EFFECTS OF NOISE ON PEOPLE
DECEMBER 31, 1971
U.S. Environmental Protection Agency
Washington, D.C. 20460
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EFFECTS OF NOISE ON PEOPLE
DECEMBER 31. 1971
Prepared by
THE CENTRAL INSTITUTE FOR THE DEAF
under
CONTRACT 68-01-05000
for the
U.S. Environmental Protection Agency
Office of Noise Abatement and Control
Washington, D.C. 20460
This report has been approved for general availability. The contents of this
report reflect the views of the contractor, who is responsible for the facts
and the accuracy of the data presented herein, and do not necessarily
reflect the official views or policy of EPA. This report does not constitute
a standard, specification, or regulation.
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PREFACE
This manuscript was prepared by Dr. James D. Miller,
Central Institute for the Deaf, St. Louis, Missouri
and has been reviewed and approved for publication
by the following members of the NAS-NRC Committee
on Hearing, Bioacoustics, and Biomechanics: Hallowell
Davis, Karl D. Kryter, William D. Neff, Wayne Rudmose,
W. Dixon Ward, Harold L. Willima, and Jozef J. Zwislocki,
Of course, the final responsibility for the contents
of this paper lies with Dr. Miller as author.
111
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FOREWORD
It has not been demonstrated that many people have had their lives
shortened by noise. While undoubtedly there have been accidental in-
juries and deaths when auditory warning signals were misunderstood or
not heard because of the effects of noise, the prevalence of these has
not been evaluated. Perhaps the stress of continued exposure to high
levels of noise can produce disease or make one more susceptible to
disease, but the evidence is not convincing. There are only hints of
relations between exposure to noise and the incidence of disease. In
other words, the effects of noise on people have not been successfully
measured in terms of "excess deaths" or "shortened lifespan" or "days
of incapacitating illness." The only well-established effect of noise
on health is that of noise-induced hearing loss.
There is clear evidence to support the following statements about
the effects on people of exposure to noise of sufficient intensity and
duration.
Noise can permanently damage the inner ear with resulting permanent
hearing losses that can range from slight impairment to nearly total
deafness.
Noise can result in temporary hearing losses and repeated exposures
to noise can lead to chronic hearing losses.
Noise can interfere with speech communication and the perception
of other auditory signals.
Noise can disturb sleep.
Noise can be a source of annoyance.
Noise can interfere with the performance of complicated tasks and,
of course, can especially disturb performance when speech communication
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or response to auditory signals is demanded.
Noise and other acoustical considerations can reduce the opportunity
for privacy.
Noise can adversely influence mood and disturb relaxation.
In all of these ways noise can affect the essential nature of human
life—its quality. It is for these reasons that the recitation of facts
and hypotheses that follow may be of some importance.
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TABLE OF CONTENTS
INTRODUCTION 1
PART I. AUDITORY EFFECTS
Preliminary Statement 6
Section 1. EAR DAMAGE AND HEARING LOSS
Introduction 6
A. Ear Damage 7
How ear damage from noise is studied .7
Kinds of ear damage and major findings 9
B. Hearing Loss .15
How hearing loss due to noise is studied 15
Temporary, compound, and permanent
threshold shifts—single exposures. ....16
Noise-induced permanent threshold
shifts—repeated exposures 24
Threshold shifts from impulsive noise 28
C. Implications of Ear Damage and Hearing Loss 33
Interpretation of noise-induced hearing loss 33
Hearing aids and noise-induced hearing loss 38
Presbyacusis and environmental noise go
D. Prevention of Ear Damage and Hearing Loss from Noise 39
Section 2. MASKING AND INTERFERENCE WITH SPEECH COMMUNICATION
Introduction 43
VI
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A. Interference with Speech Communication 44
Speech and understanding speech 44
How speech reception in noise is studied 45
The major effects of noise on speech communication 48
Characteristics of people and speech
interference by noise 52
Situational factors and speech
interference by noise 52
B. Implications of Masking and Interference
with Speech Communication 54
Masking of auditory signals 54
Interference with speech communication 54
PART II. GENERAL PSYCHOLOGICAL AND SOCIOLOGICAL EFFECTS
Preliminary Statement 58
(|ection 3^) INTERFERENCE WITH SLEEP
Introduction 58
A. Methods for Studying Sleep Disturbance by Noise. 60
Field studies 60
Laboratory studies • 60
B. General Properties of Sleep 61
Sleep stages and a night of sleep 61
Sensory responses to stimulation during sleep 65
Arousal. 66
C. Noise and Sleep 66
Effects of brief noises 66
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Motivation to awake and intensity level of the noise 69
Fluctuating noise levels 70
Steady and rhythmic sounds 72
Sound quality and sleep disturbance 73
Sleep deprivation and sleep disturbance 73
Difference between men and women 73
Age and sleep disturbance by noise 74
Sleep stage and accumulated sleep. 75
Stimulus meaning and familiarity 75
Adaptation to sleep disturbance by noise. 75
Other factors 76
D. Noise, Sleep Disturbance, Health, and the Quality of Life.... 77
Section 4. LQUBNESS, PERCEOTSD NOISINESS, AND UNACCEPTABILm
Introduction. 79
A. Measurement of Auditory Dimensions 80
Field studies. 80
Laboratory methods. 81
B. Loudness, Perceived Noisiness, and the
Physical Characteristics of Sounds. 83
Loudness 83
Perceived noisiness (unacceptability) , 84
C. Verbal Descriptions of Sound and Auditory Experience 90
Section 5. ANNOYANCE AND COMMUNITY RESPONSE
Introduction «, 93
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A. How Annoyance and Community Response
to Noise are Studied • 93
Case histories. • • 93
Social surveys 94
B. Acoustical and Situational Factors. 95
Acoustical factors 95
Relations between situational and
acoustical variables 98
Physical measurements of noise exposure 99
\
^'CV Annoyance, Attitudes, and Disruption of Activities 101
\_y
Annoyance and noise 101
Annoyance and attitudes 102
Examples of activities disturbed by aircraft and
traffic noises as measured in social surveys 107
Adaptation to noise 108
D. Community Response 108
E. Concluding Statement 113
OTHER POSSIBLE PSYCHOLOGICAL AND SOCIOLOGICAL EFFECTS
Introduction 117
A. Noise and Performance 118
B. Acoustical Privacy 121
C. Time Judgments 122
D. Effects on Other Senses 122
E. Mental Disorders 123
F. Anxiety and Distress 123
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PART III. GENERAL PHYSIOLOGICAL
Preliminary Statement
Section ?. TRANSIENT AND POSSIBLE PERSISTENT PHYSIOLOGICAL RESPONSES
TO NOISE
A. Transient Physiological Responses to Noise ................... 125
Responses of the voluntary musculature .................... 125
Responses of the smooth muscles and glands ................ 129
Orienting and defense reflexes ............................ 129
Neuro-endocrine responses ..... ........................ .... 130
B. Possible Persistent Physiological Responses to Noise ......... 130
Section 8. STRESS THEORY, HEALTH, AND NOISE
A. Stress Theory ................................................ 133
B. Noise and General Health ..................................... 135
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LIST OF FIGURES
Figure 1. Drawings of the human organ of Corti are shown that
illustrate the normal state, Panel A, and increasing degrees
of noise-induced permanent injury, Panels B, C, and D 10
Figure 2. Photomicrographs of cross-sections of the human organ
of Corti 12
Figure 3- Hypothetical growth of threshold shift after various
single and continuous exposures to noise 20
Figure 4» Hypothetical recovery from threshold shift after
various single and continuous exposures to noise 22
Figure 5« Median noise-induced threshold shifts for jute weavers
with one to over 40 years of occupational exposure to noise
with an A-weighted sound level of about 98 decibels 26
Figure 6. Noise-induced permanent threshold shifts (NIPTS)
plotted against years of occupational exposure to noise
for workers in three levels of noise 27
Figure 7« The distribution of noise-induced threshold shifts
of jute weavers exposed for various numbers of years 29
Figure 8. Idealized pressure waveforms of impulse sounds..... 31
Figure 9- Upper limits of acceptable exposure to impulse noise
as defined by Working Group 57 of the NAS-NRC Committee on
Hearing, Bioacoustics, and Biomechanics 32
Figure 10. Permanent threshold shifts produced by a single
exposure to a firecracker explosion 34
Figure 11. Median hearing loss in habitual sports shooters and
age-matched controls 35
Figure 12. Guideline for the relations between the average
hearing threshold level for 500, 1000, and 2000 hertz and
degree of handicap as defined by the Committee on Hearing
of the American Academy of Ophthalmology and Otolaryngology 36
XI
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Figure 13. The dependence of the accuracy of speech communica-
tion on the relations between the intensity level of the
speech in relation to the intensity level of the noise .....
Figure 14. Quality of speech communication as dependent on the
A-weighted sound level (dBA) of the background noise and
the distance between the talker and listener ................... 49
Figure 1$. Simplified chart that shows the quality of speech
communication in relation to the A-weighted sound level of
noise (dBA) and the distance between the talker and the
listener ........................ ...... ......... ..... ........... 50
Figure 16. The nocturnal sleep pattern of young adults ............. 64
Figure 1?. Awakenings to sound from various laboratory and
questionnaire studies .................................... • ..... 68
Figure 18. Average scores on an annoyance scale for persons
exposed to various levels of aircraft noise ............. . ..... 103
Figure 19. Average annoyance scores for persons exposed to
various levels of traffic noise ....... . ........ . ....... . ...... 104
Figure 20. The relation between community response and noise
exposure ........... . ...................... . .......... ... ...... 109
Figure 21. Relations between community noise levels (measured
in CM or NEF), judgments of unacceptability^ and community
responses ............................ ........ . ..... . .......... no
Figure 22. Differences between the percentages of physiological
problems of those who work in two different levels of noise... 132
XII
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INTRODUCTION
An old riddle asked, "What comes with a carriage and goes with
a carriage, is of no use to the carriage and yet the carriage
cannot move without it?" The answer: "A noise."
And yet (sound) is of great use to us and to all animals.
Many events of nature, whether the meeting of two objects or
the turbulent flow of air, radiate a tiny part of their energy
as pressure waves in the air. A small fraction of the energy
that is scattered enters our ears, and we hear it and thus we
know of the event. Hearing is a late development in evolution
but it has become the sentinel of our senses, always on the
alert.
But hearing does more. The ear and the brain analyze these
sound waves and their patterns in time, and thus we know that
it was a carriage, not footsteps that we heard. What is more,
we can locate the position of the carriage, and tell the direc-
tion in which it is moving.
Many birds and animals have also learned to signal one another
by their voices, both for warning and for recognition. But we
humans, with good ears and also mobile tongues and throats, and
above all, our large complex brains, have learned to talk. We
attach arbitrary and abstract meanings to sounds, and we have
language. We communicate our experiences of the past and also
our ideas and plans for future action. For human beings, then,
the loss of hearing brings special problems and a special tragedy-
... human society creates a special problem even for those
with perfect hearing—the problem of unwanted sound, of noise,
which is as much a hazard of our environment as disease germs or
air pollution.
All of (these subjects) are important. Sounds may
be small and weak, but civilization could not have grown without
them.
(introduction by Hallowell Davis, M.D., to Sound and Hearing,
1965, Time, Inc., courtesy of TIME-LIFE BOOKS.)
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Biologically, man has not changed for many thousands of years.
His responses to sound today rest on the same biological heritage as they
did in the far distant past. The ear, the auditory nervous system, and
the interrelations between the auditory system and the remainder of man's
bodily and behavioral functions developed to meet the demands for adapta-
tion to the environment—the environment of the past.
It is interesting to contrast the visual and auditory systems in
this regard. ¥ith each day there are and have been enormous, sustained
changes in the amount of light at any point on the surface of the earth.
Visual animals that engage in important activities during both day and
night developed visual mechanisms that function, without damage, during
sustained periods that differ greatly in luminance. Daily changes of
luminance equivalent to about 100 decibels have occurred for as long as
the earth has rotated on its axis. The eyes are provided with lids that
can block out light, pupils which vary in size and thus control the
amount of light entering the eyes, and sensory receptors that have mecha-
nisms to alter their sensitivity with these very large changes in luminance.
