instrument
set to the A scale is reported as 60 dBA. As rough rules
of thumb it may be useful to note that, if, for a given
noise broadly distributed in frequency, the readings taken
wilh all three scales in turn differ by less than 3 dB
the noise is mainly composed of relatively high frequencies
(above 500 Hz); but if the readings taken using the C
A5-6
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weighting exceed those using the A and B scales by more
than some 6 dB the noise contains mainly low-frequency
components (below 500 Hz).
AS.1.8 Subjective noise meters. Some instruments, called
subjective noise meters (Anderson, 1960), have been
developed which, using various frequency-weighting or
limiting, networks, purport to measure directly the sub-
jective noisiness, loudness or speech interference level
of noise. Caution should be exercised when using such
instruments or interpreting readings obtained with them;
for the network characteristics may not have been stan-
dardized or even have been generally accepted by acousti-
cians. Subjective noise meters are inappropriate to the
evaluation of hearing hazard.
A5.1.9 Noise exposure meters (noise cumulators and dosi-
meters) . Integratxng noise meters (dosimeters) can be
constructed which total acoustic irradiation of a measuring
point taking place in a selected period of time. Some
instruments can be adjusted so as to integrate only the
exposure above a selected level (Cox, 1959; Benson, 1964;
Strong & Neely, 1968) . The measuring may be done at a
fixed point such as a workplace; or the meter can be
carried on the person so as to total the individual's noise
exposure.
A5.1.10 Surveying noise. It is important to appreciate
that an isolated reading taken with a sound level meter,
yielding a single weighted level or an octave band spectrum,
is very rarely adequate to define a noisy environment; for
the sounds encountered in most instances change substantially
from time to time and from place to place. A familiar example
is the noise from aircraft flying over a city, disturbing
people to an extent which depends not only upon the maximum
sound levels reached at given points on the ground during
fly-overs but also upon the time, frequency and regularity
of operations and upon many other factors affecting the
propagation of noise from moving sources.
A5.1.11 Broad-band frequency analysis. A coarse frequency
analysis is adequate for many purposes, for instance, the
appraisal of the sound fields of aircraft or vehicles in
order to define what is required by way of noise control.
When the noise is broadly distributed in frequency, and when
the spectrum may be presumed to be continuous, as in the
cases of jet aircraft noise and much industrial noise,
octave-band analysis is precise enough for many purposes.
Using this technique, band-pass filters covering contiguous
frequency bands, nominally one octave wide and based upon a
logarithmic series of center-frequencies, are inserted
A5-7
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sequentially into the measuring equipment. An octave
spans frequencies having a ratio of 2:1. For example,
the octave centered on 1000 Hz extends nominally from
707 to 1414 Hz. The energy in each filter pass-band is
recorded and may be plotted graphically as a spectrum of
band pressure level against frequency. After many years
of debate amongst acousticians, a preferred series of
octave-band center-frequencies based on the series 1000,
2000, ... Hz has now been generally accepted and has
received standardization (ISO, 1962). However, some
acoustical instruments and test codes are still in wide-
spread service using non-standard filter sets such as
those covering the octave bands 37.5-75 Hz, 75-150 Hz
and so on, which have different center-frequencies. In
audiometry (see Appendix 6), audiograms are customarily
plotted on a preferred frequency scale running in octaves
from 125 to 8000 Hz, with the interpolation of the audio-
metrically significant frequencies 3000 and 6000 Hz.
When the noise level varies substantially with fre-
quency, perhaps having steep slopes and sharp peaks in its
spectrum, an approach to narrow-band analysis may be
necessary. A practical compromise between imprecision and
complexity of analysis is achieved by the use of one-third
(1/3) octave band analysis (some equipment, not now in
common use, divides the spectrum into half-octaves). Con-
veniently, ten 1/3-octave bands span a decade of frequency^-
(eg, 1000 Hz to 10 kHz). Moreover, the frequency ratio 10
is sufficiently close an approximation to the ratio 2^/3
(actually the ratios are 1.2589 and 1.2599 respectively) for
the spacing of the standardized bands to be based upon the
former value. A typical 1/3-octave band filter set, avail-
able commercially, covers the acoustical range 25 Hz to
22 kHz in 30 steps, centered at the ISO preferred frequencies
for acoustical measurements already cited.
The type of frequency analysis used must be specified
when sound spectra are interpreted and compared. For identi-
cal noises yield spectra which differ according to the degree
of resolution of the analysis. The shape and overall level
of the spectrum depend upon the attenuating characteristics
(ie, the sharpness and linearity of cut-off outside the pass-
band) of the filter or filters used in the analyzer and, to
a lesser extent, upon the type of microphone and amplifiers
used to pick up and process the noise signal. Ideally, an
analytical filter should let through all the energy falling
within its nominal pass-band and discriminate totally against
energy at frequencies outside it. Unfortunately, like most
ideals, such a filter cannot exist. Electronic filters
possess finite powers of discrimination against frequencies
outside the pass-band and, theoretically, they pass some
A5-8
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energy at all frequencies. An insertion loss of up to 4 dB
at the center frequency occurs when octave-band filters are
used, depending upon the type of filter. Moreover, the
insertion loss in the pass-band is greater for narrower-
band filters than for broad-band filters of similar charac-
teristics (for example, the loss can be up to 6 dB in the
case of a 1/3-octave band filter of conventional design),
which is one of the reasons why the overall level of the
spectrum of a given distributed noise appears higher on
broad-band than upon narrow-band analysis. Insertion loss
is corrected for by calibration or computation. National
and international standards (IEC, 1966) have been laid down
defining the characteristics and precision of band-pass
filters for use in acoustical work.
A5.1.12 Spectrum level. This is sometimes defined as the
band pressure level of noise that would be measured using
an ideal analyzer with flat frequency-response and a band-
width of only 1 Hz. Readings taken from analyzers having
differing characteristics and using different analytical
bandwidths can be compared upon conversion to spectrum
levels. Computational data for carrying out this conver-
sion may be found in acoustical handbooks or in the infor-
mation provided by the manufacturers of sound analyzing
instrumentation. The band pressure level of distributed
noise recorded in bands of different widths differs by a
factor related to the logarithm of the ratio of the band-
widths and it decreases with decreasing bandwidth.
A5.2 Impulsive (transient) noise
A5.2.1 Varieties of impulsive noise. These were defined
(according to Coles and Rice, 1971) in Section III. The
two main varieties (see Figure A5-1) are Type A (Friedlander
wave) and Type B (oscillatory decay). Neither of these types
can be measured adequately with a conventional time-averaging
sound level meter: use of such an instrument may grossly
underestimate the hazard to hearing associated with the high
peak level of an impulsive noise. The error rises with de-
creasing duration of the peak (eg, in the noise of gunshots
or metallic impacts in industry). Coles and Rice have,
however, distinguished a third variety of noise, Type C,
which although impulsive in origin, may be treated as essen-
tially continuous in nature, and hence amenable to measurement
using a sound level meter. Further details are given below.
A5.2.2 Type A noises. The important features to be measured
are the peak level (AB in Figure A5-1) and the duration of the
positive phase of the wave, AC; and of course the number of
impulses and their frequency of occurrence. The waveform can
A5-9
-------
only be measured with precision by oscillographic analysis
of the discrete pressure signal at grazing incidence upon
the microphone (Coles & Rice, 1971). The peak sound
pressure level may be expressed in decibels (dB) with
respect to the standard reference zero for SPL measure-
ments. In certain application (eg, sonic boom work), it
is customarily expressed dimensionally as an absolute
pressure measurement.
A5.2.3 Type B noises. Here again the notional peak value
(BD in Figure A5-1) and the duration of the wave envelope
(AD) must be measured oscillographically when precise
determinations are called for. It may also be appropriate
to extend the oscillographic analysis to yield spectral
information (ie, to identify the oscillatory composition
of the impulse at the point of measurement).
A5.2.4 Type C noises. It is debatable at what point a
series of closely-spaced Type A or B noises becomes , for
practical purposes, a Type C noise, and so amenable to
treatment as continuous noise (ie, quantifiable in dBA).
Coles and Rice (1971) have suggested that merged impulses
having a repetition rate exceeding 10 per second may be
so treated and their averaged level specified in dBA
provided that the repeated decay from peak to minimum
level in the wave-envelope (see Figure A5-1) does not
exceed 6 dB.
A5.3 Bibliography on noise measurement*
A5.3.1 General references.
Coles, R R A & Rice, C G. Assessment of risk of hearing loss
due to impulse noise. In: Occupational hearing loss,
ed. Robinson, D W, London and New York: Academic press.
1971. 71-77.
Environmental Protection Agency- Fundamentals of noise:
measurement, rating schemes, and standards. US Environ-
mental Protection Agency, Office of Noise Abatement,
Washington, DC: NTID.300 15. Washington: US Government
Printing Office. 1971.
Guignard, J C & King, P F. Aeromedical aspects of vibration
and noise. NATO/AGARD, Neuilly-sur-Seine, France:
AGARDograph AGARD-AG-151, 1972.
Peterson, A P G & Gross, E E. Handbook of noise measurement.
Concord: General Radio Company. 7th edn. 1972.
* Other references may be found in the main bibliography.
A5-10
-------
Robinson, D W. The concept of noise pollution level.
National Physical Laboratory, Teddington, England:
Aero Reports Ac38 and Ac39. 1969.
Robinson, D W. Estimating the risk of hearing loss due to
exposure to continuous noise. In: Occupational
hearing loss, ed. Robinson, D W. London and New York:
Academic Press, 1971. 43-62.
A5.3.2 Relevant standards.
International Electrotechnical Commission. Precision sound
level meters. IEC Publication 179. Geneva: IEC 1965,
International Electrotechnical Commission. Octave, half-
octave and third-octave band filters intended for
the analysis of sound and vibration.IEC Publication
225. Geneva: IEC. 196^6.
International Organization for Standardization. Expression
of the physical and subjective magnitudes of sound
or noise~.ISO Recommendation ISO/R131. Geneva: ISO.
1959.
International Organization for Standardization. Preferred
frequencies for acoustical measurements. ISO/R266.
Geneva: ISO. 1962.
International Organization for Standardization. Power and
intensity levels of sound or noise. ISO/R357.
Geneva: ISO. 1963.
International Organization for Standardization. Standard
reference zero for the calibration of pure-tone
audiometers.ISO/R389.Geneva: ISO. 1964(with
addendum 1 to ISO/R389-1964: Additional data in
conjunction with the 9A coupler. 1970).
A5-11
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(A)
B
(B)
(O
\MM/W'
Figure A5-1.
Types of impulse noise (diagrammatic).
(A) simple diphasic wave. AB = peak
pressure; AC = effective duration.
(B) Oscillatory wave. AB = peak
pressure; BD = effective duration.
(C) Wave envelope of quasi-steady-
state series of closely-spaced
impulses. (After Coles & Rice, 1971.)
A5-12
-------
Appendix 6
AUDIOMETRY
A6.1 Pure-tone audiometry
The hearing may be tested clinically in several ways,
ranging from simple tests of the subject's ability to
detect or distinguish tones, spoken words or other sounds,
to pure-tone and speech audiometry. Pure-tone audiometry
is most widely used, with recourse to speech audiometry
for certain diagnostic purposes. In pure-tone air conduction
audiometry, the subject is presented with a succession of
sounds at discrete frequencies (pure tones). The sounds
generated by the audiometer are heard through earphones, by
means of which the two ears are customarily tested separately
in a regular sequence. The level of each tone is varied
progressively, the purpose being to find the lowest level at
which the subject can just detect the tone in prescribed con-
ditions of listening. The subject's hearing threshold level
is defined with respect to a standard reference level.
The selection of tones and the variation of level may
be accomplished manually or, in some techniques, by a semi-
automatic program (Bekesy audiometry).. The subject's
threshold response for each ear is plotted on a chart as a
function of frequency (the audiogram). As to the frequencies
tested, most commercially available audiometers cover the
range 250 to 8000 Hz in octave or, in some instruments (more
rarely used) half-octave steps. The interpolated frequencies
3000 and 6000 Hz are often included in the test series.' The
range 250 to 8000 Hz (some would say 500 to 6000) is gener-
ally deemed to be sufficient for purposes of screening
audiometry such as in industrial hearing conservation programs.
For many purposes in clinical audiology, concerned particularly
with the assessment of disability due to occupational noise-
induced hearing loss, the hearing changes at 500, 1000 and
2000 are used as indices of impairment or handicap. For
particular applications in research or diagnostic audiometry
a wider audiometric range of frequency may be investigated
including 125 Hz at the lower end of the scale and frequencies
up to 12 kHz or even higher at the upper end. Testing above
8 kHz is more difficult to perform accurately and usually
requires special testing facilities. It is a sine qua non of
audiometry that the instrument be properly calibrated and used
in accordance with prevailing standards. National and inter-
national standards have been laid sown, defining the charac-
teristics and the condition and method of operation of
audiometexs, as well as the reference zero to be used in
pure-tone audiometry (ISO, 1964).
A6-1
-------
A6.2 Speech audiometry
Spondaic words (in standard lists) are used instead of
pure tones to test the subject's speech reception threshold
(SRT). At suprathreshold levels, speech audiometry using
spondees may also be used to determine the optimum loudness
for speech reception. Phonetically balanced (PB) word lists
are also used in speech audiometry, mainly to determine the
subject's ability to discriminate speech at suprathreshold
levels. These functions are of particular value in appraising
the efficiency of hearing data. An important purpose of
speech audiometry is to test the patient's ability to discri-
minate speech in noise.