The situation for sound and hearing is quite different. As the ear
developed it did not need to contend with large daily variations in aver-
age sound levels. Indeed, one imagines that only rarely were intense
sounds sustained for very long periods of time. To be sure, the ear had
to be able to withstand the intense but brief sounds of thunder, the mod-
erately intense sounds of windstorms and sustained rain, but these rarely
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lasted more than a few hours. In general, the evolving ear did not have
to cope with either frequent, very intense sounds or even moderately
intense sounds that were maintained day after day. Only near some beaches,
waterfalls, or areas with sustained winds would moderately intense sound
levels have continued for prolonged periods of time. It is interesting in
this regard that ancient travelers noted that villagers who lived near the
cataracts of the Nile appeared to have hearing loss (Ward, 19?0a).
Hearing evolved to play a role in both individual and social adapta-
tion to the environment. For individual efforts at survival, hearing is
indeed the "sentinel of our senses, always on the alert." By hearing, man
can detect a sound-making object or event, day or night. Often man can
localize the direction of an object or event and sometimes identify it by
its sound alone. To increase the chances of identifying objects or events
and to insure appropriate preparation for response, evolution has closely
tied hearing to man's activating and arousal systems. These systems ener-
gize us. In addition, specific auditory-muscular reflexes cause one to
orient his head and eyes in an appropriate direction to aid recognition
and identification of the sound-making object or event.
Hearing is also involved in social mechanisms of adaptation to the
environment. With our voices and ears we can "communicate our experiences
of the past and also our ideas and plans for future action." In addition,
language, dialect, and manner of speech are important determiners of the
actions and cohesiveness of social groups.
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The close ties of hearing to arousal, muscular actions, and social
relations provide the biological foundations for the mood-influencing
esthetic properties of auditory experience. For hearing not only serves
as an ever-vigilant warning system and as the avenue of speech reception,
but also acts to influence man's moods, feelings of well-being, and
esthetic sensibilities. Many of these responses to sound are culturally
determined and represent learned attitudes, but surely there are biological
bases for development of music with its associated emotional responses along
with the muscular responses of rhythmic movement and dance. Some of these
biological bases stem from adaptative interrelations between the auditory
system and the arousal and muscular systems. Others may be simply acci-
dents of the evolution of the auditory system.
Thus, it is clear that sound is of great value to man. It warns him
of danger and appropriately arouses and activates him. It allows him the
immeasurable advantage of speech and language. It can be beautiful. It
can calm, excite, and it can elicit joy or sorrow. The recent discovery
that five-day-old infants will work to produce a variety of sounds
(Butterfield and Siperstein, 1970) only reinforces our everyday observa-
tions that man enjoys hearing and making sounds.
But not all sound is desirable. Unwanted sound is noise. The defi-
nition of noise includes a value judgment, and for a society to brand
some sounds as noises requires an agreement among the members of that
society. Sometimes such agreements can be achieved readily. Other times
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considerable analysis and debate is required before agreement can be
reached.
For example, while machines are useful and valuable, they often pro-
duce as a by-product too much sound, noise. On the other hand, since
machines can be dangerous, undoubtedly they should make enough sound to
warn us of their approach or of the danger from their rapidly moving,
powerful parts. But how much and what kinds of sound? Also, sounds that
are valuable in one location may travel to places where they may not only
serve no desirable purpose, but they may interfere with and disrupt useful
and desirable activities. Some sounds seem to serve no useful purpose,
anywhere or anytime to anyone. These sounds are unwanted and they clearly
are noises. Other sounds are noises only at certain times, in certain
places, to certain people. It is these complexities that require consid-
erable analysis and thought to enable us to reach agreement about what is
noise and what is not. Scientists and citizens have engaged in such anal-
ysis and thought and some of the results of their efforts are described in
this report.
The effects of noises of such low frequency (infra-sound) or of such
high frequency (ultra-sound) that they cannot be heard by people are not
considered in this paper. Furthermore, this paper is not addressed to
the extent of the noise problem either in terms of the number of people
affected or in terms of the resulting social or economic costs of noise.
Rather it is the relations between the properties of noise and its effects
on people that are presented.
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PART I. AUDITORY EFFECTS
Preliminary Statement
The auditory system is exquisitely sensitive to sound. The acousti-
cal power at the eardrum associated with a sound so loud as to produce
discomfort (120 decibels) is only about 1/10,000 of a watt. The sound
power of the same sound impinging over the entire surface of the body is
of the order of 1.0 watt. Furthermore, the boundary between the skin of
the body and the surrounding air is such that little of the acoustical
power of audible sound is actually transmitted into the body. Even for
very loud sounds only a small amount of acoustical power actually reaches
the body. Therefore, it is not surprising that noise has its most obvious
effects on the ear and hearing since these are especially adapted to be
sensitive to sound.
One set of auditory effects is noticeable after a noise has passed;
these are temporary hearing loss, permanent hearing loss, and permanent
injury to the inner ear. Another set of auditory effects is noticeable
while a noise is present; these are masking and interference with speech
communication. Both of these sets of adverse auditory effects are discus-
sed below.
Section 1. EAR DAMAGE AND HEARING LOSS
Introduction
Exposure to noise of sufficient intensity for long enough periods of
time can produce detrimental changes in the inner ear and seriously
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decrease the ability to hear. Some of these changes are temporary and
last for minutes, hours, or days after the termination of the noise.
After recovery from the temporary effects, there may be residual permanent
effects on the ear and hearing that persist throughout the remainder of
life. Frequent exposures to noise of sufficient intensity and duration
can produce temporary changes that are chronic, though recoverable when
the series of exposures finally ceases. Sometimes, however, these chron-
ically maintained changes in hearing lose their temporary quality and
become permanent.
The changes in hearing that follow sufficiently strong exposure to
noise are complicated. They include distortions of the clarity and quality
of auditory experience as well as losses in the ability to detect sound.
These changes can range from only slight impairment to nearly total deaf-
ness.
A. Ear Damage
How ear damage from noise is studied. Conclusive evidence of the
damaging effects of intense noise on the auditory system has been obtained
from anatomical methods applied to animals. One group of animals is exposed
to noise and a comparable control group is not. After a wait of a few
months, both groups of animals are sacrificed and their inner ears are
prepared for microscopic evaluation. The primary site of injury is found
to be in the receptor organ of the inner ear. Modern quantitative methods
allow an almost exact count of the numbers of missing sensory cells in the
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inner ears of noise-exposed animals. These can be compared to the num-
bers of missing cells in the inner ears of control animals. Other signs
of injury such as changes in the accessory structures of the inner ear
can also be observed.
These anatomical methods are limited for two reasons. The integrity
of crucial structures, such as the connections between the hairs of the
hair cells and the tectorial membrane, cannot be evaluated, and also the
functional properties of cells that are clearly present cannot be assessed.
That is,when a cell is clearly present, the anatomist can only guess at
its functional state. The absent cell is clearly identifiable and the
interpretation of its function is obvious.
The inner ears of human beings have also been examined. Some patients
with terminal illness have volunteered their inner ears to temporal bone
banks. Such specimens are collected at the time of a post-mortem examina-
tion. The anatomist tries to relate the condition of the human ear to the
patient's case history after making allowances for post-mortem changes in
the inner ear and possible pre-mortem changes associated with the terminal
illness or its treatment. In spite of these difficulties, observations
of human cochleas are extremely important and in combination with animal
experiments provide a fairly clear description of the damaging effects of
noise on the inner ear.
Because of the limitations of anatomical methods and the lack of
complete knowledge of the relations between hearing abilities and the
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anatomy of the auditory system, it is not possible to predict completely
the hearing changes from the anatomical changes. However, physiological
observations which include measurement of changes in biochemical state
and electrical responses of the cochlea and auditory nerve help to reveal
the functional changes produced by exposure to noise.
Kinds of ear damage and major findings. The outer ear, eardrum, and
middle ear are almost never damaged by exposure to intense noise. The ear-
drum, however, can be ruptured by extremely intense noise and blasts
(von Gierke, 1965). The primary site of auditory injury from excessive
exposure to noise is the receptor organ of the inner ear. This has been
known for many years, and excellent illustrations of such damage were pub-
lished near the turn of the century (Yoshii, 1909).
The receptor organ of the inner ear is the organ of Corti, and its
normal structure is illustrated in cross-section in Panel A of Figure 1.
Here one can identify the auditory sensory cells (hair cells) and the
auditory nerve fibers attached to them, as well as some of the accessory
structures of the receptor organ. A brief account of the function of the
organ of Corti is as follows. Through a complicated chain of events,
sound at the eardrum results in an up-and-down movement of the basilar
membrane. The hair cells are rigidly fixed in the reticular lamina of
the organ of Corti which in turn is fixed to the basilar membrane. As
the basilar membrane is driven up and down by sound, a shearing movement
is generated between the tectorial membrane and the top of the organ of
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HAIR CELLS
INNER OUTER
TECTORIAL MEMBRANE
RETICULAR
LAMINA
SUPPORTING
CELLS
NERVE FIBERS
(A) NORMAL ORGAN OF CORTI
3 OUTER HAIR CELLS ABSENT
DISTORTED
PILLAR CELL
SWOLLEN
SUPPORTING CELLS
(B) PARTIAL INJURY
COLLAPSE OF ORGAN OF CORTI-
HAIR CELLS ABSENT - ACCESSORY
CELLS SWOLLEN AND DISTORTED
ORGAN OF CORTI ABSENT
NERVE FIBERS REDUCED
IN NUMBER
(C) SEVERE INJURY
NERVE FIBERS ABSENT
(o) TOTAL DEGENERATION
Figure 1. Drawings of the human organ of Corti are shown that illustrate
the normal state, Panel A, and increasing degrees of noise-induced perma-
nent injury, Panels B, C, and D.
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Corti. This movement bends the hairs at the top of the hair cells. This
bending, in turn, causes the hair cells to stimulate the auditory nerve
fibers. As a result, nerve impulses arise in the nerve fibers and travel
to the brain stem. From the brain stem, the nerve impulses are relayed
to various parts of the brain and in some unknown way give rise to auditory
sensations. The point to be made is that the integrity of the sensory
cells and the organ of Corti is important for normal hearing.
Excessive exposure to noise can result in the destruction of hair
cells and collapse or total destruction of sections of the organ of
Corti. In addition, auditory neurons may degenerate. Figure 1 illustrates
these injuries. The injury illustrated in Panel B includes absence
of 3 outer hair cells, distortion of a pillar cell, and swelling of the
supporting cells. In Panel C there is a complete collapse of the organ
of Corti with the absence of hair cells, distortion of the accessory
structures, and a reduction in the number of nerve fibers. This section
of the organ of Corti is almost certainly without auditory function. The
injury shown in Panel D is obvious; there is complete degeneration of the
organ of Corti.
On Figure 2 are shown actual photomicrographs of cross-sections of
the organ of Corti from post-mortem human specimens. These photographs
were provided by Dr. Harold F. Schuknecht of the Massachusetts 3ye and
Ear Infirmary of Boston, Massachusetts. The organ of Corti in Panel A of
Figure 2 is essentially normal and can be compared with the drawing on
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(A)
• *
NORMAL ORGAN OF CORTI
(B)
.
OUTER HAIR CELLS ABSENT
(0
• » . • .
v-v--
• .*. ^
COLLAPSE OF ORGAN OF CORTI
Figure 2. Photomicrographs of cross-sections of the human organ of
Corti are shown: Panel A, normal; Panels B and C, injuries most probably
produced by exposure to noise. Similar injuries have frequently been
seen in experimental animals after exposure to noise. (These photographs
were provided by Dr. Harold F. Schuknecht of the Massachusetts iye and
Zar Infirmary of Boston, Massachusetts.)
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Panel A of Figure 1. Shown on Panel B of Figure 2 is a cross-section of
the organ of Corti from a man who worked for a few years in small compart-
ments of boilers where for prolonged periods of time he was exposed to
the noise of riveting machines. In this cross-section the inner hair ceU
is present but only one outer hair ceU can be seen where one would nor-
mally expect to see four. The example in Panel C is from a man who worked
in the noisy environment of a steel factory. There is collapse of the
organ of Corti with complete absence of normal receptor cells.
The injuries on Figures 1 and 2 are froa selected locations within
the ear. For proper perspective it is important to know that the human
organ of Corti is about 34 millimeters in length with about 395 outer hair
(Bredberg, 196£).
cells and 100 inner hair cells per millimeter/ These total about 17,000.
Thus, the five hair cells shown in a single location represent but a small
fraction of the receptor organ. The magnitude of injury to the inner ear
and the associated hearing loss depend not only on the severity of the
injury at any one location but also on the spread of the injury along the
length of the organ of Corti.
The loss of hearing abilities depends, in a complicated way, on the
extent of the injury along the organ of Corti. Total destruction of the
organ of Corti for one or two millimeters of the total 34 millimeters may
or may not lead to measurable changes in hearing. Recent evidence from
human cases and animal experiments suggests that the loss of sensory cells
must be quite extensive in the upper part of the cochlea (that part which
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is important for the perception of low-frequency sounds) before this damage
is reflected as a change in threshold. In the lower part of the cochlea
(that part which is important for the perception of high-frequency sounds)
losses of sensory cells over a few millimeters are sometimes reflected in
changes in hearing (Bredberg, 1968).
The mechanism by which over-exposure to noise damages the auditory
receptor is not well understood. Very intense noise can mechanically
damage the organ of Corti. Thus, loud impulses such as those associated
with explosions and firing of weapons can result in vibrations of the organ
of Corti that are so severe that some of it is simply torn apart. Other very
severe exposures to noise may cause structural damage that leads to rapid
"break-down" of the processes necessary to maintain the life of the cells
of the organ of Corti. Such an injury is an acoustic trauma.
Over-exposure to noise of lower levels for prolonged periods of time
also results in the degeneration of the hair cells and accessory structures
of the organ of Corti. Such injuries are called noise-induced cochlear
injuries. Many theories have been proposed to explain noise-induced cochlear
injuries. One notion is that constant over-exposure forces the cells to
work at too high a metabolic rate for too long a period of time. As a
result the metabolic processes essential for cellular life become exhausted
or poisoned, and this leads to the death of the cells. In a sense the
receptor cells can die from overwork.
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No matter what theory is eventually found to be correct, certain facts
are established beyond doubt. Excessive exposure to noise leads to the
destruction of the primary auditory receptor cells, the hair cells. There
can be other injuries to the organ of Corti that can range from mild dis-
tortion of its structure to collapse or complete degeneration. The auditory
neurons may also degenerate. All of these cells are highly specialized.
Once these cells are destroyed, they do not regenerate and cannot be stim-
ulated to regenerate? they are lost forever.
B. Hearing Loss
How hearing loss due to noise is studied. Experiments on hearing loss
are sometimes done with animals because one would not deliberately deafen
a human subject. For these experiments it is necessary to train the animal
subjects so that their ability to detect faint tones can be measured. The
measure of this ability is the intensity level of the faintest tone that
can be detected. This is called the hearing threshold level. The greater
the hearing threshold level, the poorer the ability to hear. The hearing
thresholds of trained animals are measured by methods similar to those used
with human patients. After the animal's normal thresholds have been
measured, it is exposed to noise under controlled laboratory conditions.
After the cessation of the noise, changes in the animal's thresholds are
measured. Subsequently, its ears are evaluated by physiological and
anatomical methods.
Experiments with human subjects are limited to exposures to sound
that produce only temporary changes in the hearing mechanism. In such
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experiments, measures of some auditory capability are made prior to ex-
posure and also at Tarious specified times after its termination. One
of the advantages of laboratory studies is the fact that precise measures
of hearing are made before and after exposures to a noise whose properties
are exactly known.
Measurements of the effects of noise on human hearing are also
collected in field and clinical case studies. These data are subject
to considerable error, but several well-done field studies have been
completed or are now in progress. Threshold measurements are made on
persons who are regularly exposed to noise. These exposures usually
occur in an occupational setting. Noise levels are measured and the
progress of hearing thresholds is followed. While it is true that the
actual occupational exposures vary from day to day and moment to moment
within a day, some rather clear trends emerge when a sufficient number
of persons are carefully studied. For comparison, similar measurements
are made on persons whose life patterns include very little exposure to
noise.
Well-done studies of individual patients in the clinic have suggested
hypotheses and have also been an important source of data.
Temporary, compound, and permanent threshold shifts—single exposures.
The primary measure of hearing loss is the hearing threshold level. The
hearing threshold level is the level of a tone that can just be detected.
16
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The greater the hearing threshold level, the greater the degree of hearing
loss or partial deafness. An increase in a hearing threshold level that
results from exposure to noise is called a threshold shift.
Some threshold shifts are temporary and they diminish as the ear
recovers after the termination of the noise. Frequently-repeated exposures
can produce temporary threshold shifts that are chronic though recoverable
when the exposures cease. When a threshold shift is a mixture of temporary
and permanent components, it is a compound threshold shift. When the tem-
porary components of a compound threshold shift have disappeared (that is,
when the ear has recovered as much as it ever will), the remaining thresh-
old shift is permanent. Permanent threshold shifts persist throughout the
remainder of life.
Temporary threshold shifts can vary in magnitude from a change in
hearing sensitivity of a few decibels restricted to a narrow region of
frequencies (pitches) to shifts of such extent and magnitude that the ear
is temporarily, for all practical purposes, deaf. After cessation of an
exposure, the time for hearing sensitivity to return to near-normal values
can vary from a few hours to two or three weeks. In spite of efforts in
many laboratories, the laws of temporary threshold shifts have not yet been
completely determined. There are large numbers of variables that need to
be explored. Also, there are probably several different underlying
17
-------
processes that influence the measured threshold shifts. It may be nec-
essary to sort out the influence of each of these underlying processes
before the laws of noise-induced temporary threshold shifts will be com-
pletely understood.
Nonetheless, certain generalizations seem to be correct (Ward, 1963).
Noises with energy concentrations between about 2000 and 6000 hertz
probably produce greater temporary threshold shifts than noises concen-
trated elsewhere in the audible range. In general, A-weighted sound levels
must exceed 60-80 decibels before a typical person will experience tempo-
rary threshold shifts even for exposures that last as long as 8-16 hours.
All other things being equal, the greater the intensity level above 60-80
decibels and the longer the time in noise, the greater the temporary
threshold shift. However, exposure durations beyond 8-16 hours may not
produce further increase in the magnitude of the shift (Mills et al.,
1970; Mosko et_ al., 1970). It is also an interesting property of temporary
threshold shifts that such shifts are usually greatest for test tones 1/2-1
octave above the frequency region in which the noise that produces the shift
has its greatest concentration of energy. Finally, there is less temporary
shift when an exposure has frequent interruptions than when an exposure is
continuous.
People differ in their susceptibility to temporary threshold shifts.
Unfortunately, these differences in susceptibility are not uniform across
18
-------
the audible range of frequencies (pitches). Indeed, one person may be
especially susceptible to noises of low pitch, another to noises of
medium pitch, and another to noises of high pitch. In general, women
appear to be less susceptible to temporary threshold shifts from low-
frequency noises than are men, and this relation is reversed for high-
frequency noises (Ward, 1966; Ward, 1968a).
An impression of the quantitative facts of temporary threshold
shifts can be obtained from Figures 3 and 4- All of the dashed lines
indicate extrapolations based on current research. While it is likely
that the general trends shown on these figures will be verified by addi-
tional research, the exact values cannot be expected to be accurate.
For short durations of exposures to high intensities there may even be
some changes in the rank ordering of the initial segments of curves.
Nonetheless, these graphs provide an adequate summary of reasonable ex-
trapolations of available data.
Consider Figure 3. The time in noise is plotted along the horizontal
axis, while the amount of threshold shift measured in decibels at two
minutes after the cessation of the exposure is plotted on the vertical
axis. These curves represent probably the worst possible situation in
that the noise is in the region, 2400-4800 hertz, to which the ear is
most susceptible, and the test tone is at 4000 hertz where threshold
shifts are often large. Certain facts are obvious from the graph. The
19
-------
120
M
LLJ
110
O
O
100
UJ 90-3
(A
O on
O. 8O
X
70 -
to 60
UJ
SO
CJ
a 40
UJ
a:
to 30
LU
to
o
X
to
LU
CC.
X
20
10
HYPOTHETICAL GROWTH OF THRESHOLD
SHIFT MEASURED 2 MINUTES AFTER
A SINGLE CONTINUOUS EXPOSURE
TO NOISE
PARAMETER-- LEVEL OF NOISE -
K
O
z
o
o
UJ
cr
t
30 45 1
8 12
SECONDS
MINUTES
I
HOURS
DAYS
0.3 0.5 0.8
5 8
10
30 50 80
100
300 500 800
IK
3K 5K 8K
10K
TIME IN NOISE ( MINUTES )
Figure 3. 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 can
be found in papers by Botsford (1971), Carder and Miller (in press).
Davis et al. (1950), Miller et al. (1963), Miller et al. (1971),
Mills et al. (1970), Mosko et al. (1970), and Ward~Tl9o~0, 1970b).
20
-------
more intense the noise, the more rapidly threshold shifts accumulate as
the time in noise is extended. When the noise is only 65 decibels, a
typical person has to be exposed for several hours before any significant
threshold shift can be detected. However, when the noise is very intense,
say 130 decibels, a typical person exposed for only 5 minutes reaches
dangerous levels of threshold shift. Notice that the combinations of
intensity level and duration that produce threshold shifts greater than
about 40 decibels are said to be in the region of possible acoustic trauma.
In this region, for some people, the normal processes of the ear may
"break down" and permanent threshold shifts—hearing loss—may result from
even a single exposure to noise. Remember, however, that these relations
are for the worst possible situation where the noise is concentrated in
the region from 2400 to 4800 hertz. While exposures to other noises lead
to qualitatively similar changes in hearing thresholds and to similar
risks, the quantitative relations (even when the noise is measured in
A-weighted sound level) may be different.
Recovery from threshold shifts after the cessation of an exposure to
noise depends on a variety of factors and is not completely understood.
Sometimes recovery from a threshold shift is complete in 50 or 100 minutes.
Such rapid recovery from a threshold shift has been observed when the
threshold shift is small, less than 40 decibels, and the duration of the
exposure is short, less than B hours (Ward, e_t al., 1959a). Less rapid
recovery from threshold shifts is illustrated on Figure 4« The straight
21
-------
120
110
100
90
UJ
CD
a so
o
z
70
N
X
o
8 60
t 50
X
"> 40
Q
O
X 30
tn
UJ
cc.
H 20
10
, 1 | | ||||| 1 1 1 | 1 "1 ' ' ' ' ""1 '
HYPOTHETICAL RECOVERY AFTER A
SINGLE CONTINUOUS EXPOSURE TO ~
NOISE
„„.„_ PARAMETER: LEVEL AND DURATION
120 dB, 3 DAYS
ION OF POSSIBLE ACOUSTIC TRAUMA
1 a:
:t
•
100 dB, 3 DAYS \
"^X \
90 dB, 3 DAYS \ \
-x \ V-l
80 dB, 3 DAYS "\ \
'"''••Pi '*, ~"~~"*^. \ \ PERMANE
'••** X \ \ THRESHOLD
'".. N \ \ 40 dB
70 dB, 3 DAYS '''••. 4 \ \ \
*"**~". — --- SN>\ v
-------
dotted line indicates the course of recovery from a threshold shift that
has often been assumed (Ward et al., 1959a» 1959bj Kryter et al., 1966). The
data points (filled circles) represent the decline of threshold shift after
an exposure at 95 decibels for 102 minutes as actually measured for human
listeners. The accuracy of the extrapolation of the dotted lines beyond
the data points is unknown. Clearly, however, recovery from the exposure
of 95 decibels for 102 minutes (dotted line) is more rapid than recovery
from the exposures for 3 days (dashed lines).