A6.3 Pulsed-tone Bekesy audiometry
Comparing the use of continuous with pulsed tones in
Bekesy audiometry, McCommons and Hodge (1969) maintain the
latter to be preferable, having shown the pulsed tone tech-
nique to be superior in both the accuracy and repeatability
of threshold determinations. For optimum sensitivity and
freedom from intratest variability, these authors have
recommended the use of tones having the following charac-
teristics: a period of 500 ms; a duty cycle in the range
50 to 60%; and an attenuation rate in the range 4 to 5 dB/s.
The advantage of pulsed over continuous tone audiometry has
been ascribed by McCommons and Hodge to the greater amount
of perceptual "feedback" making the subject's task of
decision-making easier in the pulsed tone technique.
Jokinen (1969) has pointed out that presbyacusis data
may be influenced, inter alia, by the method used to test
hearing in the elderly: in old age, pulsed tone tracing
indicated better hearing than the steady tone threshold.
Manual audiometry gave poorer results than automatic testing
in inexperienced subjects. Jokinen has claimed that ad-
vancing age does not decrease the performance of Bekesy
audiometry and that the best thresholds are obtained using
an automatic pulsed technique.
A6.4 Sources of variance in audiometry
A6.4.1 Audiometry; calibration variance. An important source
of confusion in comparisons between data on hearing levels in
different populations has hitherto been the lack of universal
standardization of audiometric reference thresholds. For
example, differences between different national standard
reference zeros for audiometers (eg, ASA Z24.5: 1951 in the
USA and BS 2497: 1954 in the United Kingdom) can average more
than 10 dB and have at certain frequencies exceeded 20 dB.
A6-2
-------
For this reason, it is imperative to be clear as to which
reference zero the data in question refer to. In 1964 the
International Organization for Standardization issued a
Recommendation (ISO/R389) attempting to unify the conflict-
ing threshold values into an international standard refer-
ence equivalent threshold sound pressure level, specified
for various reference types of earphones, couplers and
artificial ears. A detailed account of the complex process
of reconciliation attempted by the ISO has been published
by Weissler (1968). It would appear that inter-instrumental
variance of several decibels is implicit in the Recommendation,
Individual physical factors can introduce an element of
uncertainty into the'actual level of test sounds delivered
to the eardrum. For example, outer-ear configuration and
canal size vary with age, sex and race. This variation may
therefore be a factor underlying differences observed
between groups in audiometric surveys. Such differences can
affect the interpretation of standardized reference zeros
(Erber, 1968). Delany (1970) has commented critically upon
the use of circumaural earphones in clinical audiometry
because of their inherent difficulty of calibration. With
regard to instrumental variance in general, however, Jackson
et al (1962), describing audiometric practice at an industri-
al plant, have maintained that this problem can be kept to an
acceptable minimum when sufficient care is taken in calibra-
tion and audiometric technique.
A6.4.2 Audiometry; subject variance. It would seem to be
axiomatic that, irrespective of any changes due to age,
noise or otological disorder, there will be some long-term
instability of an individual's audiometric threshold. Sub-
stantial replication variances (eg, 20dB2 at 3000 Hz) were
reported in non-noise-exposed industrial control subjects
by Burns and Robinson (1970) but, in properly controlled
conditions of testing, this source of variation can be kept
small, about half of it being attributable to an inherent
long-term fluctuation (Delany, 1970).
Moreover, replicate testing is apt to show changes
(usually "improvements" in hearing sensitivity) associated
with familiarization or re-familiarization with the audio-
metric procedure, even in non-naive subjects. Practice or
experience may account for a change in threshold of the
order of 1 to 3 dB. A similar order of difference can be
seen in measurements taken using pulse-tone self-recording
as against manual techniques (Delany, e_t aJL, 1966, 1970).
Learning effects can add to audiometric variance (Jackson
et al, 1962) ; moreover the practice effect can be enhanced
TTeading to lowered measured thresholds) by reward and
feedback (Zwislocki et al, 1958). The latter factor can
account for the increased sensitivity seen when using pulsed-
tone techniques.
A6-3
-------
A6.4.3 Audiometry; variance due to personality. Measure-
ments may £>e subject to minor variance arising from the
personality of the audiometrician (largely eliminated in
Bekesy audiometry) and from observer differences in the
reading of audiograms (Stephens, 1970). Observer influences
upon the subject may even affect the response in average
evoked response audiometry (an ostensibly objective tech-
nique) , in which the response can alter with the level of
arousal of the subject (Stephens, loc cit).
Although it is of uncertain practical significance,
subject personality can also be a variable: threshold
detection variance is dependent to some extent on the
subject's degree of extroversion/introversion; moreover,
differences in the pattern of excursion in Bekesy audio-
metry have been said to relate to various scores on per-
sonality inventories (Stephens, 1970). Hinchcliffe (1965,
cited by Stephens: unpublished thesis to London University,
"A psychological investigation into vertigo") has found
patients with certain otological conditions to be more
neurotic than normal. This could be a minor factor in-
fluencing the range of variability of their audiograms.
Regarding the audiological assessment of individuals
in whom NIHL is suspected, and particularly those for whom
the question of compensation arises, Hinchcliffe (1970)
has reminded us that a non-organic component in the loss
must always be excluded. He has remarked that "where
compensation is involved, it is not a question of whether
or not there is a functional component, but to what extent
there is a functional component". He has also pointed out
that, even when such a component has been excluded, and
due allowance made for presbyacusis, one is still not
justified in attributing any remaining fraction of hearing
loss entirely to noise: the possibility of loss due to a
variety of clinical conditions, irrelevant noise exposure
(in occupationally-related assessments), or to noise-equiva-
lent trauma (eg, head injury) must be borne in mind. Such
conditions may of course operate to impair hearing even in
the interval between pre-and post-noise exposure audiograms,
such as may be recorded in an occupational setting. Attri-
bution of hearing loss to noise exposure in the individual
is thus a difficult matter of balancing probabilities.
A6.5 Audiometric zero - a critique and comparison
Riley et al (19S5) have published a cogent critique of
both the concept and practical specification of audiometric
reference zero, drawing attention to the substantial dis-
crepencies standing between various standards and reference
data in current use (Table A6-I).
A6-4
-------
Table A6-I. Comparison of ASA (1951) and ISO (1964) audio-
metric zeros with hearing threshold level of
selected young (16 - 25 year old) industrial
employees. In dB SPL re 0.00002 N/m2 (data
from Riley et al, 1965). Note employees'
hearing "better" than ASA reference zero in
the Eastman-Kodak series.
Data Parameter 250 500 1000 2000 3000 4000 6000
ASA: 1951 Mode 39.5 25.0 16.5 17.0 16.0 15.0 17.5
Selected males Mean 35.3 21.5 12.6 15.1 14.2 19.9 22.5
(Eastman-Kodak) Median 31.0 18.0 9.4 11.2 9.4 15.0 17.8
Selected females Mean 34.5 20.2 11.3 13.8 10.7 14.7 16.4
(Eastman-Kodak) Median 29.3 16.6 7.8 10.2 6.5 10.7 12.3
ISO 1964
recommended Median 24.5 11.0 6.8 8.5 7.5 9.0 8.0
A6.6 Audiometric pattern of noise-induced hearing loss
Both AC and BC audiometry are necessary to distinguish
a mixed hearing loss. In typical cases, the loss begins at
4000 or 6000 Hz, characteristic "notch" deepening and widen-
ing with continued noise exposure so as to involve higher
and lower frequencies. In contrast to the typical patterns
for conductive and sensorineural hearing loss from other
causes, the audio gram of the noise damaged ear is recogniz-
able from a progressive depression of hearing from 1000 to
6000 'Hz, accompanied in some cases by a relative upswing of
the curve at 8000 Hz and higher frequencies. As noted else-
where in this document a similar pattern is seen in presbya-
cusis; and ^distinguishing diagnostic evidence, gleaned from
the patient's o.tological history, may be at best circumstantial.
Sometimes an audiogram is found to show an absolute or
relative depression of the hearing at low frequencies. In
rare cases, small amounts of threshold shift at frequencies
below 1000 Hz nay be attributable to noise damage, where the
causative noise (or bone-conducted vibration) has itself been
of very low frequency (see Appendix 10 on infrasound).
But commonly, the accuracy of such an audiogram is suspect,
because of the possibilities of a distorting artifact affecting
the measurement. Some causes of a false audiometric pattern
A6-5
-------
include excessive ambient noise in the test room; loose
or poorly fitted earphones; lack of adequate practice
when the patient is using an automatic audiometer; and
malfunction or erroneous calibration of the audiometer.
Progressive losses above 4000 Hz are warning signs
that the patient is at immediate risk of impairment if he
or she continues to be exposed to intense noise. Progres-
sive loss at 3000 Hz indicates an immediate need for
maximum protection of the hearingif necessary, by removal
from hazardous noise exposure.
A6.7 Audiometric and clinical pointers to susceptibility
Within the context of industrial hearing conservation
using noise monitoring and routine serial audiometry, Summar
(1965) has suggested some pointers to undue individual
sensitivity to noise which should be looked for. An employ-
ee may be regarded as unduly susceptible to occupational
NIHL if he:
1) Works in relatively low levels of noise (less than 85 dBA)
but reveals progressive hearing loss of noise-induced type;
2) Works in high levels without protection and shows abnormal-
ly rapid shifts; or
3) Works in high levels with protection but shows undue
continuing threshold shifts.
Other warning symptoms can be undue difficulty with speech
discrimination in noise; loudness recruitment; and tinnitus
persisting unduly after removal from the noise.
A6-6
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Appendix 7
NOISE-INDUCED HEARING LOSS: INDUSTRIAL EXPERIENCE
AND PREDICTIVE METHODS
A7.1 Effects of continuous noise on hearing; industrial
experience
Gallo and Glorig (1964) examined audiometric data from
400 men (aged 18-65) and 90 women (18-35) exposed regularly
to high-level industrial plant noise (102 dB SPL overall; 89,
90, 92, 90, 90 and 88 dB respectively in the octave bands
spanning 150 to 9600 Hz). These subjects were selected from
larger groups of 1526 male and 650 female employees, using a
screening process designed to exclude otological abnormalities
and irrelevant noise exposure (eg, to military n'oise) , and to
maintain in the men a high correlation between age and time
on the job. The purpose of the study was specifically to
investigate age and duration of steady-state noise exposure
as factors in PTS. It showed that hearing level tends to
rise relatively rapidly over the first 15 years of exposure
but then to level off at the higher audiometric frequencies,
3, 4 and 6 kHz. By contrast, hearing level at 500 Hz, 1 and
2 kHz rose more slowly but continued to rise in an essentially
linear manner over exposures up to some 40 years.
A comparison of data for 4 kHz in the men with equivalent
data from non-noise-exposed males showed that the effects of
the age and noise were not simply additive. Examination of
individual differences showed that the spread of hearing level
within groups tends to increase with both increasing exposure
time and with audiometric frequency (a similar effect was
reported by Taylor et al in 1965); and also, in the men
studied, that the time and frequency dependence of noise-
induced hearing level change is similar for most subjects.
Gallo and Glorig concluded from this study that early evidence
of PTS at 4000 Hz is the best indicator of susceptibility to
noise-induced PTS on either a group or individual basis. A
cognate study by Taylor e_t al_ (1965) in female jute weavers
supported the finding of Gallo and Glorig that noise-induced
deterioration in hearing takes place rapidly and mainly in the
first 10 to 15 years of exposure, with, however, further
deterioration at the speech frequencies continuing in later
years.
Taylor et aJL (1965) carried out retrospective audiometric
studies of groups" of women working in or retired from the jute
weaving industry in Scotland. The contributions to their
group hearing levels attributable to the regular noise
A7-1
-------
(99-102 dB SPL overall with higher peaks) to which they
had been exposed were evaluated by comparison with non-
noise exposed control subjects and by corrections for
presbyacusis using Hinchcliffe's (1959) median data.
In the main, this study reported the conclusions of Gallo
and Glorig (1964), namely, that the effect of noise on
hearing levels is greatest, earliest and most rapid at
the higher audiometric frequencies (4 and 6 kHz), where
it mostly takes place in the first 10 or 15 years of
occupational exposure; but that further deterioration
involving frequencies in the range 1 to 3 kHz (being most
marked at 2 kHz) becomes manifest during the third decade
of noise exposure. After as few as 10 ydars on the job in
high-level (90 dB SPL or higher) industrial plant noise,
men as young as 30 years old may have hearing levels
worse than non-noise-exposed men twice their age and may
in some cases already suffer impaired speech perception
(Gallo & Glorig, 1964).
PTS produced by noise exposure and PTS produced by
aging (presbyacusis) may not be distinguishable on either
a group or individual basis (Gallo & Glorig, 1964). Noise
induced PTS is found primarily among industrial workers
who have been exposed repeatedly and over a long period to
high-intensity noise: provided that the ears affected are
otologically normal, the PTS found in noise-exposed people
may be attributed to the combined effects of aging and
habitual noise exposure. Moreover, the component attri-
butable to noise exposure may be viewed as the result of
repeated noise-induced TTS exceeding a certain critical
level (about 20 dB).
Gallo and Glorig (1964) have summarized some general
characteristics of noise-induced PTS, as seen in occupational
contexts, namely, (1) the magnitude of the resulting PTS is
related to the noise levels to which the ear has habitually
been exposed; (2) the magnitude of the resulting PTS is
related to the length of time for which the ear has been
exposed; (3) the growth of occupationally related PTS at
4000 Hz is most rapid during the first 10 to 15 years of
exposure, after which it tends to slow down (see also
Passchier-Vermeer, 1968); and (4) there are large individual
differences in susceptibility to noise-induced PTS. The
data of Summar and Fletcher (1965) imply that age at the
time of exposure is probably not a significant factor in
industrial NIPTS.
A7.1.1 Differences between the sexes. In Gallo and Glorig's
study (1964) of hearing level changes in men and women exposed
regularly ro high-level industrial plant noiser the men in the
age range 18 to 35 years were found to show larger changes and
A7-2
-------
greater variability in hearing level than the equivalent
group of working women. It might be inferred from this
that men are more susceptible to noise than women. The
authors pointed out, however, that the data were likely
to have been influenced by the facts that the women's
noise exposure was more intermittent (due to mandatory
work-breaks), probably allowing some recovery from TTS;
and that (the screening procedure notwithstanding) some
of the men may already have suffered significant noise
exposure in the armed forces (see Appendix 2).
A7.1.2 Social significance of hearing loss at retirement.
Kell et al (1971)have reported that more than two thirds
of a surveyed group of elderly (mean age 64.7 years) women
who had worked as weavers (with steady daily noise exposures
of approximately 100 dBA) for up to 50 years had difficulty
with such social intercourse as understanding conversation,
using the telephone, and attending to public meetings
or church services. By contrast, fewer than one in six
age-matched women who had not been in a noisy occupation
was similarly handicapped.
A7.1.3 The reliability of the data from industrial studies.
Unfortunately, many hearing loss data from industry are
heavily contaminated by what Glorig and others (see Cohen,
Anticaglia & Jones, 1970; and Ward, 1970) have called
"sociacusic" factors (ie, undeterminable losses due to non-
occupational noise exposure in military, recreational or
other pursuits; or to disease affecting the ear); by the
effect of presbyacusis, which is inextricably bound up with
the time-dependent effect of noise exposure (and which is
presumed largely on a_ priori rather than evidential reason-
ing to be simply additive); and even within the setting of
industrial noise exposure, by lack of continuity (eg, due to
personnel changing jobs) affecting both retrospective and
prospective studies.
Ward (1970) has drawn attention to the particular
weakness of the evidence in relation to intermittent ex-
posure, pointing out that the "equal-energy" rule (probably
a reasonably satisfactory hypothesis over a wide range of
durations of continuous noise) makes no allowance for differ-
ent patterns of recovery from TTS in different patterns of
intermittency. For example, the rule cannot distinguish the
effect of a single 2-hour exposure from two 1-hour exposures
to the same noise with variable amounts of intervening quiet.
It was in an attempt to meet this shortcoming that CHABA
evolved a criterion based on the observation,of debatable
validity, that two noises producing equal temporary loss
(conventionally measured as TTS2) will produce equal ultimate
PTS (see Appendix 1).
A7-3
-------
A7.1.4 Interrupted noise exposure. A noise interruption
has been defined (Federal Register, 35 (238), 9 December
1970, pp 18671-18672) as a period of time "equal to at
least 20% of the duration of the preceding noise burst
when the noise level falls below 80 dBA". A lower figure,
say 70 dBA, may however be more appropriate where hearing
protection is the criterion. The significance of noise
interruption resides in the belief that the interruption
allows the ear to recover to some extent from depression
of the hearing caused by the noise. It follows that inter-
mittency of noise exposure allows higher noise levels to
be tolerated than when the noise is not interrupted.
Several factors may affect the auditory tolerance of inter-
mittent noise exposure. These include the number and
duration of interruptions (Schmidek et al, 1972; Ward,
Glorig & Selters, 1960); the relationships between contin-
uous and intermittent noise exposures (Ward, 1970; Ward,
Glorig and Sklar, 1959; Cohen & Jackson, 1968); and
possibly the level of noise below 80 dBA during the inter-
ruption (Schmidek et al, 1972). In the present document
interrupted noise exposure is treated as a special case of
fluctuating level (see Section IIIA).
A7.2 TTS and PTS; Prediction in the individual
A7.2.1 Burns' approach. The search for a reliable prognostic
test for individual susceptibility to PTS based on tests of
TTS continues (Burns & Robinson, 1970); and some promising
findings have recently been published by Burns (1970). He has
developed a.relative index (based on the regression of TTS on
hearing level) of susceptibility to TTS (Dtp) and, using the
predictive method of Robinson (1968), an index (D ) of PTS,
being the deviation (dB) of the individual's age-Corrected HTL
from the predicted median value of HTL for his peers in age and
noise-exposure. This index again has the advantage of being
a relative one, permitting subjects of various HTL's, ages
and noise-exposures to be grouped for purposes of correlation
with the TTS index, DT- Having determined values of DT for
3 groups of subjects distinguished by sound level (LA2 in the
range 93 to 104 dB) causing TTS, Burns has performed regressions
of Dip upon Dp for numerous combinations of audiometric test
frequencies and found a positive if rather low (not greater
than 0.34) correlation coefficient for several such combinations
Somewhat unexpectedly, the most promising result was found
when D« was based on low audiometric frequencies (1 and 2 kHz)
and Dp on high (3, 4 and 6 kHz), for reasons which the author
admitted to remain obscure. Burns considers this test to have
potentialities and has suggested possible ways of strengthening
it: its present weakness rests largely in the large residual
variance of Dy in the regression of DT upon Dp.
A7-4
-------
A7.2.2 The question of TTSo as a predictor of hazardous
noise exposure. Luz and Hodge (1971) have recently presented
complementary evidence, from studies of recovery from impulse-
noise induced TTS in monkeys and men, to show that the re-
covery is not a simple process and that, accordingly, a single
measure such as TTS2 may not be a particularly reliable
predictor in the construction of damage risk criteria for
hazardous noise exposure. Luz and Hodge have described
multiple TTS recovery patterns (cf. Ward's (1963) logarithmic
rule) and have postulated the existence of two types of
threshold shift, due to "metabolic" and "structural" auditory
fatigue respectively. They adduce the "rebound" recovery
phenomenon as strong evidence for a delayed component in
recovery from TTS (evident from other work also) and hypothe-
size with some conviction that this is related to permanent
damage.
A7.2.3 "Equal-energy" hypothesis. Some recent work by Ward
and Nelson (1970) on noise-induced threshold changes in
chinchillas appears to confirm the observations of Eldredge
and Covell (1958) in guinea pigs that there is an equivalence
of time and energy (at least within certain ranges of para-
meters) for continuous, uninterrupted noise exposure. In
other words, there is probably a limiting constant product
of intensity and time (analogous to Robinson's "immission")
for single unbroken exposures. Ward and Nelson (loc cit)
urge caution however, in extrapolation to repeated or to
interrupted exposures. They cite the findings of Miller,
Watson and Covell (1963) that frequent interruptions of noise
exposure by noise-free periods reduces both the TTS and the
PTS produced by the noise.
A7.2.4 Growth of TTS in constant noise. Miller, Rothenberg
and Eldredge (1971) have shown in the chinchilla exposed to
constant octave-band (300-600 Hz) noise at 100 dB SPL that
TTS grows in magnitude and in audiometric range with duration
of exposure over the first 1-2 days, then remains essentially
constant (asymptotic) with continuing exposures up to 7 days.
After cessation of exposures of that duration, the TTS decays
approximately exponentially over some 5 days (decay took about
2 days after identical exposures leasting only 193 minutes).
These noise exposures produced demonstrable cochlear damage,
although this was associated with only a small PTS measured
3 months after the noise exposure.
A7.2.5 TTS from prolonged noise exposure. Recent work in
the chinchilla (Carder & Miller, 1969) and in man (Mills,
Gengel et al, 1970) has confirmed that TTS due to a maintained
steady-state octave-band noise exposure reaches an asymptotic
level after some (up to 12) hours; and that recovery from
asymptotic TTS is slow (3 to 6 days for complete recovery in
man) and exponential in form.
A7-5
-------
A7.2.6 Asymptotic TTS as a function of noise level. Using
behavioral audiome try in monaural chinchillas, Mills (in
the press) has further demonstrated asymptotic threshold
shift following 4 kHz octave-band exposures of up to 9 days
(see also Carder and Miller, 1972). The magnitude of thresh-
old shift at asymptotic (TS ) was found empirically to be
predicted by the equation: 4o°
TS =1.7 (SPL - 47) ,
f£OO
where SPL is the sound pressure level in decibels relative
at 0.00002 N/m2. The frequency distribution, temporal
pattern and degree of persistence of the TTS were also found
to depend on the noise exposure level. TTS caused by 80 dB
noise was purely temporary, decaying from the asymptotic
value to zero in 3 to 6 days. Noise in the range 86 to 98 dB,
however, caused a "permanent" component to persist in the
threshold shift, which had not decayed to zero after 90 days.
The magnitude of this residual ("permanent") threshold shift
was related to noise level, being of the order of 10 dB at
the higher audiometric frequencies following 86 dB expsoure,
about 20 dB following 92 dB exposure and up to 40 dB (at
5.7 kHz) following 98 dB exposure. It cannot of course be
inferred that similar values or temporal patterns of TTS
and PTS would be caused by the same exposures in man, but
this work would appear to support a correlation between
threshold shifts measured within 20 minutes of exposure and
persistent threshold shifts measured 90 days after exposure.
The threshold shift measured 90 days following exposure may
reasonably be presumed to be a permanent threshold shift.
This work is considered in more detail in Appendix 12.
A7 . 2 . 7 Pitfalls of generalizing from animal studies to man.
Price (1968) has shown that, although the cat is regarded as
being more susceptible than man to behaviorally measurable
NIHL (see Miller, Watson & Covell, 1963), as is the chin-
chilla (Peters, 1965), the cochlear microphonic in the cat
appears to be much more resistant to alteration by noise
stress (at 5 kHz) than is the auditory threshold measured
(TTS) in man (although both changes follow a rate law which
is linear with the logarithm of time) . Price has urged
caution in drawing parallels between cochlear microphonic
and TTS data, although he has suggested that mechanical
factors in the peripheral auditory mechanism may explain
certain paradoxes in the growth of TTS resulting from high
intensity sustained versus impulse noise exposure (see Ward,
Selters & Glorig, 1961) . Price (1972) has recently pub-
lished similar findings at 500 Hz.
Poche et al (1969) have shown that impulsive (cap gun)
noise and pure tones (2 kHz at 125-130 dB SPL for 4 hours)
produce similar patterns of hair cell damage in the guinea
A7-6
-------
pig. They have pointed out, however, that no firm correla-
tion has yet been established between hair cell damage and
hearing loss either in animals (see Miller et alr 1971) or
in man.
A7.2.8 Asymptotic TTS in man. In tentative observations
upon his own ear, Mills(1970) has found evidence that TTS
in prolonged (24-48 hour) octave-band noise reaches an
asymptote in man, as in the chinchilla. The time to reach
it appears to be in the range 4 to 12 hours for man; and
the time required for complete recovery some 3 to 6 days.
A7.2.9 Miscellaneous factors considered in TTS. In 1958,
in a widely cited paper, Trittipoe maintained that pre-
exposure non-TTS-producing noise levels as low as 48 dB SPL
could enhance subsequent TTS due to a high (118 dB) brief
noise exposure. This has been taken as evidence that there
is no threshold of noxiousness for noise hazardous to the
ear. This observation and its interpretations have, however,
been disputed by Ward (1960).
Durrant and Shallop (1969) have recently reviewed
evidence for central factors in TTS; and have contended
that the state of attention can affect the acoustic reflex
and hence its protective function, in turn affecting the
pattern of TTS. It would appear very doubtful, however,
whether such an effect would be of any practical signifi-
cance in regard to the risk of PTS in noise exposures.
A7.3 Theories underlying continuous noise damage risk
criteria
Because most of our data concerning the long term
hazard of noise come from 8-hour industrial-type noise
exposures, there is a relative lack of information about
shorter term intermittent or incomplete daily exposures;
and virtually no data about continuous human exposure to
noise going on longer than 8 hours, or around the clock.
One is accordingly driven to make interpolations and ex-
trapolations on the basis of theories of noise trauma.
Two main theories have been suppor ced by substantial amounts
of field observation and experimental work. A continuing
difficulty in setting guidelines for safe noise exposure is
that equivalent predictions using these two thoeries con-
flict. Because the conflict is not resolvable in many cir-
cumstances, an empirical decision has to be faced as to
which theory to follow in evaluating a particular noise
hazard.
A7-7
-------
A7.3.1 The "equal-energy" hypothesis. Simply expressed,
this argues that the hazard to the hearing is determined by
the total energy (a product of sound level and duration) ent-
ering the ear on a daily basis. This rule is basic .to the
damage risk criteria embodied in certain important and wide-
ly used regulatory or guiding documents, notably the United
States Air Force (1956) Regulation AFR160-3 (the first formal
implementation of the rule). The "equal-energy" rule allows
a 3 dB increase in sound pressure level for each halving of
the duration (below 8 hours) of continuous daily steady-state
noise exposure. Extrapolation to prolonged durations of con-
tinuous noise exceeding 8 hours daily exposure, and extens-
ion to extremely brief exposures and impulses, has only re-
cently been proposed (Johnson, 1973; Robinson, 1971). In
practice, a cut-off is introduced by the widely recognized
mandatory absolute limit of 135 dB (BENOX, 1953) for unpro-
tected exposure, irrespective of duration. Botsford (1970)
has remarked that there is still a lack of experimental or
empirical verification for the "equal-energy" hypothesis,
except perhaps for overall durations of everyday exposure
extending over years (the only application for which the rule
was originally proposed). However, the theory possesses the
attractions of simplicity and a certain a priori reasonable-
ness (Eldred, Gannon & von Gierke, 1955).