The slow recovery from noise-induced threshold shifts illustrated on
Figure 4 by the dashed lines probably holds whenever the exposure is severe
either in terms of the total duration or in terms of the amount of thresh-
old shift present a few minutes after the termination of the noise.
Recovery from temporary threshold shift appears to be very slow when the
initial threshold shift exceeds 35-45 decibels (Ward, I960), when the expo-
sure lasts as long as about 12 hours (Mills et al., 1970; Mosko et al.,
1970), or after some long but intermittent exposures to noise (Ward, 1970b).
For example, it has been shown that exposure to a noise with an A-weighted
sound level of about 80 decibels for two days results in small temporary
threshold shifts that do not completely disappear for several days (Mills
et al., 1970).
Very severe exposures to noise can produce compound threshold shifts
from which complete recovery is impossible. After recovery from the tem-
23
-------
porary component of a compound threshold shift, there remains a permanent
threshold shift. Some examples are shown on Figure 4. The ear's recovery
from compound threshold shifts is often quite slow and this recovery prob-
ably represents a "healing" process. There can be no additional recovery
(healing) beyond two to twelve weeks after an exposure (Miller et al.. 1963).
Noise-induced permanent threshold shifts—repeated exposures. Some-
times people encounter single exposures to steady noises that produce
permanent threshold shifts. This only happens rarely as people usually
will not tolerate such severe exposures (see Figures 3 and 4)»
More commonly, noise-induced permanent threshold shifts accumulate
as exposures are repeated on a near-daily basis over a period of many
years. The best examples of such cases are from field studies of occupa-
tional deafness.
An unusually thorough study was done of jute weavers (Taylor et al.,
1965). These weavers were all women with little exposure to noise other
than that received on the job. The noise exposures had been nearly constant
in the mills for almost 52 years, and employees who had worked in the mills
for 1-52 years were available for testing. All audiometry (measurement
of hearing thresholds) was done with a properly calibrated instrument by
a trained physician. Hearing thresholds were measured after a weekend
away from the noise. This means that about 2-1/2 days of recovery were
allowed and probably only a small recoverable component remained in the
measured threshold shift (see Figure 4). Since the noise in the mill had
24
-------
an A-weighted sound level of about 98 decibels, a working-day exposure of
eight hours would be expected to produce 35-65 decibels of temporary thresh-
old shift in a typical, young adult female for a test tone of 4000 hertz.
In 2-1/2 days, this threshold shift would be expected to decay to within
about five decibels of normal (see Figures 3 and 4). Of course, wide
variations can be expected. What happens when such an exposure is repeated
about five days a week, 50 weeks a year, year after year? The results are
shown on Figure 5- These thresholds are typical for the jute weavers and
the expected changes with age have been subtracted.
Evidently, as the exposures are repeated year after year, the ear
becomes less and less able to recover from the temporary threshold shift
present at the end of each day. It also seems likely that as the exposures
are repeated, the amount of threshold shift present at the end of each
day's work might creep upward toward the asymptote appropriate to the level
of the noise as indicated on Figure 3«
In any case, as the exposures are repeated, the noise-induced tem-
porary threshold shifts become permanent or nearly so. It is also signif-
icant that on weekdays there are only 16 hours of recovery between work
exposures. Therefore, from the first day of employment, most of these
weavers will be living with a chronic threshold shift of 25-55 decibels
at 4000 hertz (see Figures 3 and 4). Only on Saturday and Sunday will
their hearing be near normal even during the first year of employment.
25
-------
CD
I
tn y=
5*
z">
00
< UJ
Q U
^ =>
2 Q
5O
40
3O
20
IO
-IO
4000 Hz
/ 3000 Hz
2000 Hz
1000 Hz
10
20
30
40
50
EXPOSURE - YEARS
Figure 5. Median noise-induced threshold shifts for jute weavers with
one to over 40 years of occupational exposure to noise with an A-weighted
sound level of about 98 decibels. These threshold shifts have been
corrected for the expected changes in thresholds with age in persons who
are not exposed to noise. (From Taylor ej, al.} 1965, with permission of
the authors and the Journal of the Acoustical Society of America*)
As the years roll by, these jute weavers become partially deaf even on the
weekends.
Similar data have been gathered on male workers in noisy industries
in the United States (Nixon and Glorig, 196l). Age-corrected threshold
shifts at 4000 hertz are shown for these workers on Figure 6. The average
26
-------
50
N
040
O
CD
O
(ft
Q.
Z
10
I I I I ' '"I
20-
I I
1 ml
i i i i
2 345
EXPOSURE
10
TIME
20 50
IN YEARS
100
Figure 6. Noise-induced permanent threshold shifts (NIPTS) plotted
against years of occupational exposure to noise for workers in three
levels of noise. These threshold shifts have been corrected for the
changes with age found in persons without occupational exposure to noise.
The graphs are for a test tone of 4000 hertz and the data points are
medians. The average A-weighted sound levels were 83 decibels for
group A, 92 decibels for group B, and 97 decibels for group C.
(Reprinted from Nixon and Glorig, 1961, with permission of the authors
and the Journal of the Acoustical Society of America.)
A-weighted noise levels for the workers in environments A, B, and C were
about 83, 92, and 97 decibels, respectively. Presumably, most of the
threshold shifts were measured 2-1/2 days after the last workday and prob-
ably contain temporary components of less than 7-10 decibels.
27
-------
The important points to notice on Figure 6 are: (a) there is an
orderly relation between the median amount of noise-induced threshold
shift and the intensity level of the noise; and (b) the amount of threshold
shift at 4000 hertz from these occupational exposures shows no further
increase after about ten years of exposure although the threshold shifts
for lower frequencies (not shown) continue to increase.
The results shown on Figures 5 and 6 are medians. These orderly
trends do not reflect the large differences among individual ears in
susceptibility to noise-induced hearing loss. In fact, within a group
of similarly exposed people some will exhibit very large threshold shifts
while others will exhibit only small threshold shifts. The extent of
these differences is shown on Figure 7« Some of the differences between
similarly exposed people are due to differences in susceptibility to noise,
and some are due to actual differences in the noise levels encountered. In
and time,
an industrial situation the measurement of noise is an average over space/
and, therefore, all workers do not necessarily receive the same exposure.
Threshold shifts, from impulsive noise. Intense impulsive noise can
be particularly hazardous to hearing. The reason is that in addition to
the processes involved in noise-induced threshold shifts there is the
added risk of a "breakdown" in the inner ear. Permanent threshold shift
due to acoustic trauma may result. Since an acoustical impulse may con-
tain only a small amount of total energy because of its limited duration,
28
-------
2000 Hz
cc
<
yj
LL
O
LJ
O
o:
LJ
o.
20
IO
O
2O
!O
O
2O
10
O
20
10
o
20
IOH
o
20-
IO-
O
n
n
15-19YR
N=32
2O-24YR
N=39
25-29YR
N=4O
3O-34YR
N=27
35-39YR
N=32
4O-52YR
• If T * " T I * I
O IO2O3O4O5O6O7O8OdB
ESTIMATED NOISE-INDUCED
THRESHOLD SHIFT
Figure 7. The distribution of noise-induced threshold shifts of jute
weavers exposed for various numbers of years (parameter). The test fre-
quency is 2000 hertz. Notice the large differences among people with
regard to the effects of noise on the magnitude of the threshold shift.
(Reprinted from Taylor et al., 1965 f with the permission of the authors
and the Journal, of the Acoustical Society of America.)
29
-------
the predicted threshold shift might be small. At the same time, a single
impulse because of its high amplitude might rip or tear a crucial tissue
barrier (say the reticular lamina which protects the hair cells and nerves
from the fluids of scala media) and a considerable degeneration of the
organ of Corti may result. Therefore, it is unlikely that description
of impulsive noise in terms of equivalent spectrum and energy of "steady
sounds" will be successful in predicting the enormous variability in
response to impulses with high peak levels. With these impulses occasional
cases of sudden severe hearing loss are observed, and these can be explained
in terms of direct mechanical injury. It may be possible that expressing
impulses in terms of equivalent spectrum and energy with steady sounds
may be successful in predicting median trends (Kryter, 1970).
When a gun is fired or a hammer strikes metal, very large peak sound
pressures may be generated at the eardrum. To follow the time course of
an impulse accurately, one records the output of a good microphone on an
oscilloscope. Idealized waveforms of impulse noises are shown on Figure 8.
On Figure 9 are shown the combinations of peak sound pressure and duration
that can be allowed if as many as 100 impulses were delivered to the ear
over a period of four minutes to several hours each day. It is presumed
that only 5% of the persons receiving a criterion exposure would have tem-
porary threshold shifts that exceed ten decibels at 1000 hertz or below,
15 decibels at 2000 hertz, or 20 decibels at 3000 hertz or above. Details
30
-------
to
_l
ID
QL
LU
CC
D
co
CO
LU
CC
Q_
co
ID
O
f
CO
(a)
B =
1 2
TIME
Figure 8. Idealized pressure waveforms of impulse sounds. On line (a)
is shown a single, well-damped impulse. Its duration is taken as the time
period indicated by the letter A. On line (b) is shown an impulse that
has several oscillations. Also shown is a single reflection. Its dura-
tion is taken as the time period B » bi + ... + bn. The amplitudes of
both types of impulse noises is taken as the peak value, P, expressed in
decibels. For more details see Ward (I968b).
31
-------
-------
of these criteria and their derivation can be found elsewhere (Ward,
1968b).
Samples of permanent threshold shifts produced by a single fire-
cracker explosion or the repeated firing of guns are shown on Figures
10 and 11.
C. Implications of Sar Damage and Hearing Loss
Interpretation ofnoise-induced hearing loss. There has been and
continues to be considerable debate about the implications and signifi-
cance of small amounts of ear damage and hearing loss. The most recent
statement of the Committee on Hearing of the American Academy of
Ophthalmology and Otolaryngology on Hearing Handicap is given on Figure 12.
Prior to 1965, this group had used the terms hearing impairment, hearing
handicap, and hearing disability almost synonymously and in accordance
with the categories displayed in Figure 12.
In 1965, this committee offered these definitions of terms related
to hearing loss. Hearing Impairment; a deviation or change for the worse
in either structure or function, usually outside the normal range. Hearing
Handicap: the disadvantage imposed by an impairment sufficient to affect
one's efficiency in the situation of everyday living. Hearing Disability;
actual or presumed inability to remain employed at full wages.
By these definitions, any injury to the ear or any change in a hearing
threshold level that places it outside of the normal range constitutes a
33
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FREQUENCY OF TEST TONE IN HERTZ
250 500 1000 2000 4000 8000
10
LU
59 20
o
UJ
5 30
40
o
I 50
cr
I
60
70
80
TIME AFTER ACCIDENT
1 WEEK
•—• 1 MONTH
X x 2 YEARS
HEARING LOSS FROM
FIRECRACKER EXPLOSION
NEAR THE EAR
250
500
1000
2000
4000 8000
Figure 10. Permanent threshold shifts produced by a single exposure to
a firecracker explosion. The change in hearing is shown by the difference
between the thresholds taken before and after the accident. The firecracker
was an ordinary flashlight cracker about two inches in length and 3/16
inch in diameter. It was about 15 inches from the patient*s right ear
when it exploded. (After Ward and Glorig, 1961, with the permission of
the authors and Laryngoscope.)
34
-------
HUNTERS N«3Z
OL GROUP N'9
70
50
FREQUENCY
TONE IN
4000 6000
HERTZ
8000
Figure 11. Median hearing loss in habitual sports shooters and age-matched
controls. (From Taylor and Williams, 1966, with the permission of the
authors and Laryngoscope.)