A7.3.2 The "equal temporary effect" hypothesis. This theory,
originally based largely on the work of Ward, Glorig and Sklar
(1958; 1959) argues that the long-term hazard (of PTS) due
to steady-state noise exposure is predicted by the average
TTS produced by the same daily noise in the healthy young
ear. As Botsford (1970) has noted in a recent review, this
hypothesis is plausible because (unlike the "equal-energy"
rule) it relates to an observable physiological function
of the ear. Moreover, recent work suggests that a unifying
(but as yet unproven) hypothesis of metabolic insufficiency
induced in the hearing organ by noise my underlie both the
temporary and permanent hearing defects caused by excessive
noise (Hawkins, 1971). The essence of the supporting data
is that noise intense enough to cause PTS in the long run
is intense enough to produce TTS in the normal ear; while
noise which does not produce measurable TTS is not associated
with NIPTS (Ward, 1960). TTS studies also tend to support
the observation (reflected in industrial studies of PTS) that
intermittent noise is less harmful than constant exposure
to steady-state noise at the same level (Sataloff, 1969;
Cohen & Jackson, 1968). Adoption of this theory has led to
a number of current criteria, including that of CHABA (1965),
considered below.
A7.3.3 CHABA criterion for steady-state noise exposure.
CHABA's criterion is based essentially upon the hypothesis of
"equal temporary effect" already alluded to. In essence it
states that a noise exposure is unsafe if, upon testing the
normal ear two minutes after the cessation of the exposure,
A7-8
-------
an average TTS2 of 10 dB is exceeded at audiometric frequen-
cies up to 1000 Hz; 15 dB at 2000 Hz; or 20 dB at 3000 Hz
and above (Kryter, 1963; CHABA, 1965). According to Ward
(1970) this criterion reflects the empirical observation
that in most normal-hearing people, a TTS2 of 20 dB or less
recovers completely within 16 hours (when the worker would
be due to renew a typical 8-hour industrial exposure).
The corollary to that is that it is deemed unlikely that
any PTS is building up when the TTS recovers completely
before the commencement of the next working day. (A fraction
of "sensitive" ears, of course, will not recover completely.)
This makes no allowance, however, for non-occupational ex-
posure outside working hours.
A7.4 "Industrial" methods of predicting long-term hazard
from daily continuous noise exposure
Predictive methods were alluded to in Section IIIA and
are considered in detail in Supplement 2. These methods
permit predictions of the amount of noise-induced change in
hearing level to be predicted for designated fractions of
otologically normal working adult populations notionally
exposed day by day to steady-state industrial type noise,
as a function of average noise level (or equivalent continuous
sound level - see Appendix 8).
A7.4.1 Method and data of Passchier-Vermeer (1968). In order
to determine the influence of steady-state broadband noise on
the hearing levels of people exposed to noise for 8 hours a
day, at least 5 days a week, literature data were analyzed
from 20 groups of employees (about 4600 people) by Passchier-
Vermeer (1968) . Noise-induced shifts of hearing levels were
considered for exposure times between 10 and 40 years and
for noise with Noise Ratings (NR) for 500 to 2000 Hz between
75 and 98, or sound levels between 79 and 102 dBA.
The median noise-induced hearing losses (E>5Q%) at 500,
1000, 2000, 3000, 4000, 6000 and 8000 Hz were first examined
as a function of exposure time. The increase of D5Q% with
exposure time varied with frequency. The findings were:
(1) D50% at 4000 Hz remained constant at exposure times of
at least 10 years. The only exception was the female
group, for which DSQ% increased slightly after 10 years
of exposure.
(2) E>50% at 2000 Hz was a linearly increasing function of
exposure time from the very beginning of exposure.
A7-9
-------
(3) DSO% at 500, 1000 and 3000 Hz increased, for exposure
times of at least 10 years, year by year with respec-
tively 2%, 2.5% and 1% of the median hearing loss
caused by an exposure to noise for 10 years. If D50%
after 10 years is zero, then 1)50% remains zero for
longer exposure times.
(4) D50% at 6000 and 8000 Hz did not increase after 10
years of exposure, provided that the NR for 500 to
2000 Hz was at most 92. At higher NR's for 500 to
2000 Hz, D50% at 6000 and 8000 Hz increased year by
year with about 0.3(X-92)% of the median noise-induced
hearing loss occurring after 10 years of exposure
(where X is equal to the NR for 500 to 2000 Hz).
The analysis showed that the relation between noise and
median noise-induced hearing losses is most accurate, when
the NR for 500 to 2000 Hz is taken as a parameter of the
noise. Passchier-Vermeer (1968) has stated that the sound
level in dBA may also be used to estimate the median noise-
induced hearing losses, if the octave band spectrum contains
sound pressure levels in the two highest octave bands (mid-
frequencies 4000 and 8000 Hz) that are relatively low
compared with the sound pressure levels in the other octave
bands; if the sound pressure levels in those two octave
bands are about as high as the other sound pressure levels,
then the median noise-induced hearing losses will be over-
estimated .
The only difference found between the median noise-
induced hearing losses of men and those of women, was a
slight increase of 1)50% at 4000 Hz for the female group
for longer exposure times. At the other frequencies there
was no difference between the Dso%-values of men and women.
However, the data from only one female group could be
considered.
Because mean hearing levels of four groups were given
in the literature studied by Passchier-Vermeer (1968), the
mean noise-induced hearing losses of those four groups were
calculated. It appeared that there was no difference
between those four mean values and the median values of the
noise-induced hearing losses, if all these values were
related to the NR for 500 to 2000 Hz. However, it was not
possible to exclude the possibility that curves based on
mean values only have a shape different from curves based on
median values only.
Considering the median hearing losses caused by an
exposure to noise for 10 years as a function of frequency,
Passchier-Vermeer's analysis showed that these hearing losses
A7-10
-------
are maximal at 4000 Hz and decrease with increasing and
decreasing frequency. After 10 years of exposure, differ-
ences between the hearing losses at 3000 Hz and 4000 Hz
are slight. With increasing exposure time the median
noise-induced hearing loss moves from 4000 Hz towards
lower frequencies at higher NR's for 500 to 2000 Hz.
(At NR 98 the median hearing loss caused by an exposure
to noise for 40 years was maximal at 2000 Hz.) If the
NR for 500 to 2000 Hz is at most 80, the median noise-
induced hearing losses are generated during the first 10
years of exposure. At NR 85, D5Q% increases for longer
exposure times - mainly at 2000 Hz. At higher NR's for
500 to 2000 Hz the increase of D5n% at all frequencies,
except 4000 Hz, is considerable after 10 years of ex-
posure time, at least for the values limited by the
hearing levels not exceeded in 75% and 25% of the people
exposed to noise. The spread of the hearing levels depends
simply upon the NR for 500 to 2000 Hz; it is an increasing
function of the NR for 500 to 2000 Hz, at each frequency,
except at 4000 Hz. At 4000 Hz the spread is a decreasing
function of the NR for 500 to 2000 Hz. A consequence is
that, at NR's of at most 80 at all frequencies, except
4000 Hz, the spread of the hearing levels of people exposed
is just as large as that of people not exposed. The
reverse occurs at high NR's (at least 96) where the spread
of the hearing levels at 4000 Hz is the same for people
exposed and people not exposed, but at the other frequencies
the spread of the hearing levels of the people exposed is
larger. If the spread of the hearing levels of people
exposed to noise is larger than that of people not exposed
to noise, this increase in spread must be caused by noise.
Passchier-Vermeer's analysis showed that, in addition to
an increase in median hearing level, it is possible that
the spread of the hearing levels also increases as a result
of exposure to noise.
"Approximations" of the noise-induced hearing losses
not exceeded in 75% and 25% of the people, were also cal-
culated by Passchier-Vermeer (1968). The median hearing
level of those people not exposed to noise who had the same
mean age as the people exposed was subtracted from the
hearing levels not exceeded in 75% and 25% of the people
exposed. This implies an approximation, because the spread
in the hearing levels of people not exposed to noise was
not taken into account. Calculation of the exact values
of the noise-induced hearing losses not exceeded in 75% and
25% of the people is impossible, because it is not known
exactly what would have been the hearing level of a person,
when he had not been exposed to noise. However, it is
possible to calculate the noise-induced increase in the
A7-11
-------
hearing levels not exceeded in 75% and 25% of the people.
Designating these shifts as D75% and I>25%' "then D7c%f
D50% and D25% are e
-------
50
4000 Hz
40
ra
~
c
H
(I
:-
O
in
0
30
20
10
MEDIAN HEARING LOSS CAUSED BY
EXPOSURE TO NOISE FOR 13 YEARS,
AS A FUNCTION OF THE NOISE RATING
FOR 500 TO 2000 HERTZ.
75
85
95
105
dBA
Figure A7-1.
Median NIHL at 10 years as a function of
level of noise exposure (Passchier-Vermeer, 1968)
A7-13
-------
8000 Hz),, noise exposure level in dBA (80 through 100 in 5dBA
steps) and years of exposure (10, 20, 30, 40)*. Linearly in-
creasing median values may be assumed to take place for years
from 0 to 10. The values of dBA have been assumed to equate
with Passchier-Vermeer's values of "NR 512" when 5 is added
to the latter (eg, 75 NR = 80 dBA: see*Appendix 5). For
comparison, corresponding tables are appended in which the
values were derived using Passchier-Vermeer's Table B (ie,
the method yielding 025% and D75%). The methods of Passchier-
Vermeer are considered further by Johnson (1973).
Table A7-I.
Passchier-Vermeer's (1968) Tables A and B for
predicting median, 25% and 75%-ile noise-induced
hearing level changes from the data illustrated
in Figure A7-1.
Frequency
500 Hz
1000 "
2000 "
3000 "
4000 "
6000 "
8000 "
Table A
Increase of DSQ% in relation to D5Q%(T=10)
for exposure times of at least 10_yjears
0.28
0.37
2
2,5
10
1
0
0
(NR-92)
0
(NR-93)
% per year
n
n
n
n
n
n
n
n
NR
NR
NR
NR
92
92
92
92
NR for 500
to 2000 Hz
75
80
85
90
94
98
NR for 500
to 2000 Hz
Table B
Addition (dB) to be made to
500 1000 2000 3000 4000
to obtain 075%
6000 8000 Hz
0
0
0
0
0
0
0
0
0
0
0
0.5
0
1
2
3
4.5
7
0
0
2
4
4
4
.5
.5
.5
.5
4
3
3
2
0
0
.5
.5
0
1
2.
3.
4
5
5
5
0
1
2
3
3
3
Subtraction (dB) to be made from 050% to obtain 025%
500 1000 2000 3000 4000 6000 8000 Hz
75
80
85
90
94
98
0
0
0
0
0.5
1.5
0
0
0
0
0.5
1.5
0
0
0.5
3
4
5
1
1
2.5
3.5
3.5
3.5
5
5
5
4
2
1
1
3.5
6
7
7.5
8
0
0
0
0
0
0
10 to 40 years
5 days/week.
of exposure typically occurring for 8 h/day,
A7-14
-------
Table A7-II,
Passchier-Vermeer's Table C, for use with the
"approximate" method of predicting noise-induced
hearing level changes (1968).
Table C
NR for 500
to 2000 Hz
75
80
85
90
94
98
NR for 500
to 2000 Hz
75
80
85
90
94
98
Addition (dB) to be
500 1000 2000
4
5
5
5
5
5
5
5
5
5
5
5.5
Subtraction
500 1000
4
4
4
4
4.5
5.5
4
4
4
4
4.5
5.5
5.5
7
8
9
10.5
13
(dB) to
2000
3
3
5.5
8
9
10
made to
3000
6
8
11.5
13.5
13.5
13.5
be made
3000
9
9
10.5
11.5
11.5
11.5
D50%
4000
13
12.5
12
11
9.5
7.5
from
4000
13
13
13
12
10
9
to obtain 0*75%
6000 8000 Hz
8
10
11.5
12.5
13
14
D50% to
6000
9
11.5
14
15
15.5
16
8
10
11
12
12
12
obtain V 25%
8000 Hz
6
7
7.5
7.5
8
8.5
Table A7-III.
Composite table of median, 10%, 25%, 75% and
90%-ile values of noise-induced hearing level
change predicted as a function of length of
exposure, using the direct (first series) and
"approximate" (second series) methods of
Passchier-Vermeer (1968) with the modifications
alluded to in this appendix. The predictions
are given as a function of average noise level
in dBA for a given audiometric test frequency
on each of the following pages. (Pages A7-16
through A7-29.)
A7-15
-------
First series (500 to 8000 Hz): direct method
500 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA)
85 " > an
90 " )" A11
95 " )
100 dBA 50- 5555
75 4444
90 3333
A7-16
-------
1000 Hz
Exposure (Years)
Exposure
Level #ile 10 20 JO 4-0
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
All
All
All
All
50-
75
90
0
0
0
5
8
7
6
0
0
0
6
10
9
8
0
0
0
8
12
11
10
0
0
0
9
14-
13
12
A7-17
-------
2000 Hz
Exposure
Level
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
*ile
All
10
25
50+
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
0
2
1
0
6
4
2
1
0
11
8
5
2
0
20
15
10
6
2
Exposure
20
0
4
2
0
8
6
4
3
2
16
13
10
7
4
30
25
20
16
12
(Years)
30
0
6
3
0
1C
8
6
5
4
21
18
15
12
9
40
35
30
26
22
40
0
8
4
0
12
10
8
7
6
26
23
20
17
14
50
45
40
36
32
A7-18
-------
3000 Hz
Exposure (Years)
.Exposure
Level
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
iSile
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
7.