35
-------
CLASS
A
B
C
D
E
F
DEGREE OF
HANDICAP
Not significant
Slight Handicap
Mild Handicap
Marked Handicap
Severe Handicap
Extreme Handicap
AVERAGE HEARING
THRESHOLD LEVEL FOR
500, 1000 AND 2000 Hz
IN THE BETTER EAR
MORE THAN
25 dB
40 dB
55 dB
70 dB
90 dB
NOT
MORE THAN
25 dB
40 dB
55 dB
70 dB
90 dB
ABILITY TO
UNDERSTAND SPEECH
No significant difficulty
with faint speech
Difficulty only with
faint speech
Frequent difficulty with
normal speech
Frequent difficulty with
loud speech
Can understand only
shouted or amplified speech
Usually cannot understand
even amplified speech
Figure 12, Guideline for the relations between the average hearing
threshold level for 500, 1000, and 2000 hertz and degree of handicap
as defined by the Committee on Hearing of the American Academy of
Ophthalmology and Otolaryngology. (From Davis, 1965, with the permission
of the author and the Transactions of the American Academy of Ophthalmology
and Otolaryngology.)
36
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hearing impairment* Whether a particular impairment constitutes a hearing
handicap or a hearing disability can only be judged in relation to an
individual's life pattern or occupation.
The guideline for the evaluation of hearing handicap shown on Figure 12
uses only thresholds for tones in the region most important for the recep-
tion of speech (500, 1000, and 2000 hertz), and judgments of handicap are
based on the associated ability to understand connected speech in quiet
surroundings. While most authorities agree that a person in Category B
or worse has a hearing handicap, there is debate over whether handicap
exists when a person in Category A also has large hearing threshold levels
above 2000 hertz.
Examples of audiograms that would fall into Category A and also exhibit
levels
large hearing threshold/ above 2000 hertz are shown on Figures 10 and 11.
Notice that the guideline of Figure 12 indicates that such audiograms do
not represent a significant handicap. Those who question the guideline
of Figure 12 rally certain facts. For example, some individuals with
sizable hearing threshold levels above 2000 hertz may experience consider-
able difficulty in understanding speech in moderate levels of background
noise even though their average hearing threshold levels at 500, 1000,
and 2000 hertz do not exceed 25 decibels (Niemeyer, 1967). Also, persons
with hearing loss primarily above 2000 hertz may not be able to distinguish
the sounds of certain consonants. Sometimes hearing loss above 2000 hertz
37
-------
may be especially important to a person; for example, piccolo players or
specialists concerned with bird song may experience handicap whereas many
others might not.
More generally, individuals will react differently to a hearing loss.
One may be particularly upset by his inability to understand his children;
another may feel handicapped by his inability to participate in rapid
verbal patter; and others may miss the sounds of music or those of nature.
There is little room for controversy over the question of handicap
when losses become as severe as those of Category C of Figure 12. Persons
with losses this severe or worse are aware that they have lost part or all
of a precious gift.
Hearing aids and[ noise-induced hearing loss. People with partial
deafness from exposure to noise do not live in an auditory world that is
simply "muffled." Even those sounds that are heard may be distorted in
loudness, pitch, apparent location, or clarity. While a hearing aid some-
times can be useful to a person with noise-induced hearing loss, the result
is not always satisfactory. The modern hearing aid can amplify sound and
make it audible, but it cannot correct for the distortions that often
accompany injury to the organ of Corti.
Presbyacusis and environmental noise. With age, people almost
uniformly experience increasing difficulty in understanding speech.
Undoubtedly, some of this loss is due to the degeneration of neurons in
38
-------
the brain which generally accompanies advancing age. Some of this loss
is due to changes in middle or inner ears. Some of the changes in the
inner ear are due to normal aging processes} some are undoubtedly due to
toxic'drugsj some are due to disease processes; and some are due to inci-
dental, recreational, and occupational exposures to noise. Clear evidence
is available that noises with A-weighted sound levels,above 80 decibels
can contribute to inner ear damage and eventual heardjng handicap if such
noises are frequently and regularly encountered. Beyond this, the evidence
does not warrant stronger statements about the role of noise in progressive
hearing loss with age. Theoretical grounds do suggest that frequent expo-
sures of sufficient duration to noises with A-weighted sound levels greater
than 70-&0 decibels could contribute to the "normal loss of hearing with
age."
At least some aspects of hearing loss with age seem to add to hearing
loss from noise exposure (Glorig and Davis, 196l). This means that a small
loss of hearing from exposure to noise may be insignificant when one is
middle-aged, but might, when combined with other losses due to age, become
significant * as one reaches an advanced age.
D. Prevention of Ear Damage and Hearing Loss from Noise •
Hearing loss and ear damage due to noise can be eliminated if
exposures to noise are: (l) held to sufficiently low levels; (2) held to
sufficiently short durations; or (3) allowed to occur only rarely.
39
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The regulation of the acoustic environment in such a way that hearing
loss and ear damage from noise are eliminated poses several problems. For
example, the chances that a person will develop a hearing impairment due
to noise depends on the pattern of exposure from all sources of noise that
he happens to encounter. Some of these exposures from particular sources may
be innocuous in isolation. But these same noises, which are innocuous by
themselves, may combine with noises from other sources to form a total sequence
of noises sufficient to produce hearing impairment (Cohen et al., 1970).
While it may be possible to control the total exposure in an occupational
setting during a day's work, it is nearly impossible to control an individual's
activities and exposure to noise while he is away from work. Thus, one must
turn to the regulation of sources of noise.
In general, any source with an A-weighted sound level of 70-80 decibels
has the potential to contribute to a pattern of exposure that might produce
temporary threshold shifts (see Figure 3) and this could lead to permanent
hearing impairment. Therefore, it seems desirable to have as few sources
as possible that expose people to A-weighted sound levels in excess of 70-80
decibels. But people can tolerate many brief exposures in excess of 70-80
decibels if they are widely spaced in time. For example, a shower bath may
have an A-weighted sound level of about 74 decibels, but one would have to
shower for over an hour before a temporary threshold shift would appear
40
-------
(see Figure 3). Clearly, regulation must not eliminate all sources of noise
with A-weighted sound levels in excess of 70-80 decibels. On the other
hand, if such sources are allowed to proliferate without bound, then vast
numbers of persons will suffer chronic threshold shifts.
Sources with A-weighted sound levels in excess of 80 decibels have the
potential to contribute to the incidence of hearing handicap. The argument
about regulation of such sources runs exactly parallel to that of the pre-
vious paragraph.
Finally, from studies of hearing loss from occupational exposures to
noise, one can identify exposures that, in and of themselves, increase the
incidence of hearing handicap (Kryter et al., 1966; Radcliff, 1970). Sources
that provide exposures as severe as these should be avoided, eliminated, or
controlled.
Part of the problem of the evaluation of hearing hazard from various
sources of noise is this. While knowledge has accumulated about the effects
of schedules of noise exposure such as those encountered in the occupational
setting, very much less is known about the effects of other, irregular
schedules such as those associated with occasional use of home tools and
recreational devices (snowmobiles,for example). Here much more research is
needed.
Another approach to the protection of hearing from noise is the use
of ear plugs and earmuffs when hazardous noises are encountered. Effective
devices are available for this purpose, but they must be carefully selected
41
-------
and properly used. In spite of the effectiveness of earplugs and earmuffs,
people will often refuse or neglect to use them for reasons of appearance,
discomfort, and bother.
-------
Section 2. MASKING AND INTERFERENCE WITH SPEECH COMMUNICATION
Introduction
Man has a formidable ability to "hear out" one sound from a background
of other sounds. For example, often one can hear the doorbell over a back-
ground of music and conversation. But there are very definite limits to
this ability to "hear out" a signal. Unwanted sounds, noises. can interfere
with the perception of wanted sounds, signals. This is called masking. By
masking, an auditory signal can be made inaudible or the signal can be changed
in quality, apparent location, or distinctiveness. Masking has been studied
extensively in the laboratory, and, consequently, the effects of noise on the
perception of auditory signals can be calculated for many environmental
conditions. Descriptions of the masking of auditory signals by noise can be
found elsewhere (Hirsh, 1952; Jeffress, 1970; Kryter, 1970; Scharfj 1970,
and Ward, 1963).
Much of the research on auditory masking has been motivated by auditory
theory. From their research, scientists hope to learn the basic laws of
the analytic capacities of human hearing. The study of the masking of speech
by noise has been undertaken to meet both practical and theoretical goals.
While it is important for everyday life to be able to understand generally
the perceptibility of auditory signals, most would agree that the understand-
ing specifically of the problem of speech perception has great significance
for the quality of human life. If speech is totally drowned out by a masker,
43
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the speech is said to be inaudible or below the threshold of detectability.
If the presence of the speech can be detected, but it is indistinct or
difficult to understand, the speech is said to be above the threshold of
detectability and to have poor intelligibility or discriminability. Intel-
ligibility or discriminability refers to the clarity or distinctness with
which speech can be heard over a background noise and it is usually measured
in the percentage of messages that a listener can understand.
A. Interference with Speech Communication
Speech and understanding speech. A talker generates a complicated
series of sound waves. This series is called the speech stream. It is
not possible to assign a particular acoustic pattern to each of the "sounds"
of the English language in a one-to-one fashion. Rather, the "speech stream"
carries the cues for the "sounds" of English and the listener decodes the
"speech stream" by a complicated, synthetic process that not only relies
on the acoustic cues carried by the "speech stream," but also relies on the
listener's knowledge of the language and the facts of the situation. Not
all of the cues carried by the "speech stream" are known. Also, the syn-
thetic processes by which the "speech stream" is decoded and "heard as
speech" are not fully understood. Nonetheless, much is known about which
regions of the audible range of frequencies carry the cues for the intel-
ligibility of speech.
Cues in the speech stream can be found at frequencies as low as about
100 hertz to as high as about 8000 hertz. Most of the acoustical energy
44
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of the speech stream is concentrated between 100 and 6000 hertz. But,
the most important cue-bearing energy falls between about 500 and 2000
hertz. The speech stream carries much extra information. It is redundant.
Therefore, speech can be heard with high intelligibility even when some of
the cues have been removed.
How speech reception in noise is studied. There are many variables
that iJafluence the accuracy of speech communication from talker to listener
in an experiment. The characteristics of the talker; the test materials;
the transmission path from talker to listener; the background noise; the
spatial locations of the talker, noise source, and listener; and the in-
tegrity of the listener's auditory system all can be important. The outcome
of such an experiment is usually measured in the percentage of messages
understood, and this percentage is taken as a measure of intelligibility
or discriminability of the speech. Other measures are sometimes used.
Among these are ratings of the quality or the naturalness of speech, recog-
nition of the talker, or recognition of the personality or psychological
state of the talker.
In no one experiment are all of the variables studied. Rather, most
are held constant and the effects of a few are evaluated. The experiments
of Miller e_t al. (1951) provide a good illustration. Only two subjects
were used and they alternated roles as talker and listener. The subjects
were located in different rooms and could only communicate via a microphone-
45
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amplifier-earphone system which passed only frequencies between 200 and
3000 hertz. Noise could be added into this communication link and the
•
ratio of speech power to noise power could be controlled. In one experi-
ment, the test materials were one-syllable words. The talker always said,
"You will write ," with the test item read at the blank. He monitored
his voice level with an appropriate meter and, thus, the speech intensity
at the microphone was held constant. The level of the speech and noise at
the listener's ear was controlled by the experimenter through appropriate
adjustments of the electronic equipment. Of major interest in this experi-
ment were the relations between the speech power and noise power, the number
of possible messages (one-syllable words), and the percentage of messages
understood. For some tests, the message could be one of two alternatives
known to the listener; for other tests the message could be one of four,
eight, sixteen, thirty-two, two hundred fifty-six, or any of one thousand
possible one-syllable words. The results are shown on Figure 13.
It can clearly be seen that the more intense the speech in relation
to the noise the greater the percentage of messages correctly understood.