3
3
2
1
6
6
6
5
4
18
15
12
9
6
31
26
21
17
13
42
37
32
28
24
20
5
3
3
2
1
7
7
7
6
5
19
16
13
10
7
33
28
23
19
15
45
40
35
31
27
30
4
4
4
3
2
7
7
7
6
^
20
17
14
11
8
35
30
25
21
17
48
43
3b
34
30
40
4
4
4
3
2
8
8
8
7
6
22
19
15
12
9
37
32
27
23
19
51
46
42
38
34
A7-19
-------
WOO Ez
-xposure
Level
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
*ile
10
25
50
75+
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
13
9
5
0
18
14
10
5
0
22
19
16
11
6
30
28
26
22
18
40
39
38
36
34
Exposure
20
13
9
5
0
18
14
10
5
0
22
19
16
11
6
30
28
26
22
18
40
39
38
36
34
(Years)
30
13
9
5
0
18
14
10
5
0
22
19
16
11
6
30
28
26
22
18
40
39
38
36
34
40
13
9
5
0
18
14
10
5
0
22
19
16
11
6
30
28
26
22
18
40
39
38
3o
34
A7-20
-------
6000 Hz
Exposure
Level
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
*ile
10
25
50
75
90
1C
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
10
4-
4-
4-
3
2
9
8
7
3
0
18
15
12
6
0
26
22
18
11
4-
32
28
24-
16
8
Exposure
20
4-
4-
4-
3
2
9
8
7
3
0
18
15
12
6
o
26
22
18
11
4-
33
29
25
17
9
(Years)
30
4-
4-
4-
3
2
9
8
7
3
0
18
15
12
v./
0
26
22
18
11
4-
33
29
25
17
9
40
4-
4-
4-
3
2
9
8
7
3
0
18
15
12
6
0
26
22
18
11
4-
34-
30
26
18
10
A7-21
-------
8000 Hz
Exposure (Years)
exposure
Level
80 dEA
85 dBA
90 dBA
95 dBA
100 dBA
gile
All
10
25
50+
10
25
50+
10
25
50+
10
25
50+
10
0
3
2
1
8
6
4
14
11
8
20
17
14
2C
0
3
2
1
8
6
4
14
11
8
21
18
15
30
0
3
2
1
8
6
4
14
11
8
21
18
15
40
0
3
2
1
8
6
4
14
11
8
22
19
16
A7-22
-------
Second series (500 to 8000 Hz): approximate method
500 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA
10
25
50+
8
4
0
8
4
0
8
4
0
8
4
0
90 dBA
10
25
50+
10
5
0
10
5
0
10
5
0
10
5
0
100 dBA
10
25
50
75+
15
10
5
0
15
10
5
0
15
10
5
0
15
10
5
0
A7-23
-------
1000 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA)
85 dBA)-
90 dBA)
95 dBA
100 dBA
10
25
5 OH-
IO
25
50
75
90
10
25
50
75
90
10
5
0
15
10
5
1
0
18
13
8
3
0
10
5
0
16
11
6
2
0
20
15
10
5
1
10
5
0
18
13
8
4
0
22
17
12
7
3
10
5
0
19
14
9
5
1
24
19
14
9
5
A7-24
-------
2000 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
10
25
50+
10
25
50+
10
25
50
75
90
10
25
50
75
90
10
25
50
75
90
11
-6
0
14
7
0
18
10
2
0
0
23
14
5
0
0
31
21
10
1
0
11
6
0
14
7
0
20
12
4
0
0
28
19
10
2
0
41
31
20
11
2
11
6
0
14
7
0
22
14
6
0
0
33
24
15
7
0
51
41
30
21
12
11
6
0
14
7
0
24
16
8
2
0
38
29
20
12
4
61
51
40
31
22
A7-25
-------
3000 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA 10
25
50
75+
85 dBA 10
25
50
75+
90 dBA 10
25
50
75
90
95 dBA 10
25
50
75
90
100 dBA 10
25
50
75
90
15
9
3
0
22
14
6
0
35
24
12
1
0
48
35
21
9
0
59
46
32
20
9
15
9
3
0
23
15
7
0
36
25
13
2
0
50
37
23
11
0
62
47
35
23
12
16
10
4
0
23
15
7
0
37
26
14
3
0
52
39
25
13
2
65
50
38
26
15
16
10
4
0
24
16
8
0
39
28
16
5
0
54
41
27
15
4
68
53
42
30
19
A7-26
-------
4000 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA 10
25
50
75+
85 dBA 10
25
50
75+
90 dBA 10
25
50
75
90
95 dBA 10
25
50
75
90
100 dBA 10
25
50
75
90
31
18
5
0
35
23
10
0
40
28
16
3
0
48
37
26
14
2
57
48
38
28
18
31
18
5
0
35
23
10
0
40
28
16
3
0
48
37
26
14
2
57
48
38
28
18
31
18
5
0
35
23
10
0
40
28
16
3
0
48
37
26
14
2
57
48
38
28
18
31
18
5
0
35
23
10
0
40
28
16
3
0
48
37
26
14
2
57
48
38
28
18
A7-27
-------
6000 Hz
Exposure (Years)
Exposure
Level %ile 10 20 30 40
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
10
25
50
75+
10
25
50
75+
10
25
50
75+
10
25
50
75
90
10
25
50
75
90
20
12
4
0
27
17
7
0
35
24
12
0
43
31
18
4
0
50
37
24
8
0
20
12
4
0
27
17
7
0
35
24
12
0
43
31
18
4
0
51
38
25
9
0
20
12
4
0
27
17
7
0
35
24
12
0
43
31
18
4
0
51
38
25
9
0
20
12
4
0
27
17
7
0
35
24
12
0
43
31
18
4
0
52
39
26
10
0
A7-28
-------
8000 Hz
Exposure
Level
80 dBA
85 dBA
90 dBA
95 dBA
100 dBA
%ile
10
25
50+
10
25
50
75+
10
25
50
75+
10
25
50
75+
10
25
50
75
90
10
16
8
0
21
11
1
0
26
15
4
0
32
20
8
0
38
26
14
6
0
Exposure
20
16
8
0
21
11
1
0
26
15
4
0
32
20
8
0
39
27
15
7
0
(Years)
30
16
8
0
21
11
1
0
26
15
4
0
32
20
8
0
39
27
15
7
0
40
16
8
0
21
11
1
0
26
15
4
0
32
20
8
0
40
28
16
8
0
A7-29
-------
continuous industrial-type noise. Based on recent British
data relating hearing levels to continuous industrial-type
noise exposure, Robinson (Robinson & Cook, 1968; Robinson,
1971) has devised a method for estimating the magnitude of
NIPTS due to a known or predicted noise exposure to be
expected in designated fractions of an otologically normal
population. Assuming that the noise is of the same general
type as is found in manufacturing industries , where the
worker's ear is exposed throughout every shift to a fairly
constant (steady-state) assault, Robinson's formula may be
used to determine the A-weighted noise immission level , E^,
as a measure of the total equivalent exposure*. Noise
immission level has been defined by Robinson according to
the formula :
EA = LA + 10 Io9
-------
Appendix 8
EQUIVALENT CONTINUOUS SOUND LEVEL
A8.1 Calculation of equivalent continuous sound level, L
- ^ - L eq
The equivalent continuous sound level is a notional
sound level which , in an 8-hour period, would cause the
same total A-weighted sound energy to be received as that
received from the actual noise during the day (24 hours) .
This quantity, Leq, is yielded by the formula:
Leq = + 90, where
F = gf antilog [p.l(L - 90)] , in which
t is exposure time in hours and L is the sound level of
the exposure in dBA.
The value of Leg for a daily exposure to on-going
noise may be obtained from the nomogram at the end of
this Appendix, using the following procedure:
(1) For each component of the total exposure,
determine the fractional exposure value, F,
from the central scale by connecting the
values of L (level in dBA) and t (duration
of exposure component) in the nomogram.
(2) Sum the values of F for all the exposure
components received in the day.
(3) Read off the value of Leq corresponding to
the total value of F on the central scale
of the nomogram.
A8.1.1 Continuous exposure to constant (steady-state) noise
for 8 hours* In this case the equivalent continuous sound
level is numerically equal to the measured or predicted
sound level in dBA.
AS . 1 . 2 Single continuous exposure to constant (steady-state)
noise for other durations. Using the nomogram, a single
value of F is found for the duration in question (up to 24
hours) . The corresponding value of I^g is then found from
the central scale. Example ; a 90 dBA noise lasting for just
one hour in a day has an equivalent continuous sound level,
L , of 81 (to the nearest decibel) .
A8-1
-------
A8 . 1 . 3 Noise of fluctuating level (including "intermittent"
noise) . In this case the value of Leq is found from the sum
of the values of F for each component of the exposure.
Example : Suppose that the day's exposure comprises the
following components (with corresponding values of F) .
Duration of Sound Level, F (from
Component Exposure L (in dBA) nomogram
5 hours 85 0.2
1 hour 95 0.4
10 minutes 115 6.5
The total value of F is 7.1 in this case, giving a value for
of 99 (to the nearest decibel) .
A8-2
-------
Appendix 8a
SUPPLEMENTARY METHOD OF CALCULATION
ASa.l Calculation of equivalent continuous sound level ,
Step 1: Add to (or subtract from) each measured sound level
the adjustment value found from Table A8a-I.
Table A8a-I. Adjustments to measured sound level for cumu-
lative noise exposure.
Additions
Subtractions
Total Exposure
per day (h)
24
16
12
11
10
9
8
Adjustment
(dB)
+4.8
+3.0
+1.8
+1.4
+0.9
+0.5
0
Total Exposure
per day (h)
Adjustment
(dB)
7
6
5
4
3
2
1
0.5
-0.6
-1.2
-2.0
-3.0
-4.3
-6.0
-9.0
-12.0
Step 2: Convert adjustment levels to values of F, using
Table A8a-II.
Step 3; Add up the values to obtain total value of F.
Step 4; Reconvert total value of F to Leq/ using Table
A8a-II in reverse.
-------
Table A8a-II. Values of F for adjusted sound level
(fractional exposure) or equivalent
continuous sound level.
Level
(dBA)
120
119
118
117
116
115
114
113
112
111
110
109
108
107
106
105
104
103
102
101
100
Fractional
Exposure
(F)
1000
794
630
501
398
316
251
200
158
126
100
79.4
63.1
50.0
39.8
31.6
25.1
19.9
15.9
12.5
10.0
Level
(dBA)
99
98
97
96
95
94
93
92
91
90
89
88
87
86
85
84
83
82
81
80
Fractional
Exposure
(F)
7.9
6.3
5.0
4.0
3.2
2.5
2.0
1.6
1.3
1.0
0.8
0.6
0.5
0.4
0.3
0.3
0.2
0.2
0.1
0.1
-------
A8a.2 Conversion of octave-band sound pressure levels in
dB to sound level in dBA
Sound level in dBA may be calculated from octave-band
pressure level measurements in decibels using the following
method and tables (pages A8-6 and A8-7).
-------
CONVERSION OF OCTAVE BAND SPL (dB) TO A-WEIGHTED SOUND LEVEL (dBA)
(1) Subtract or add the appropriate A-weighting correction given in Table I for each octave
band SPL.
Table I. Relative response for sound level meters (IEC, 1965)
Frequency (Hz): 16 31.5 63 125 250 500 1000
Correction (dB): -56.7 -39.4 -26.2 -16.1 -8.6 -3.2 0
Approximation (dB): -57 -39 -26 -16 -9 -3 0
2000 3150 4000 8000 16000
+1.2 +1.2 +1.0 -1.1 -6.6
+1 +1 +1 -1 -7
(2) Convert the A-weighted band levels to arbitrary intensity units, I],jan(j> using Table II:
Table II. Values of I. , corresponding to octave band SPL from 50 dB to 129 dB
band
dB
50
60
70
80
90
100
110
120
0.
0.
0.
0.
1.
10.
100
1000
00010 0
0010 0
010 0
100 0
00 1
0 12
126
1260
.00013 0.
.0013 0.
.013 0.
.126 0.
.26 1.
.6 15.
158
1580
00016 0.
0016 0.
016 0.
158 0.
58 2.
8 20.
200
2000
00020 0.
0020 0.
020 0.
200 0.
00 2.
0 25.
251
2510
00025 0
0025 0
025 0
251 0
51 3
1 31
316
3160
.00032 0
.0032 0
.032 0
.316 0
.16 3
.6 39
398
3980
.00040 0
.0040 0
.040 0
.398 0
.98 5
.8 50
501
5010
.00050 0
.0050 0
.050 0
.501 0
.01 6
.1 63
631
3160
.00063 0
.0063 0
.063 0
.631 0
.31 7
.1 79
794
7940
.00079
.0079
.079
.794
.94
.4
-------
(3) Add up the values of Iband (£lband).
(4) Re-entering Table II, reconvert the total value fclv>and i-nto tne A-weighted sound level in
dBA (if the S.Iband falls between two values in Table II, take the higher value).
Example; The first two columns of the Table below give the octave band sound pressure
levels of a typical industrial noise (riveting machine). Column 3 shows the A-weighting
corrections taken from Table 1 which are added to the levels in column 2 to obtain the
A-corrected octave band levels in column 4. These are converted in to values of Iband
using Table 2, added together, then converted into A-weighted sound level using Table 2.
Octave band
centre frequency
Hz
Octave band
sound pressure
level
dB
A-weighting
correction
(from Table 1)
dB
A-corrected
octave band
level
dB
(from Table 2)
125
250
.500
1000
2000
4000
8000
88
85
77
78
80
70
69
-16
-9
-3
0
+1
+1
-1
72
76
74
78
81
71
68
.016
.040
.025
.063
.126
.013
.006
Total value of Iband=0-289
From Table 2, A-weighted sound level 85 dBA (to the nearest decibel)
-------
1251
-eq
120
120-
115
110
110
105
100
95
90
100
90
80-
70-
65-
= 1000
500
400
300
200
= 100
50
40
30
20
- 10
0.5
0.4
0.3
0.2
= 0.1
0.05
0.04
0.03
0.02
0.01
Duration
24 i
16
12 -
4
0.001
85
80
60
-S40H
c
o
«/> 20
15
10
j- 80-
60
-40
20
f 10
6
4
- 2
75*
Nomogram for calculating equivalent continuous sound
level, Leq (from P = * antilog [0.1(L-90>] , where t
is duration in hours and F is fractional exposure
value). For each noise exposure, connect sound level,
L, in dBA with exposure duration, t, and determine
fractional exposure, F, from the scale at center, right.