Also, the fewer the number of alternative messages the greater the per-
centage of correctly understood messages. It is important to realize that
the absolute percentage of correct messages transmitted for each speech-to-
46
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MONOSYLLABLES
•15 -12 -9 -6 -3
SIGNAL-TO-NOISE RATIO
0 + 3 +6
IN DECIBELS
figure 13. The dependence of the accuracy of speech communication on the
relations between the intensity level of the speech in relation to the
intensity level of the noise. The several curves are for various numbers
of possible messages. When the message could be one of two possible
words, the scores were high. When the message could be one of approximately
1000 one-syllable words, the scores were low. (From Miller et al., 1951,
with permission of G. A. Miller and the Journal _of Experimental'^sychology.)
47
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noise ratio will depend on the talker, the exact nature of the noise, its
spectrum and intensity, and on the way in which the speech and noise intensities
are measured.
The ma.ior effects of noise on speech communication. Many of the facts
of speech communication in noise can be understood in terms of a single
graph. This graph is given on Figure 14 and in simplified form on Figure 15.
The vertical axis is the A-weighted sound level of background noise measured
in decibels. The horizontal axis is the distance between talker and listener
in feet. The regions below the contours are those combinations of distance,
background noise levels, and vocal outputs wherein speech communication is
practical between young adults who speak similar dialects of American-English.
The line labelled "expected voice level" reflects the fact that the usual
talker unconsciously raises his voice level when he is surrounded by noise.
Consider the example of a talker in the quiet who wishes to speak to a
listener near a running faucet. The A-weighted sound level of the back-
ground noise may be about 74 decibels for the listener. If the talker is
20 feet awayy it is clear from Figures 14 and 15 as well as from everyday
experience that communication would be difficult even if the talker were
to shout. But, if the talker were to move within one foot of the listener,
communication would be practical even when a normal voice is used. It can
be seen that at 15-20 feet, distances not uncommon to many living rooms or
classrooms, A-weighted sound levels of the background noise must be below
50 decibels if speech communication is to be nearly normal.
48
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X
C3
X
110 -
TALKER TO LISTENER DISTANCE IN FEET
5 10 15 20 25 30
co
T3
LU
CO
o
z
Q
Z
3
O
EC
O
o
CO
AREA OF
NEARLY NORMAL
SPEECH COMMUNICATION
INTIMATE
PERSONAL-
USUAL TYPE OF COMMUNICATION
Figure 14. Quality of speech communication as dependent on the A-weighted
sound level (dBA) of the background noise and the distance between the
talker and listener. (Modified from Webster, 1969.) The heavy data
points represent scores of 90fo correct with tests done with phonetically
balanced lists of one-syllable words (Waltzman and Levitt, 1971). The
types of speech communication typical of various talker-listener dis-
tances are based on observation (Hall, 1959).
49
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COMMUNICATION
IMPOSSIBLE
COMMUNICATION
DIFFICULT
EH™™-- COMMUNICATION ESK^S^vv^^vviS^
™ POSSIBLE
%AREA OF
-^NEARLY NORMAL
^SPEECH COMMUNICATION
^SJ
0 5 10 15 20 25 30
TALKER TO LISTENER DISTANCE IN FEET
Figure 15. Simplified chart that shows the quality of speech communica-
tion in relation to the A-weighted sound level of noise (dBA) and the
distance between the talker and the listener.
50
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People vary their voice levels and distances not only in accordance
with the level of background noise and physical convenience, but also in
accordance with cultural standards (Hall, 1959). Distances less than about
4-1/2 feet are reserved for confidential or personal exchanges usually with
a lowered voice. Distances greater than about 5 feet are usually associated
with a slightly raised voice and reserved for messages that others are
welcome to hear. Thus, levels of background noise that require the talker
and listener to move within less than 4 feet will be upsetting to persons
who do not normally have an intimate association. Even for close friends
there may be some embarrassment if the message would not normally require
such nearness. When the content of the message is personal, there will be
reluctance to raise the voice level even if the background noise demands
it for intelligibility.
In one-to-one personal conversations the distance from talker to
listener is usually of the order of 5 feet and nearly normal speech com-
munication can proceed in A-weighted noise levels as high as 66 decibels.
Many conversations involve groups and for this situation distances of 5-12
feet are common and the intensity level of the background noise should be
less than 50-60 decibels. At public meetings or outdoors in yards, parks,
or playgrounds distances between talker and listener are often of the order
51
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of 12-30 feet and the A-weighted sound level of the background noise must
be kept below 45-55 decibels if nearly normal speech communication is to
be possible.
Characteristics of people (speech, age, and hearing) and speech
interference by noise. The contours on Figures 14 and 15 represent conditions
for young adults who speak the same dialect when they are in a diffuse noise
field. The location of these contours would shift in accordance with many
variables. Lower noise levels would be required if the talker has imprecise
speech (poor articulation) or if the talker and the listener speak different
dialects. Children have less precise speech than do adults (Eguchi and Hirsh,
1969) and also their knowledge of language often makes them less able to "hear"
speech when some of the cues in the speech stream are lost. Thus, adequate
speech communication with children under about 13 years of age probably requires
lower noise levels than are required for adults. One's ability to understand
partially masked or distorted speech seems to begin to deteriorate at about
age 30 and declines steadily thereafter (Palva and Jodinen, 1970). Generally,
the older the listener, the lower the background noise must be for nearly
normal communication. It is well known that persons with hearing losses
require more favorable speech-to-noise ratios than do those with normal
hearing. This group again requires lower noise levels for adequate speech
communication than do young adults with normal hearing.
Situational..factors, (message predictability, opportunity for lip
reading,, spatial arrangements and reverberation, and kinds of noise) and
52
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speech interference by noise. Of course, adequate communication in higher
noise levels than those indicated on Figures 14 and 15 can occur if the
possible messages are predictable. Thus, at ball games, vie may be able to
discriminate the umpire's "ball" and "strike" at much greater distances and
in more intense levels of noise than indicated on the chart. This factor
accounts for the success of communication in many industrial situations
with high levels of noise. Success may give way to failure, however, when
an important but unpredictable message must be coraaunicated. For example,
firemen in a high-level noise may have little difficulty with standard
communications about the use of equipment, but may encounter grave difficulty
communicating about unexpected events that occur at the scene of the fire.
The opportunity to lipread or use facial or bodily gestures in support
of hearing will improve the success of communication in background noise.
Almost everyone has some small amount of lipreading skill which they often
use without awareness of its contribution to intelligibility.
Spatial variables also may facilitate speech communication in noise.
If the source of noise is clearly localized in a position different from
that of the talker, speech communication may be possible under noise condi-
tions less favorable than those indicated on Figures 14 and 15. On the other
hand, spatial factors can sometimes reduce the intelligibility of speech.
If a space produces many reflections of sound it is said to be reverberant
or lively. Noise interferes with speech communication more in a very rever-
berant space than in one that is not.
53
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Sometimes unusual acoustic conditions can make our voices clearly
audible at great distances. If one raises his voice to talk to a nearby
person over the sound of a power lawn mower or outboard motor, he can
sometimes be heard more clearly by a distant accidental spectator than
by the nearby friend.
The exact characteristics of the noise are also important for pre-
dicting speech communication. While the A-weighted noise level is an
adequate measure of many noises, some situations and noises demand a more
complicated analysis of the noise. A discussion of the use of the various
methods of measuring noise to predict speech interference can be found
elsewhere (Kryter, 1970, p. 70-91).
B. Implications of Masking and Interference
with Speech Communication
Masking of auditory signals. Many auditory signals serve important
functions in our lives and these functions may be lost in noise. While
the masking of a doorbell because of noise may only be a source of incon-
venience and annoyance, the masking of signals can interfere with the per-
formance of tasks. In some cases, the masking of a signal such as that of
an approaching vehicle can lead to property damage, personal injury, or
even death.
Interference with speech communication. The implications of reduced
opportunity for nearly normal speech communication are considerable.
Those who must work in high levels of background noise claim that
they "get used to it." There is evidence, however, that they adopt a
54
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"non-communicating life style" and increase their use of non-verbal
communication through gestures, posture, and facial expression. Even
though non-verbal communication is important, it is unlikely that it is
nearly as important as verbal communication. Many subtleties of life are
lost when verbal communication is restricted.
Among adults, free and easy speech communication is probably essential
for full development of social relations and self.
For very young children, there may be an additional problem. They
gradually induce their knowledge of language and its subtleties from the
speech to which they are exposed. Also, as previously stated, because
their knowledge of language is still developing, children probably have
more difficulty understanding speech in noise than do adults. Because noise
can reduce the amount of speech used at home, in the yard, or on the play-
ground and because noise can make speech difficult to understand, it is
possible, though unproven, that the language development of early childhood
might be adversely affected. From this, difficulty in learning language
and learning to read may ensue. One can only guess at how severe the noise
must be to produce such effects; nearly continuous A-weighted sound levels
in excess of 70 decibels might be required. Such conditions do exist at
some residences in urban areas near freeways. When contemplating possible
increases in general levels of community noise, one should give considera-
tion to these possible effects on the linguistic development of children.
Later, school-age children probably encounter more difficulty in
55
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noisy classrooms than, for example, do sailors in noisy enginerooms who
exchange a limited number of prescribed technical messages. With regard
to the impact of noise on formal education, the Jamaica Bay Environmental
Study Group of the National Academy of Sciences summarized their findings
as follows:
Within the present impacted area (NEF 30 or greater) there
are 220 schools attended by 280,000 pupils. With normal
school-room usage, this implies about an hour's interruption
of classroom teaching each day and the development by the
teachers of the "jet pause" teaching technique to accomodate
the impossibility of communicating with the pupils as an
aircraft passes overhead. The noise interference goes beyond
the periods of enforced non-communication, for it destroys
the spontaneity of the educational process and subjects it to
the rhythm of the aeronautical control system. Given the
advanced age of many of these schools, noise-proofing (where
possible) would cost an appreciable fraction of their replace-
ment cost.
Any casual observer of intimate family life is aware of the irritation
and confusion that can arise when simple, everyday messages need frequent
repetition in order to be understood. Noise does not cause all of these
occurrences, but surely it causes some.
The enjoyment of retirement and later life can be hampered by masking
noises. It is well known that speech reception abilities deteriorate with
age and clinical observations clearly indicate that older persons are more
susceptible to the masking of speech by noise than are young adults.
It is likely that one must somehow "work harder" to maintain speech
reception in noise than in quiet. Thus, successful speech communication
in noise probably has its cost. If the cost is too high, the number of
56
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verbal exchanges probably declines.
In a highly intellectual, technical society, speech communication
plays an extremely important role. Background noise can influence the
accuracy, frequency, and quality of verbal exchange. In excessive back-
ground noise, formal education in schools, occupational efficiency,
family life styles, and the quality of relaxation can all be adversely
affected.
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PART II. GENERAL PSYCHOLOGICAL AND SOCIOLOGICAL EFFECTS
Preliminary Statement
Noise not only has direct effects on auditory function as decribed
in PART I, but it also produces other behavioral effects of a more general
nature. Included among these effects are INTERFERENCE WITH SLEEP, Section
3j the general evaluation of auditory experience included under LOUDNESS,
PERCEIVED NOISINESS, AND UNACCEPTABILITY, Section 4» and ANNOYANCE AND
COMMUNITY RESPONSE, Section 5. All of these areas have been investigated
and certain clearcut patterns have emerged. Plausible, but less thoroughly
studied behavioral effects of noise are discussed under OTHER POSSIBLE
PSYCHOLOGICAL AND SOCIOLOGICAL EFFECTS, Section 6.
Many of the psychological and sociological effects of noise can be
traced to the role of hearing in man's evolutionary development as described
in the INTRODUCTION. Others may be linked more specifically to the auditory
effects described in PART I or to the general physiological responses to be
described in PART III. Because of these interrelations among the effects
of noise on people, the organization of topics is necessarily somewhat
arbitrary.
Section 3. INTERFERENCE WITH SLEEP^
Introduction
From everyday experience it is evident that sound can interfere with
3JThe effects of noise on sleep are discussed at greater length in this
paper than are other effects of equal or greater importance. This was
done because other revie\irs of the effects of noise on people have given
relatively less attention to the subject of sleep disturbance.