Determine total F by summing all values received in day.
Head off value of Leg from scale at center, right.
-------
Appendix 9
PHYSIOLOGICAL FACTORS AFFECTING THE
NOISE SUSCEPTIBILITY OF THE EAR
1 State of health and the general physiological state
It is possible that debilitatory diseases and certain
acute systematic diseases that reduce the body's general
resistance to noxious agents might increase a person's
susceptibility to NIHL. It may further be postulated that
this is particularly likely to be so when the effect of the
disease (eg, anemia from any cause) is to reduce the level
of oxygen circulating in the bloodstream; for there is an
association between local hypoxia and haircell damage in
the inner ear (Misrahy et al, 1958; Hawkins, 1971). In the
absence of reported studies of noise-susceptibility in
relation to the general state of health, however, such a
relationship remains speculative. Evidence for a link
between "stress" disorders, particularly high blood pressure
(Rosen et al, 1962, 1971; Bergman, 1966; Sataloff e_t aj^,
1969) , and~Eearing loss (although not necessarily noise-
susceptibility) has been reported but is similarly incon-
clusive. It is mostly based on studies comparing the
hearing of noise-exposed populations living and working
in urban or industrial settings with that of other groups
living in quiet rural conditions. It is clearly debatable
whether audiometric differences between such groups are due
to differences in the prevailing stress or noise exposure
alone, for many factors, including genetic, cultural,
dietary and nonacoustical environmental differences, may
underlie differences in the patterns of hearing found in
dissimilar populations or communities who are widely
separated geographically or culturally.
A9.1.1 Cochlear hypoxia and NIHL. Hawkins (1971) has
recently reviewed metabolic theories to explain the temporary
and permanent effects of intense noise upon the hearing organ;
and has reported new micropathological observations in guinea-
pigs exposed to noise at 118 to 120 dB SPL for 8 to 110 hours.
He describes noise-induced capillary vasoconstrictions which
which would appear to be a plausible basis for attributing
TTS to a regional relative hypoxia occurring in the organ of
Corti during excessive acoustic stimulation. Hawkins
theorizes that the local vascular response may have the
physiological functions of moderating the distribution of
blood supply in the organ of Corti to meet the metabolic
demands imposed by "natural" levels and patterns of noise;
but that excessive and prolonged noises (mostly generated by
mechanized human activity) render the response pathological,
-------
leading to irreversible vasospastic damage in susceptible
regions of the cochlea (including both the hair cells
themselves and the capillaries carrying their essential
blood supply). Hawkins1 hypothesis is still speculative,
however, and the way in which repeated vasoconstrictive
reactions causing TTS may accumulate to establish permanent
damage remains an open question.
A9.2 Otological health
Abnormalities of the middle ear affect the flow of
acoustic energy to the inner ear and hence might be ex-
pected to affect the development of NIHL. However, there
is no unequivocal evidence to show an enhancement of in-
trinsic susceptibility to PTS in cases of middle ear
disease or abnormality. Specific studies have shown that
stapedectomized ears are no more susceptible than normal
ears, at least with respect to TTS (Steffen et al, 1963;
Fletcher & King, 1963). Indeed, some middle ear conditions
may, in effect, protect the hearing organ from NIHL by
cutting down the acoustic input. Dieroff (1964) has re-
ported such an effect in the "protected" ear in cases of
unilateral otosclerosis; related data have been published
by Gerth (1966). Chronic perforation of the tympanic
membrane may have a similar protective effect in some
workers exposed to industrial noise (Dohi, 1953).
It is possible (but unproven) that inner ear infection
(which can be the sequel to acute otitis media) might lead
to an enhanced susceptibility of the hair cells to noise
damage. It remains likewise an open question whether
ototoxic drugs can render the inner ear significantly more
prone to damage by noise (Dayal et al, 1971).
A9.2.1 Noise susceptibility of already diseased ears. As
a general rule (Hinchcliffe, 1967) the diseased ear (with
the possible exception of endolymphatic hydrops) is probably
less susceptible to NIHL, or no more so, than is the normal
ear. This is likely to be so particularly in cases of
conductive defects, provided there is no sensorineural com-
plication; for middle ear obstructions are apt to act^as an
ear protector. However, there can be exceptions, making an
individual with conductive deafness abnormally susceptible
to NIHL. One is loss of the acoustic reflex (as in oto-
sclerosis) ; another condition is a reduction of pneumatization
of the mastoid bone (leading to a loss of air-damping of
tympanic membrane movement) associated with chronic middle
ear infections (Link, Handl and others cited by Hinchcliffe,
1967). It appears that stapedectomized patients are not
unusually susceptible to NITTS (Ferris, 1965) although there
-------
A9.2.2 Non-acoustic causes of sensorineural hearing loss
resembling NIHL. It has been known since ancient times
(Hinchcliffe, 1967; King, 1972) that blows to the head,
falls with head injury and direct injury to the ear can
cause partial or total hearing loss. Hearing loss associa-
ted with head injury is audiometrically identical with
noise-induced hearing loss, characterized typically by
the 4000 Hz notch in the audiogram; and, accordingly, may
be difficult to exclude, except inferentially, in clinical
assessments of disability attributed to noise exposure.
The hearing organ (apart from the middle ear mechanism)
can also be damaged by abrupt and severe changes in atmos-
pheric or extra-tympanic pressure, as in aviation, diving,
or exposure to blast waves from explosions (Coles, 1965;
King, 1972). In this connection, Hinchcliffe has distin-
guished five classes of "insult" to the hearing organ,
namely:
1) Noise-induced hearing loss
2) Acoustic trauma (see Glossary)
3) Otitic blast injury (Commonly damaging the
middle ear as well as the organ of Corti)
4) Acoustic accident (see Glossary)
5) Head injury
(Of these, 1, 2 and 4 are generically linked.)
A9.3 Physiological defensive mechanisms
A9.3.1 The acoustic (middle-ear) reflex. The middle ear
contains two small muscles, stapedius and tensor tympanir
which act upon the ossicular chain. Both muscles have
been shown to contract in response to acoustical stimuli.
They also respond to mechanical irritation of the external
auditory canal or of the skin of the face or neck around
the ear. Functionally, these miniature skeletal muscles
have opposing actions: stapedius acts to withdraw the foot
of the stapes from the oval window, while tensor tympani
tends to pull the handle of the malleus (attached to the
tympanic membrane) inwards. The resulting combined action
is to increase the stiffness, and probably also the damping,
of the ossicular chain, thereby changing the impedance of
the middle ear in a way which principally discriminates
against the conduction of intense low-frequency sound to
A9-3
-------
is also called the intra-aural or middle-ear reflex) can be
observed in man by measuring the acoustical impedance of the
ear (tympanomanometry) and the change in impedance can be
used diagnostically for audiometric and otological purposes.
Because the acoustic reflex is a physiological response
with a latency of at least 10 ms, it is of little value in
protecting the ear against unanticipated noises which are
impulsive or of extremely rapid onset (eg, gunfire). Some
protection against such noises may be obtainable by alerting
the reflex with a moderate premonitory noise (Fletcher &
Riopelle, 1960).
It is evident that the acoustic reflex protects against
PTS in the same way that it can be shown to protect against
TTS, but the threshold and the protective efficiency of the
reflex vary substantially between individuals (Ward, 1962).
Coles and Knight (1965) showed that, in 40 newly trained
Marines tested with a Madsen acoustic impedance meter, those
with poorer high-tone hearing exhibited a higher reflex
threshold than the men with "good" hearing for a 4000 Hz
test zone. It appears, however, that the variability of
the reflex and its innate fatiguability (ie, its tendency
to decay in the presence of maintained or repetitive
stimulation) make it doubtful whether, except in certain
circumstances (eg, intense short-term noise exposure), it
is a significant factor in practice.
Loeb et al (1965) have adduced some slightly atypical
evidence tEat the acoustic reflex may in fact have some
power of protection against high frequency (above 600 Hz)
TTS produced by impulse noise from an arc-generator. (The
TTS observed after this type of noise was not linearly re-
lated to number of impulses but somewhat less than logarith-
mically. Moreover, reflex-activating sounds were not
reliable in diminishing the TTS produced by the impulses.)
A9.3.1.1 Acoustic reflex and critical bandwidth. Searching
for -(and finding) confirmatory evidence for a peripheral
auditory mechanism underlying the critical bandwidth in
loudness summation, Flottorp et al (1971) have shown that
the acoustic reflex thresholdTexpressed in dB re 0.00002
is approximately constant as the bandwidth of an exciting
noise is increased up to a critical bandwidth, beyond which
the threshold falls with widening bandwidth at a rate of about
3 to 6 dB/octave. This finding would appear to lend support
to the idea that the reflex is more readily excited or
sustained by (and is hence of greater protective value in)
broadband than narrow band or tonal noise (see Cohen & Baumann,
1964).
A9-4
-------
A9.3.1.2 Acoustic reflex and tonal noise. Cohen and
Baumann (1964) systematically varied the relative strengths
of tonal components (in the range 500 to 4000 Hz) with
respect to a background of broadband noise (overall SPL =
105 dB in all 20-minute exposures) and measured TTS in men
exposed to these combinations. They found that tonal
components (some exceeding the AF160-3 criterion) were most
"noxious" (ie, they caused greatest TTS) when they were of
low frequency (below 2 kHz) and contained most of the energy
of the combined noise (when the tone was most intense, the
background broadband noise was made least intense, to yield
the same overall level). This finding supports the hypothe-
sis that prominent low-frequency tonal components may be
unduly noxious because of failure of the relatively moderate
background random noise to sustain the acoustic reflex (which
rapidly adapts to a tonal stimulus).
Mills and Lilly (1971), using TTS measurements at 1000 Hz,
have recently shown that the acoustic reflex has a differen-
tially protective effect against low frequency (710 Hz) tones
compared with narrow-band noise with the same upper cutoff
frequency (both noises lasted 10 minutes at 110 dB SPL) .
Comparing normal subjects with six others who lacked an
acoustic reflex (having been stapedectomized), they found that
the normals had 10 dB more TTS2 following tonal than equivalent
1/8-octave band noise, which latter noise presumably caused
a stronger or more maintained reflex contraction. In the
stapedectomized subjects this differential effect was not seen:
the TTS2 in those subjects was about the same following both
types of noise.
A9.3.1.3 Noninteraction of TTS's; and the acoustic reflex.
Ward (1961]showed that distinct TTS' s produced at well
separated audiometric frequencies by correponding high (2400-
4800 Hz) and low frequency (600-1200 Hz) noises do not inter-
act. That is to say, the presence of one TTS has no effect
upon the growth or recovery of the other. Ward attributed
this finding to the relative independence of fatigue processes
(presumably associated with regional vasoconstriction leading
to a relative hypoxia of each part of the organ of Corti
see Hawkins, 1971) at different locations in the basilar
membrane. He pointed out that the protection afforded by the
acoustic reflex against one such noise is not affected by
prior exposure to the other.
A9.3.1.4 Acoustic reflex protection against impulse noise.
Fletcher (Fletcher & Riopelle, 1960) and Ward (1962; 1970)
have suggested the use of a provocative tone, preceding the
noise, to induce the acoustic reflex to contract as a defense
against impulsive sounds of known time of onset (eg, when
shooting) . Ward has shown (1962) that at least 150 ms is
required for the reflex to take full effect; but that some
A9-5
-------
protection begins substantially earlier (eg, 1 dB at 25 ms;
5 dB at 62 ms; 13 dB at 100 ms). Individual and temporal
differences in the protective value of the reflex are large:
Ward (1962) found this to vary from zero to some 15 dB. He
expressed doubt, moreover, as to whether specific (ie, stan-
dardizable) values of the protective effect of the reflex
could be arrived at, because of the great variability of
the response with both intrinsic (physiological) and
extrinsic (acoustic) factors.
Cognate studies by Cohen et al (Cohen, Kylin & LaBenz,
1966) had earlier implicated tEe" acoustic reflex as a
variable factor influencing the patterns of TTS and contra-
lateral remote masking (CRM), an indirect measure of
acoustic reflex response, in various combinations of im-
pulsive and steady-state noise. They showed that, while
the addition of moderately strong (90 to 100 dB SPL) broad-
band steady-state noise reduced the effect of impulses
(order of 124 to 127 dB peak SPL), the addition of impulses
also, somewhat paradoxically, reduced the temporary noxious
(TTS-producing) effect of a higher level (110 dB SPL) of
the same steady-state noise.
A9.3.2 Generalized aversiye response. Intense noise (above
about 100 dB SPL), especially if it is of rapid onset and
of high frequency, can evoke a generalized, initially
involuntary, response of tensing, grimacing and covering
the head and ears with the arms and hands. Animals with
movable pinnae (eg, cats) characteristically flatten their
ears and crouch or run during sustained intense noise (but
erect the ears in the startle response). Some people feel
a compelling urge to hide or run away from noise of extreme
intensity. This response can be enhanced by conditioning,
for example, in those who have been exposed to the sounds
of battle in highly stressful circumstances: the provocative
noise in these cases does not necessarily have to resemble
closely the sound of the sufferer's combat experience.
Abnormal sensitivity or responsiveness to loud noise is
sometimes associated clinically with certain acute or chronic
systemic disorders (eg, febrile states and idiopathic hyper-
tension) .
A9.4 Ear protectors and NIHL
Several authors have recently reviewed the principles
practical use and protective value of insert and circumaural
ear defenders (Michael, 1965; King, 1972; Rice & Coles, 1966).