58
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sleep. Almost all have been waked or kept from falling to sleep by loud,
strange, frightening, or annoying sounds and it is commonplace to be waked
by an alarm clock or clock radio. But it also appears that one can "get
used to"sounds and sleep through them. Possibly, environmental sounds only
disturb sleep when they are unfamiliar. If so, disturbance of sleep would
depend only on the frequency of unusual or novel sounds. Everyday experience
also suggests that sound can help to induce sleep and, perhaps, to maintain
it. The soothing lullaby, the steady hum of a fan, or the rhythmic sound of
the surf can serve to induce relaxation. Perhaps certain steady sounds can
serve as an acoustical eyeshade and mask possibly disturbing transient sounds.
Common anecdotes about sleep disturbance suggest an even greater
complexity. A rural person may have difficulty sleeping in a noisy urban
area. An urban person may be disturbed by the quiet, the sounds of animals,
and so on when sleeping in a rural area. And how is it that a mother may
wake to a slight stirring of her child, yet sleep through a thunderstorm?
These observations all suggest that the relations between exposure to sound
and the quality of a nightfs sleep are complicated. They are. Nonetheless,
research is beginning to untangle the story and certain trends do appear.
Before these studies are described, it will be necessary to consider
the problem of the nature of sleep. There has been significant headway
in the description of a night of sleep. Sleep is a complicated series of
states rather than a single, uniform state. Experiments verify the common
belief that sleep is essential for normal functions while awake. But the
59
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"hows" and "whys" are unknown and therefore it is difficult to state flatly
that this or that alteration in sleep is harmful. One must rely on every-
day wisdom for these judgments.
A. Methods for Studying Sleep Disturbance by Noise
Field studies. One of the most obvious and direct methods is to
interview people who live in areas that receive various exposures to noise.
People can be asked whether the noise either prevents them from falling
asleep or whether it wakes them from sleep. Of course, if such direct
questions are embedded in a series of questions concerning noise and
sound, the answers may be biased by the person's attitude toward the source
of sound. It may be better to ask about the quality of sleep, the number
of hours slept, judgments about well-being upon arising, and so on in the
context of a survey unrelated to noise.
Laboratory studies. Topically, a subject sleeps in a special laboratory
bedroom where his physiological state can be monitored from electrodes attached
to his body, and calibrated sounds can be presented by loudspeakers or by
other sound-making instruments. By these techniques subtle responses to
sounds or subtle changes in the pattern of sleep can be recorded and measured,
Furthermore, a variety of instructions and adaptation procedures can be tested.
However, such research is very slow, hard work; the required apparatus is
expensive! usually only a few subjects can be studied; and the routine is
demanding on the experimenter. Furthermore, even though the subjects are
adapted to the routine, they are not at home and they are constrained by
60
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electrodes and wires. In spite of these difficulties, however, some rather
clear trends have emerged.
B. General Properties of Sleep
Sleep stages and a night of sleep. Examination of brain waves,
other physiological measures, responsiveness, behavior, and the sequence
of events during a night's sleep have led to the concept of sleep stages
or states. There are recognizably different patterns that occur during a
period of sleep. Since these patterns blend from one to another, there are
several schemes for categorizing them into sleep stages or states. A popular
set of categories is labelled I, II, III, IV, and I-REM. Another set of
stages is defined in a slightly different manner. They are labelled A, B,
C, D, and 3. Some authors even combine the two sets of definitions.
Perhaps the easiest approach to these stages is to follow an idealized
progression as one falls asleep. As one relaxes and enters a stage of
drowsiness, the pattern of the electroencephalogram (EBG) changes from a
jumble of rapid, irregular waves to the regular 9-12 hertz pattern known
as the alpha rhythm. One is relaxed, but not asleep. Later, the alpha
rhythm diminishes in amplitude and intermittently disappears. This is sleep
stage A. As time progresses, the alpha rhythm is present less and less
often until it disappears and is replaced by a low-voltage, fast, irregular
pattern in the BEG; this is stage B. In the Roman numeral system, stage I
corresponds to the late portions of stage A and all of stage B.
Next, there appear quick bursts of larger amplitude waves known as
61
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spindles or the spindles of sleep. Mixed with these spindles there will
appear small-amplitude, low-frequency (1-5-3 hertz) waves known as delta
waves. This stage is known as stage C or stage II. In the next stage of
sleep, the spindles disappear and the delta waves become more regular and
grow in amplitude. This is known as stage D. Later, the delta waves become
even larger and of lower frequency (0.6-1 hertz). This is stage B. The
Roman numeral system III and IV include stages D and S but the criterion
for division is different. Stages D and S or III and IV are often referred
to as deep or delta sleep.
The purpose is not to confuse the reader with two sets of sleep stages,
but rather to communicate the idea that there is a progression of sleep
stages. One can reasonably divide this progression by various criteria.
Generally, in stage A or early I, man is drowsy, but awake. In stage B,
or late I, one drifts or "floats" back and forth between waking and sleeping.
When awakened at this stage of sleep, one is not quite sure whether he has
been asleep. Stages C, D, and E or II, III, and IV represent definite sleep.
The remaining stage, which has been of great interest, is the so-called
Rapid Eye Movement (REM) stage of sleep. In REM sleep, the sleeper exhibits
characteristics of stage I (late A and B). There are: fast, low-voltage
brain waves; other evidence of variable but definite physiological activa-
tion; and rapid eye movements. Consequently, this stage is usually tagged
I-REM. While dreaming and mental activity can take place in all sleep stages,
it is during I-RSM that most dreams occur.
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A typical night's sleep initially follows a progression, with
occasional reversals, from stages A (l) to stages D and E (IV). This
progression usually occurs within the first 80 minutes of sleep. After
about 90 minutes of sleep, one has left stage IV and has had a period of
I-REM. A 90-minute cycle from I-REM to I-RM tends to recur throughout
the period of sleep. There are, however, some irregularities and some
systematic changes.
Roughly equal amounts of time are spent in I-RM and in IV. Early
in the sleep period more time is spent in IV than in I-REM and later in
the sleep period more time is spent in I-REM than in IV. Generally, after
the first 80-90 minutes of sleep, more and more time is spent in the
"lighter" stages of sleep. These facts are summarized in Figure 16.
Overall, sleeping young adults distribute sleep as follows: Stage I—5$;
Stage I-REM— 20-25$; Stage 11—50$; and Stages III and IV—20$ (Berger,
1969).
Even after falling asleep, one awakens during the night. Roughly, five
per cent of the total period of "sleep" is spent awake from adolescence to
about age 40. From ages 40 to 90 the time awake during "sleep" increases to
nearly 20 per cent (Feinberg, 1969). The number of awakenings that occur after
falling asleep increases from an average of about two at age six to six at
age 90 (Feinberg, 1969).
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AWAKE
REM-
YOUNG ADULTS
345
HOURS OF SLEEP
8
Figure 16. The nocturnal sleep pattern of young adults is shown. During
the later part of the sleep period stage IV is absent and more time is
spent in stage II and in REM. Notice the two brief periods that the
sleeper spontaneously awoke, (From Berger, 1969, in Sleep; Physiol-
ogy and Pathology. A. Kales, Editor, with the permission of the author,
editor, and the J. B. Lippincott Company.)
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Sensory responses to stimulation during sleep. The sense organs are
just as sensitive to their appropriate physical stimuli during sleep as
they are during wakefulness. One may wonder whether mechanisms near the
periphery of the nervous system somehow "block" the sensory pathways
during sleep. Such mechanisms would prevent the neural messages from the
sense organs from reaching the higher centers of the brain. Available
research (Koella, 196?) does not support this view. Rather, one can state
quite strongly that information from the sense organs does reach the highest
centers of the brain even during deepest sleep. This conclusion is based
on the fact that electrical responses to stimuli can be recorded in the
highest centers of the brains of sleeping or anesthetized men and animals.
These responses usually are of brief duration and have latencies of 0.01-
0.8 second.
Therefore, the apparent indifference to stimulation during sleep is
not a simple "shutting out" of the neural messages at or near the periphery
of the nervous system close to the sense organ. Rather, this apparent
indifference to external stimulation is due to a complicated reorganization
of brain processes during sleeping as opposed to waking states. It is also
true that when the eyelids are closed, an ear is on a pillow, or the middle-
ear muscles are contracted, responsiveness to the environment can be reduced
because the magnitude of the stimulus that reaches the sense organ is not
as great. But these physical conditions are no more related to the basic
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nature of sleep tnan are reduction of light by eye patches or the attenua-
tion of sound by ear plugs.
Arousal. Sensory messages reach the highest centers of the brain,
but whether or not they influence the sleeper will depend on a compli-
cated set of circumstances. Many theorists believe that mechanisms in
the brain busily carry out "sleep work" throughout the sleeping period.
These mechanisms assess the significance of incoming sensory messages and
adjust the state of the brain in accordance with the sensory message and the
whole situational complex. This view is supported by everyday experience
as well as by scientific investigation.
Arousal from sleep can be recognized by brief changes in physiological
function? by shifts from deeper to lighter stages of sleep; or by behavioral
evidence of awakening. Some of the properties of arousal mechanisms will
become apparent as the effects of noise on sleep are discussed.
C. Noise and Sleep
Effects of brief noises. In the area of sleep disturbance by noise
it is the effects of relatively brief noises (about 3 minutes or less) on
a person sleeping in a quiet environment that have been studied most
thoroughly. Typically, presentations of the sounds are widely spaced
throughout a sleep period of 5-7 hours.
A summary of some of these observations is presented on Figure 1?.
The heavy dashed lines are hypothetical curves which represent the per cent
awakenings under conditions in which the subject (l) is a normally rested
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young adult male who has been adapted for several nights to the procedures
of a quiet sleep laboratory, (2) has been instructed to press an easily-
reached button to indicate that he has awakened, and (3) has been moderately
motivated to awake and respond to the noise (such motivation can be estab-
lished by instructions which imply that somehow the subject's ability is
being tested). A datum for sleep stage II is indicated by an Arabic two,
2. A datum for sleep stages III and IV is indicated by a Greek delta, ,A.
While in stage II, subjects can awake to sounds that are about 30-40 deci-
bels above the level at which they can be detected when subjects are con-
scious, alert, and attentive. While in deep sleep, stages III or IV, the
stimulus may have to be 50-80 decibels above the level at which they can
be detected by conscious, alert, attentive subjects before they will awaken
the sleeping subject.
The solid lines are data from questionnaire studies of persons who
live near airports. The percentage of respondents who claim that flyovers
wake them or keep them from falling asleep is plotted against the A-weighted
sound level of a single flyover (Wyle Staff, 1971). These curves are for
the case of approximately 30 flyovers spaced over the normal sleep period
of 6-8 hours. The filled circles represent the per-
centage of sleepers that awake to a 3-minute sound at each A-weighted
sound level (dBA) or lower. This curve is based on data from 350 persons,
each tested in his own bedroom (Steinicke, 1957). These measures were
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100
90
80
O 70
60
50
LU
cr
LU
30
20
10
0
AWAKENING 1
FROM STAGE II —v
SINGLE NOISE J \
V
AWAKENING
OVERALL
SINGLE NOISE
NOISE WAKES
ME UP"
30 NOISES
"NOISE KEEPS
ME FROM GOING
TO SLEEP"
30 NOISES
AWAKENING
STAGES III & IV
SINGLE NOISE
10 20 30 40 50 60 70 80 90 100 110 120
dBA-HMOOORS-BRIEF SOUNDS {UNDER 3 MINUTES)
Figure !?• Awakenings to sound from various laboratory and questionnaire
studies are shown. The horizontal axis gives the approximate A-weighted
sound level (dBA) of the noise. The curves labelled "awakening" are from
normally rested young adults who were sleeping in a laboratory and were
moderately motivated to awake in response to sound. The percentage of
awakening responses will depend not only on the intensity of the sound
but also on the definition of "awakening," the motivation of the subject
to awake in response to sound, and the sleep stage (I, II, III, IV, or
I-REM) when the stimulus is presented. The questionnaire results, "Noise
wakes me up/1 and"Noise keeps me from going to sleep," are derived from the
Wilson Report (1963) for the case of 30 brief noises distributed through-
out the night. The laboratory results are from various studies. The
filled circles were gathered throughout the night without regard to sleep
stage (Steinicke, 1957)• Data from sleep stage II are represented by
2's; those from sleep stages III and IV by deltas,-A?s. The circles with
unbroken borders are from Williams et al. (1964). The circles with bro-
ken borders are from Williams et al. (1965). The boxes with solid borders
are from Rechtshaffen et al. (1966"7« The boxes with broken borders are
from Lukas and Kryter TT970). The broken arrow is from Watson and
Rechtshaffen (1969). The solid arrows are from Kryter and Williams (1970).