Now that the use of these devices is becoming customary, if
not mandatory in many occupational and some non-occupational
A9-6
-------
noise exposure situations, it is to be expected that ear
protection will have some effect upon the NIPTS measurable
or predictable in groups. Summar and Fletcher (1965; and
Summar, 1965) reported that almost complete protection
against industrial noise can be achieved. As a rule,
however, such an overall protective effect is inherently
difficult to estimate; it is moru likely than not to be
overestimated, because in practice ear defenders are not
universally worn, even where their use is mandatory (Kopra,
1957), and they are frequently misused or worn with an
imperfect fit which reduces their theoretical effectiveness.
In the present state of knowledge, therefore, it is difficult
to allow for the use of ear defenders when predicting the
effect of noise exposures upon the hearing of groups or
individuals.
A9-7
-------
Appendix 10
INFRASOUND, VIBRATION AND ULTRASOUND
A10.1 Infrasound and mechanical vibration
Various transient physiological reactions have been
observed during human exposure to airborne infrasound in
the range 0.5 to 16 Hz. These responses generally resemble
those seen during whole-body vibration (Alford et al, 1966;
Cole et al, 1966; Guignard, 1972; Mohr et al, 1965; Nixon &
Johnson, 1973) and are mostly of a non-specTfic nature,
resembling reactions to mild stress or alarm. However, a
variety of bizarre sensations can be experienced during
exposure to airborne infrasonic waves, including fluttering
or pulsatile sensations in the ear. It is debatable
whether true auditory sensations are elicited in the 1 to
15 Hz range (Yeowart et al, 1967). There is evidence
(Parker et al, 1968) that intense infrasound can stimulate
the vestlFular system, as can low-frequency vibration, lead-
ing to disequilibrium if the stimulation is severe enough,
but little evidence that the hearing organ is affected at
intensities likely to be encountered in practical situations
(Mohr et al, 1965; Nixon & Johnson, 1973). Low-frequency
mechanical vibration of the whole body at severe intensities,
however, can be shown to produce a small TTS in man involving
the lower audiometric frequencies (Guignard & Coles, 1965)
and it may be inferred from this that airborne infrasound at
equivalent intensities could do likewise, constituting a
potential hazard to the hearing.
Few data exist, at present, to serve as the basis of a
hearing damage risk criterion for infrasound. It may be
argued by extrapolation from data pertaining to the low
audible range, however, that no unprotected ear should be
exposed to infrasound exceeding 135 dB SPL, irrespective
of either the subjective magnitude or the duration of the
disturbance; nor should unprotected whole-body exposures
to levels exceeding 150 dB SPL at frequencies above 0.5 Hz
be permitted, whether or not ear protectors are worn (Nixon
and Johnson, 1973).
In the event of prolonged exposure (eg, daily occupational
or constant exposure) to infrasound (0.5 to 16 Hz) at levels
exceeding 130 dB SPL (approximately 80 dBA at 20 Hz) , specific
measurement and narrowband analysis of the infrasound should
be carried out to appraise the hazard, which may be treated
conventionally as a noise hazard if its frequency or spectral
maximum lies within the 8 to 16 Hz octave band.
A10-1
-------
A10.2 Ultrasound
A substantial review by Parrack (1966) and some recom-
mendations by the same author and co-workers (1968, 1969)
contain essentially all the available data pertaining to
hearing hazard from airborne ultrasonics in man. Notwith-
standing the fact that some people may perceive and claim
to be disturbed by various sensations when in the vicinity
of ultrasonic equipment, we agree with Parrack's conclusion
that airborne ultrasound is not a practical threat to the
ear except perhaps in very unusual circumstances (Skillern,
1965; Acton & Carson, 1967).
Nevertheless, it is important to recognize that a
hazard may exist due to subharmonic or adventitious noise
generated at audio-frequencies (below 20 kHz) when ultra-
sonic equipment is running. The hazard in that case is
the same as that associated with audible noise at the same
level from any other source. Parrack (1969) has made some
recommendations concerning human exposure to the acoustic
and ultrasonic output from ultrasonic devices. A summary
of those recommendations is given in Table A10-I below.
Table A10-I. Recommended maximum acceptable levels of oc-
cupational exposure to airborne ultrasound
(Parrack, 1968, 1969). Note; the figures
pertain to third-octave band pressure levels
measured at the subject's ear in the normal
operating position with respect to an ultra-
sonic generator. For general adventitious
exposures of people working within 15 feet
(5 meters) of the operator's position, a re-
duction of 10 dB is recommended.
1. Frequency range 18,000 to 45,000 Hz.
Center-frequency of 1/3 octave band sound
1/3 octave band level in dB
20,000 Hz 105 dB
25,000 Hz 110 dB
31,500 Hz 115 dB
40,000 Hz 115 dB
2. Frequencies above 45,000 Hz.
No limit specified. Note;Parrack (1968) has stated
that to set a limit for this range is scarcely prac-
ticable, because of measurement limitations and the
fact that the rapid attenuation of ultrasound in air
at such high frequencies renders the hazard from
practical generators negligible.
A10-2
-------
Appendix 11
NON-OCCUPATIONAL NOISE EXPOSURE: PARTICULAR SITUATIONS
All.l Aircraft noise and hazard to the community
This hazard has been investigated by Parnell, Nagel
and Cohen (1972) with equivocal results. Hearing in a
community living near a major airport was compared with
that in a demographically equivalent group living in a
relatively quiet rural setting. The airport neighbors
were subjected to flyover noise of which the maximum
levels ranged from 76 to 101 dBA with a median value of
88 dBA. The noise levels in the quiet area rarely exceed-
ed 60 dBA and were commonly below 50 dBA. The people
exposed to jet noise showed marginally poorer (and more
variable) hearing when tested at 3000, 4000 and 6000 Hz.
Interestingly, in relation to the importance attached by
many otologists to the measurements at the speech frequen-
cies, the differences between the two communities' hearing
at 500 Hz, 1 and 2 kHz were insignificant. (Probably
because of the areas studied, both groups in this particular
study yielded average hearing levels as good as or, at some
frequencies, slightly better than the levels predicted
(Glorig & Roberts, 1965) from the 1960-62 Public Health
Survey). The authors were unable to say with certainty
that the jet noise was damaging the hearing of the noise-
exposed residents but did conclude from the evidence ob-
tained that the possibility of such risk should not be
neglected.
All.1.1 Auditory hazard from sonic booms. In 1966 von
Gierke reviewed the available information on the effects of
actual and simulated sonic booms on people, citing a tabular
summary of effects upon the ear (Nixon, 1965). This summary
is reproduced in Table All.l.
According to Rice and Coles (1968) the maximum sonic
boom overpressures likely to occur in civil aviation are of
the order of 10 Ib/sq ft (equivalent SPL: 148 dB) ; and in
practice booms are only rarely to be expected which exceed
2 Ib/sq ft (134 dB) . Testing young men with normal hearing
(within 20 dB of British Standard 2497 zero) Rice and Coles
concluded, on the evidence of minor amounts of transient TTS
following simulated booms, that sonic booms up to 17 Ib/sq ft
(152 dB) could be disregarded as a potential auditory hazard.
The criterion used was that of Garinther et al (1966). A
threshold of hazard (PTS) due to sonic boom-type impulses was
however deemed to lie somewhere above 44 Ib/sq ft (160 dB) .
This is a somewhat more conservative estimate of the safe ex-
posure limit than that (144 Ib/sq ft * 171 dB) published by
von Gierke (1966).
All-1
-------
Table All-I. Observed and predicted auditory responses to
sonic booms. (Summary of all observations
reported.)
NATURE OF AUDITORY
RESPONSE
Rupture of the tympanic
membrane
SONIC-BOOM EXPERIENCE
OR PREDICTION
None expected below 720 Ib/sq ft
None observed up to 144 Ib/sq ft
Aural pain
None observed up to 144 Ib/sq ft
Short temporary full-
ness , tinnitus
Reported above 95 Ib/sq ft
Hearing loss: permanent
None expected from frequency and
intensity of occurrence
Hearing loss: temporary
None measured
(1) 3-4 h after exposure up to
120 Ib/sq ft
(2) immediately after boom up
to 30 Ib/sq ft
Stapedectomy
No ill effects reported after
booms up to 3.5 Ib/sq ft
Hearing aids
No ill effects reported after
booms up to 3.5 Ib/sq ft
All-2
-------
Sonic booms may, if exposure to substantial over-
pressures is frequently repeated, cause hair-cell loss
which is not necessarily observable behaviorally, at least
in animals. Majeau-Chargois e_t al (1970) showed that hair-
cell loss and scarring in the apical turn of the cochlea was
an immediate consequence of exposing guinea-pigs to a long
series of rapidly repeated simulated booms. The animals
were subjected to a 1000 booms in the form of N-waves
having an amplitude of ±130 dB applied once per second.
The duration of the pressure signature (ranging from 2 to
125 ms) did not appear to influence the pattern of injury.
Although all the 24 animals tested showed similar injury
to autopsy, their behavioral response to test tones in the
range 250 to 16,000 Hz (Preyer reflex) was not altered by
the booming.
All.2 Hearing hazard from pop music
Several recent studies have confirmed that the overall
sound levels of very loud rock and roll and similar pop
music frequently exceed current hearing damage risk criteria
and can produce large amounts of TTS in both musicians and
listeners (Dey, 1970; Fletcher, 1972; Lebo & Oliphant, 1968;
Lipscomb, 1969; Rintelmann & Borns, 1968). Typical rock
music can be regarded, from the point of view of the hair
cells, if not of their owners, as a steady-state noise with
interruptions. Typically, the maximum acoustic output from
the bands' amplifiers lies in the region of 2000 Hz. Dey
(1970) found that typical exposures averaging 100 to 110 dBA
for up to 2 hours could produce TTS2 exceeding 40 dB in 16%
of young adults tested. Rintelmann and Borns (1968) measured
representative levels of 105 dBA and found that some 5% of
musicians (mostly quite young) showed evidence of NIPTS
presumably attributable to their music. Clearly, the hazard
is an occupational one for the performer and usually a recre-
ational one for the listener.
Lipscomb (1969) has demonstrated cochlear damage in
guinea-pigs exposed to 88 hours of recorded rock and roll
music played at 122 dBA, a somewhat exceptional level which
can however be exceeded at the ears of musicians and nearby
listeners in some instances where excessive amplification
of the music is used in reverberent rooms or dance-halls.
Dangerous levels can also be reached near domestic stereos
(Lipscomb, 1969). In a comparative study of the noise
hazard to young people in various recreations, Fletcher (1972)
found playing in rock-bands to be exceeded in degree of
hearing hazard only by motorcycle and drag racing and by
intensive sport shooting with inadequate ear protection.
Fletcher observed incidentally that young men and women
are equally at risk of hearing damage when exposed to over-
amplified rock music. A similar conclusion was reached by
Smitley and Rintelmann (1971).
All-3
-------
All.3 Impulsive noise from childrens* toys
Gjaevenes (1967) has cited Scandinavian data showing
that between about 1 and 4% of teenaged children may show
hearing injuries resulting from the impulsive noise from
crackers or other noisy toys, and has argued that this
degree of risk accords with a DRC of 155 dB peak pressure
for impulsive toy-noise. He points out that there is no
evidence that childrens1 ears are more easily damaged by
impulsive noise than are those of adults: all the data
upon which existing impulse noise damage risk criteria are
based (see Section IIIB) have come of course from adults
(mostly exposed to gun noise).
All-4
-------
Appendix 12
FURTHER DATA (PREPARED BY J. H. MILLS) ON
THE PATTERN OF THRESHOLD SHIFT PRODUCED
BY PROLONGED NOISE IN ANIMALS AND MAN
A12.1 Exposures longer than 8 hours and continuous exposures
Considerable attention has been given to hearing losses
produced in human subjects by noise exposures of 8 hours or
less. Hearing losses produced in human subjects by longer
noise exposures have received very little attention. Thus, in
attempting to deal with the problems created by long exposures
to noise it is necessary to turn to studies completed with
sub-human subjects, and to make comparisons with human sub-
jects when possible.
A12.1.1 Studies with sub-human subjects. Available data have
been obtained from monaural chinchillas trained in be-
havioral audiometry (see Carder and Miller, 1972). After pre-
exposure audiograms are obtained the animals are exposed to
noise in a diffuse-sound field for durations of 2 to 24 days.
At regular intervals throughout the exposure the animals are
removed from the noise, placed in a quiet room where auditory
thresholds are measured, and then returned to the noise. The
decay of threshold shift is measured after cessation of the
exposure. At a post-exposure time of 2-3 months audiograms
are obtained and compared with pre-exposure audiograms to re-
veal the presence of any permanent threshold shifts. Then,
electrophysiological measurements are made, the animal is
sacrificed, and the inner ear is processed for later anatomical
study. The results of such experiments are summarized below.
Threshold shifts measured at a post-exposure time of 4-11
minutes increase for the first 24 hours of exposure and then
reach an asymptote (Miller, Rothenberg and Eldredge, 1971;
Carder and Miller, 1972; Mills and Talo, 1972; Mills, 1973).
Threshold shifts at asymptote (TS^) , in the frequency region
of maximum effect, can be described by the equation
TSoB-'l.e (SPL - C) (Eq 1)*
where the subtractive constant C depends upon the particular
band of noise. For an octave-band centered at 4.0 kHz the
value of C is 47.0 and for an octave-band centered at 0.5 kHz
the value of C is 65.0. The 18 dB difference between the sub-
tractive constants can be attributed to the acoustic propert-
ies of the external and inner ears (Mills and Talo, 1972).
In Mills (1973) a slope of 1.7 is used,
A12-1
-------
The durations and levels of noise for which the asympt-
ote can be maintained are not known. It has been established
that the asymptote can be maintained for up to 9 days for
threshold shifts as great as 80 dB (Mills, 1973) and for up
to 21 days for threshold shifts as great as 30 dB (Carder and
Miller, 1972).
In the frequency regions of maximal effects, exposure to
high- or low-frequency noise produces highly similar growth
and decay curves (Carder and Miller, 1972), providing the lev-
els of the noises are adjusted for the acoustic properties of
the external and inner ears (Mills and Talo, 1972). However,
the spread of threshold shifts along the frequency dimension
is substantially different. Whereas the threshold shifts pro-
duced by a high-frequency noise have a distinct maximum in the
frequency region within and just above the band of noise,
threshold shifts produced by a low-frequency noise are more
widespread.
Recovery from asymptotic threshold shifts is slow, and
it depends upon characteristics of the exposure. It may be
possible, however, to place the various recovery patterns into
one of three categories. First, for exposures that produce
threshold shifts at asymptote of less than 55 dB and are short-
er in duration than 22 days, recovery to zero requires between
3 and 6 days (Carder and Miller, 1972; Mills, 1973). Second,
for complex exposures of 24 days where the threshold shift at
asymptote is about 55 dB for the last six days of the 24-day
exposure, recovery to zero or near-zero values requires about
7-15 days. Third, when the threshold shift at asymptote exceeds
about 55 dB, recovery to zero does not occur. That is, a
permanent threshold shift is produced (Mills, 1973). In this
case, recovery to the final threshold value takes about
15-30 days. Thus, decay of asymptotic threshold shifts requires
3-6 days, 7-15 days, or 15-30 days depending upon character-
istics of the exposure.
Recovery from an asymptotic threshold shift in the quiet
(<30 dBA) is somewhat faster than the recovery from an asympt-
otic threshold shift in the presence of a low-level noise (57
dB SPL). This difference, however, is small both in terms of
time (1-4 days) and magnitude (3-7 dB) (Mills, Talo and Gordon,
1973).
Anatomical and electrophysiological data have been reported
for chinchillas who participated in some of the behavioral ex-
periments. A major result is the presence of anatomical and
physiological injuries in the ears of animals with normal audit-
ory thresholds (Eldredge, Mills and Bohne, 1972). Specifically,
10 to 250 outer hair cells may be missing, input-output funct-
ions of the cochlear microphonic are decreased, and whole-nerve
A12-2
-------
action potentials range from normal to as much as two standard
deviations below normal. Thus, normal auditory thresholds
measured after exposure to noise are no assurance that the ex-
posure did not permanently injure the inner ear. As yet, the
characteristics of exposure to noise that will produce cochlear
injuries that are not detectable by audiometry are not known.
Pathologic physiology associated with asymptotic threshold
shifts is most likely to be found at a peripheral level of the
auditory system, specifically at the level of the hair-cell
modulating mechanism (Benitez, Eldredge and Templer, 1972).
A12.1.2 Studies with human subjects. For human subjects,
threshold shifts, measured a few minutes post-exposure, increase
for the first 8-16 hours of exposure and then reach an asympt-
ote (Mills et al, 1970; Mosko e_t al, 1970; Melnick, 1972).
Threshold shTfts at asymptote,~Tn the frequency of maximum ef-
fect, increase about 1.6 dB for every 1-dB increase in the lev-
el of noise above about 75 to 80 dB. This result is for an
octave-band noise centered at 0.5 kHz (Mills et al, 1970; Mel-
nick, 1972). Recovery from asymptotic threshold shift is slow,
requiring from 1 to 6 days.
A12.1.3 Comparison of human and sub-human data. For man or
chinchilla and for continuous exposures to noise, it may be
possible to describe the growth of TTS to asymptote and its
subsequent decay after cessation of exposure by growth and de-
cay functions with one or two exponential terms (Carder and
Miller, 1972). The major differences between the species ap-
pear to be in the value of the time constants and in the mini-
mum levels of noise that produce TTS (i.e., the subtractive
constants). These differences have been summarized by Carder
and Miller (1972), page 621:
" TTS grows faster for man than for chinchilla
(time constant is 2 - 5 hr for man and 6 - 12 hr
for chinchilla); the decay of TTS may be faster for
man than for chinchilla (time constant is 4 - 24 hr
for man and about 29 hr for chinchilla); TTS at
asymptote increases with the level of the noise at
about the same rate for man and chinchilla; and it
appears that the subtractive constants, that is the
minimum noise levels that produce TTS, are higher
for man than chinchilla (10 - 25 dB)."
A12.2 Continuous exposure to noise; estimated growth and decay
of threshold shifts in human subjects
Based upon available data Miller (1971) has provided an
impression of the quantitative facts of the growth and decay
of threshold shifts caused by long exposures to noise (Figures
A12-1 and A12-2). Many of the exposure conditions shown in
A12-3
-------
Figures A12-1 and A12-2 can not be used with human subjects
for ethical reasons. Other conditions can be used with human
subjects but as yet the necessary data are not available.
Figures A12-1 and A12-2 are probably the best estimates cur-
rently available of the auditory effects of continuous expos-
ure to noise in human subjects (see Miller, 1971, pages, 19-
23).
Al2.3 References
Benitez,L. D., Eldredge, D.H., and Templer, J. W. (1972).
n Temporary Threshold Shifts in Chinchilla: Electro-
physiological Correlates," J. Ac oust, Soc. Amer«
£2, 1115-1123 "
Botsford, J. H. (1971). " Theory of Temporary Threshold
Shift," JT. Ac oust* Soc» Amer. i|-9>
Carder, H.M., and Miller, J. D. (1972). " Temporary Threshold
Shifts from Prolonged Exposure to Noise, " J. Speech
Hear. Res. l£, 603-623.
Davis, H., Morgan, C.T., Hawkins, J.E., Galambos, R., and
Smith, P. (1950). Tt Temporary Deafness Following
Exposure to Loud Tones and Noise, " Acta Oto-
Laryngol. 88, £7 pp.
Eldredge. D. H., Mills, J.H., and Bohne, B. A. ( 1972).
" Anatomical, Behavioral, and Electrophysiological
Observations on Chinchillas after Long Exposures
to Noise," Internat. Symp. Otophyslol. (Karger,
White Plains, N.Y., in press).
Melnick, W. (1972). " Asymptotic Temporary Threshold
Shift from 16 hours of Continuous Exposure,"
ASHA Ik, No.9, 14.76. (A).
Miller, J.D. (1971). Effects of Noise on geopj-6* u- s*
vironmental Protection Agency, ~"N*TID30O. 7, 153 pp.
Miller, J.D., Rothenberg, S.J., and Eldredge, D.H. (1971).
" Preliminary Observations on the Effects of
Exposure to Noise for Seven Days on the Hearing
and Inner Ear of the Chinchilla, " £0, 1199-1203 .
( £. Acoust* Soc . Amer . )
Miller. J. D., Watson, C.S., and Covell, W. (1963) n Deaf-
ening Effects of Noise on the Cat," Acta Oto-Larynp:ol.
Suppl. 176, 91 pp.
A12-4
-------
Mills, J. H, (1973)* M Temporary and Permanent Threshold
Shifts Produced by Nine- Day Exposures to Noiser"
J7. Speech Hear. Res* { submitted).
Mills, J.H., Talo, S.A., and Gordon, G.S. (1973). " Decay of
Temporary Threshold Shift in Noise," £. Speech* Hear*
Res* ( submitted). "~
Mills, J.H., and Talo, S.A. ( 1972). " Temporary Threshold
Shifts Produced by Exposure to High-Frequency Noise,"
J. Speech.Hear. Res. 15, 62l].-631.
Mills, J.H., Gengel, R.W., Watson, C.S., and Miller, J.D.(1970).
"Temporary Changes of the Auditory System Due to Ex-
posure to Noise for One or Two Days," J. Ac oust. Soc.
Amer. ij.8, 52l|.-53 0. ~~
Mosko, J.D., Fletcher, J.L., and Luz, G.A. (1970). "Growth
and Recovery of Temporary Threshold Shifts following
Extended Exposure to High-Level, Continuous Noise,"
Port Ehox, Ky.: U. S. AMRL Rept. 911.
Ward, W.D. (1970). " Temporary Threshold Shift and Damage-
Risk Criteria for Intermittent Noise Exposure,"
£* AC oust. Soc. Amer. 1^.8, 56l-57ii-«
Ward, \7.D. (1960). " Recovery from High Values of Temporary
Threshold Shift," J. Acoust. Soc. Amer. 32, 1^97-^00.
Ward, W.D., Glorig, A., and Sklar, D.L. (1959) " Temporary
Threshold Shift from Octave-B'hid Noise: Applications
to Damage-Risk Criteria," J. Acoust. Soc. Amer.
31, 522-528. -
A12-5
-------
120
M
30 50 80
100
10K
TIME IN NOISE ( MINUTES )
Figure A12-1* Hypothetical growth of threshold shift after various single
and continuous exposures to noise. These curves represent predictions
for an average, normally-hearing young adult exposed to a band of noise
or pure tone centered near 4000 hertz. These are "worst-case" condi-
tions as the ear is most susceptible to noise in this region. These
hypothetical curves were drawn to be consistent with current facts and
theory. They are for an average ear; wide differences among individuals
can be expected. In many cases extrapolations had to be made from appro-
priately corrected data from animals (cats and chinchillas). The data
points are from Ward, Glorig, and Sklar (l959a). Other relevant data car,
be found in papers by Botsford (1971)T Carder and Miller (In press),
Davis et ail. (1950), Miller et al. (1963), Miller et al. (1971),
Mills et al. (1970), Mosko et al. (1970), and Ward~Tl960, 1970b).
A12-6
-------
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** ^ ^^^ 10 dB
i i i i i i i i i i i i i i i ti i 1 1 i
-
-
1
1/41/2 1 2 4 8 12 16 1 23451 2 3 4 56 8 10
1 HOURS " DAYS " WEEKS
, , I , , . , , ,,,! , 1 , , , i , ...1 ,,
i
1
8 20 40 80 200 400 800
10 100 IK
2K 4K 8K 20K 40K 80K 200K
10K 100K
TIME AFTER NOISE ( MINUTES
Figure A12-2.
Hypothetical decay of threshold shift
(see legend for Figure A12-1) .
A12-7
-------
BIBLIOGRAPHY
B.I Arrangement of Bibliography
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zero for audiometers. Jacoust Soc Amer, 44.
264-275.
Wernick, J S & Tobias, J V (1963). Central factor in pure-
tone auditory fatigue. Jacoust Soc Amer, 35,
1967-1971.
Wright, H N (1959). Auditory adaptation in noise. Jacoust
Soc Amer, 31, 1004-1012.
Yeowart, N S, Bryan, M E & Tempest, W (1967). The monaural
M.A.P. threshold of hearing at frequencies from
1.5 to 100 c/s. J Sound Vib, 6_, 335-342.
Yeowart, N S, Bryan, M E & Tempest, W (1969) . Low-frequency
noise thresholds. J Sound Vib, 9_, 447-453.
Zwislocki, J J & Hellman, R P (1967). Comments on "Use of
sensation level in measurements of loudness and
of temporary threshold shift" and on courtesy in
writing. J acoust Soc Amer, 41, 714-715.
Zwislocki, J, Maire, F, Feldman, A S & Rubin, H (1958).
On the effect of practice and motivation on the
threshold of audibility. J acoust Soc Amer, 30,
254-262.
B.3 Relevant standards
B.3.1 American National Standards Institute (ANSI*).
American standard criteria for background noise in audiometer
rooms. ASA S3.1-1960.
American national standard preferred reference quantities for
acoustical levels. ANSI SI.8-1969.
American national standard specification for audiometers.
ANSI S3.6-1969.
B.3.2 International Electrotechnical Commission (IEC)**.
Pure tone audiometers for general diagnostic purposes. IEC
Publication 177. Geneva: IEC. 1965.
* formerly USASI; previously ASA.
** Publications available from ANSI, New York.
B-28
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Pure tone screening audiometers. IEC Publication 178.
Geneva: IEC. 1965.
Precision sound level meters. IEC Publication 179.
Geneva: IEC. 1965.
Octave, half-octave and third-octave band filters intended
for the analysis of sound and vibration. IEC Publication 225,
Geneva: IEC. 1966.
B.3.3 International Orgarization for Standardization (ISO)*,
Expression of the physical and subjective magnitudes of sound
or noise. ISO Recommendation ISO/R131. Geneva: ISO. 1959.
Normal equal-loudness contours for pure tones and normal
threshold of hearing under free field listening conditions.
ISO Recommendation ISO/R226. Geneva: ISO. 1962.
Preferred frequencies for acoustical measurements. ISO/R266.
Geneva: ISO. 1962.
Power and intensity levels of sound or noise. ISO/R357.
Geneva: ISO. 1963.
Standard reference zero for the calibration of pure-tone
audiometers. ISO/R389. Geneva: ISO. 1964 (with addendum
1 to ISO/R389-1964: Additional data in conjunction with the
9A coupler. 1970).
Assessment of occupational noise exposure for hearing con-
servation purposes. ISO Recommendation ISO/R1999. Geneva:
ISO. 1971.
B.4 Additional references
The following papers were received after completion of
the present report and bibliography. They are relevant to
Appendix 6 and Appendix 7 respectively.
Berry, B F (1973). Ambient noise limits for audiometry.
National Physical Laboratory (England): NPL Acoust-
ics Report Ac60.
Robinson, D W & Shipton, M S (1973). Tables for the estim-
ation of noise-induced hearing loss. National
Physical Laboratory (England): NPL Acoustics Re-
port Ac61.
Publications available on application to American
National Standards Institute, New York.
B-29
oU.S.Government Printing Office: 1973 758-426/77
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