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made between 2:00 and ?:00 AM, and it is reasonable to assume that most of
the subjects were roused from stages II or I-EEM.
Motivation to awake and intensity level of the noise. There is clear
evidence that motivation to awake can influence the probability of awaken-
ing to noise (Williams et al., 1965; Watson and Rechtschaffen, 1969; and
Wilson and Zung, 1966). The effects of motivation, however, depend on the
stage of sleep and the intensity level of the noise. For weak stimuli,
motivation may have a strong influence on arousal only during light sleep
(Williams et al., 1965). For moderately strong stimuli, motivation to
awake may have a powerful effect on the probability of an upward shift in
sleep stage (probably awakening also) from all depths of sleep (Wilson and
Zung, 1966). With very intense stimuli it is likely that motivation would
have little influence; for example, brief noises with A-weighted sound
levels of 100-120 decibels awaken nearly everyone from any stage of sleep.
The effects of motivation are illustrated indirectly on Figure !?•
The results of Lukas and Kryter (1970) are the boxes with broken borders
that lie towards the lower right of the graph. Here awakening is defined
in the experimental setting by instructions that imply "if you happen to
wake up^push the button." The button is located on the headboard of the
bed and requires that the subject find it (often having to turn over to
do so) and press it. This definition of awakening is similar to a typical
kind of night awakening.
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The ascending series of stage 2 awakenings for stimuli of 30-40
decibels (encircled by broken lines on Figure 1?) are from Williams et al.
(1965). The ascending percentage of awakenings is correlated with the
sleeper*s motivation as controlled by instructions and punishments for
failure to respond by pushing a convenient button. As the motivation to
awake was increased, the percentage of awakenings showed a five-fold increase
from less than 11$ to about 55$ of the presentations of the same noise at the
same stage of sleep.
Fluctuating noise levels. A very important and extensive study of
the effects of noise on sleep was done at the Centre d'Etudes Bioclimatiques
du CNRS in Strasbourg, France (Schieber, Mery, and Muzet, 1968). Several
measures of the quality of sleep were used. These included: the amount
of time in each of the sleep stages; the numbers of brief awakenings as
evidenced by the appearance of alpha waves in the electroencephalogram;
the number of bodily movements; the degree of muscular tension; the occur-
rence of perturbations in heart rate; the presence of eye movements; and
the occurrence of various components of the electroencephalogram such as
K-complexes, sleep spindles, alpha waves, theta waves, and delta waves.
Artificial sounds (crescendos of white noise that rose to about 80 decibels
in 10 seconds and were terminated abruptly), sounds of aircraft flyovers
with peak values of 72 and 89 decibels (either 16 or 33 per night), or
traffic noises were used in various experiments. The time required to fall
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asleep was longer for noise than control conditions. Under control
conditions, about 26 minutes elapsed between going to bed and the first
occurrence of stage IV. Under traffic noise, the delay between going to bed
and the first occurrence of stage IV was 33 or 52 minutes depending on
the type of noise. When noises were presented, there was a tendency for
sleep to be much lighter than normal for the first half of the night and
slightly deeper than normal for the second half of the night. Thus, there
was a tendency to compensate for the loss of deep sleep in the early part
of the night by an increase in deep sleep in the later part of the night.
Nonetheless, almost all measures of sleep disturbance indicated that sleep
was disturbed overall and throughout the sleep period.
The results with traffic noise were of particular interest. These
sounds were actually recorded in a bedroom near a busy street. One set
of recordings was made between 10:00 PM and midnight. Another was made
between midnight and 4:00 AM. The 10:00 PM to midnight sample represented
about 4.3 vehicles passing per minute, while the midnight to 4:00 AM sample
had only-about 1.8 vehicles per minute. The peaks in both samples reached
A-weighted sound levels of nearly 80 decibels, but the long-term averages
were ?0 decibels for the high-density traffic and only 6l decibels for the
low-density traffic. The control night had steady ventilation noise with
a median A-weighted sound level of 48 decibels. The interesting fact was
that the low-density traffic pattern was more disruptive of sleep than
was the high-density pattern. However, both traffic patterns were more
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disruptive than the control noise.
These results strongly suggest that fluctuations in the noise levels
and degree of fluctuation are important factors in determining sleep
disturbance by sound.
Steady and rhythmic sounds. It seems plausible that steady, periodic,
or rhythmic sounds might improve the quality of sleep. Certainly, anecdotal
evidence suggests that steady sounds can mask out brief disturbing sounds
and that some periodic or rhythmic sounds have certain soothing qualities.
Investigations along these lines are badly needed. Pertinent questions
are: (l) At what levels do steady sounds begin to adversely influence
sleep patterns? (2) Can a moderate amount of masking noise reduce the
influence of brief sounds on sleep, or are brief sounds that suddenly emerge
above a masking noise more disturbing than those that simply join the usual
rise and fall of community noise? (3) Can sleep be induced and maintained
by particular rhythms of sound?
One investigation of complaints about noises produced by air-condition-
ing and heating equipment may be relevant to the effects of steady noise
on sleep (Blazier, 1959). From complaint files, conversations with dealers
and distributers, and field trips to problem sites, the investigator found
what types of noises in bedrooms resulted in adverse responses. He also
noted that the fewer the complaints, the greater the customer"s acceptance
of the product.
It was found that people especially objected to noises that included
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"tones" and "throbbing" or "beats." Blazier summarized the frequency of
complaints in relation to A-weighted sound levels of noises in sleeping
quarters as follows: below about 33 decibels, no complaints; 33-38
decibels, occasional complaints; 38-48 decibels, frequent complaints; and
over about 48 decibels, unlimited complaints. While it is not known
whether these complaints are due to sleep disturbance or other factors,
these results do appear to be in remarkable agreement with the trends for
sleep disturbance by brief noises shown on Figure 1?.
Sound quality and sleep disturbance. As yet we have no evidence on
the role that pitch, timbre, and temporal structure play in sleep disturb-
ance or enhancement. Until such data are forthcoming, it may be useful to
assume that those variables that influence perceived noisiness would similarly
influence sleep disturbance.
Sleep deprivation and sleep disturbance. Subjects who have been
deprived of sleep require more intense noises for awakening then do
normally rested subjects (Williams e_t al., 1965).
Difference between men and women. One study found that women tended
to awaken to noises of lower levels than did men (Steinicke, 1957).
Another study (Wilson and Zung, 1966) found a clear difference in arousal
as defined by upward shifts in sleep stage. In response to noise, women
shifted toward lighter stages of sleep much more frequently than did men.
Lukas (1971) finds that sleep disturbance from subsonic-aircraft noise or
sonic booms is greater for middle-aged women than for middle-aged men.
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Thus, it appears that women's sleep is more easily disturbed by noise than
is men1 s, even when other variables such as motivation and stage of sleep
are equated.
Age and sleep disturbance by noise. There is clear evidence that
persons over about 60 years of age are much more easily awakened or shifted
towards lighter sleep stages than are middle-aged adults or children (Lukas
and Kryter, 1970). This effect is large and dramatic. More specifically,
simulated sonic booms that awaken middle-aged adults and 7~ and 8-year-old
children on less than 5$ of their occurrences will awaken 69- to 72-year-
old adults on nearly 70$ of their occurrences. These dramatic differences
hold over all stages of sleep. Also, once awakened, an older person has
more difficulty in returning to sleep than does a middle-aged adult or a
child. There is no evidence that children are especially sensitive to sleep
disturbance by noise. On the contrary, Lukas et al. (1971) found that
7- and 8-year-old children are slightly less sensitive to noise during sleep
than are middle-aged adults. However, since general sleep disturbance in
children (enuresis, somnambulism, night terrors, and nightmares) seems to
peak between 4 and 6 years of age (Broughton, 1968; Feiriberg, 1969;
Jacobson et al., 1969; Kessler, 1966), one suspects that sleep disturbance
by noise may have a special impact on children in this age range. It is
well known, for instance, that thunderstorms can waken and frighten
children of these ages. Children in the age group of 4-6 years seem to
be particularly disturbed by sudden arousal from stage IV of sleep (Broughton,
1968).
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Sleep stage and accumulated sleep. In terms of either behavioral
awakening or an upward shift in sleep stage as indicated by the electro-
encephalogram, sleep can be influenced most easily in stages I and II and
least easily in stages III and IV. Sometimes I-REM seems to be more like
III and IV in this regard; other times it is more like stages I and II.
A person can be aroused from sleep more easily the longer he has slept no
matter what the stage of sleep (Lukas and Kryter, 1970; Rechtschaffen et al.,
1966; Williams et al., 1966).
Stimulus meaning and familiarity. The effects of stimulus meaning
and familiarity are closely bound to those of motivation and stimulus
intensity. There is considerable evidence that sleepers can discriminate
among stimuli if the differences were learned and the discrimination was
established while they were awake (Williams, et al., 1965; Wilson and Zung,
1966). In a classic experiment Oswald e_t al. (i960) demonstrated that
sleeping subjects will respond when their own names are spoken but show
few responses to other names. Generally, when auditory stimuli are faint
and similar, discriminations are probably performed better in light sleep
(I, II, and I-REM) than during deep sleep (ill and IV). The effect of
stimulus familiarity on arousal from sleep has not been studied extensively.
In one experiment, small but consistent differences were found between
familiar and unfamiliar sounds. "Familiar" sounds shifted sleep stages
less frequently than "unfamiliar" sounds (Zung and Wilson, 1961).
Adaptation to sleep disturbance by noise. Whether adaptation takes
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place is the subject of considerable debate. A reasonable guess at this
story is as follows. The stronger the stimulus, the less likely it is
that total adaptation will take place. Behavioral awakening and duration
of awakening will probably show the most adaptation. Upward shifts in sleep
stage are likely to show some adaptation, but less than behavioral awaken-
ing. Brief responses in the electroencephalogram and autonomic responses
such as changes in heart rate, blood flow, skin resistance, and so on
appear to show very little adaptation. The most significant and surpris-
ing finding has been that adaptation, even in behavioral awakening, has
been absent (Theissen, 1970) or slight (Lukas and Kryter, 1970). The
adaptation that seems apparent from everyday experience may be the result
of (l) changes in the motivation to awake; and (2) amnesia for awakening.
The last point is supported by the observation of sleep researchers that
subjects in their laboratories often cannot remember and often underestimate
the number of times that they awake during a sleep period.
There is clear evidence for adaptation to the total sleeping environ-
ment. Sleep researchers talk of the "first night" effect. Normal sleep
is rarely if ever observed during the first night in the laboratory. It
is likely then that some of the disturbance reported by the rural person
trying to sleep in an urban area and the urban person trying to sleep in
a rural area is but the "first night" effect. It is commonplace that when
we cannot sleep, for whatever reasons, we "hear" many sounds.
Other factors. There are, of course, a host of other factors related
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to sleep and arousal from sleep (Kales, 1969). These include mental and
physical disease states, drug usage, general stress, and so on. Most of
these have not been studied in relation to the problem of sleep disturbance
by noise. There is however, clear evidence that male patients suffering
from depression are more easily shifted from deeper to lighter stages of
sleep by sounds than are normal" males (Wilson and, Zung, 1966). Generally,
it seems probable that persons with disorders which result in light, rest-
less sleep or frequent awakenings will be more frequently aroused by sounds
than will normal persons or persons with disorders that produce unusually
deep and prolonged sleep. Also, it has been demonstrated that sleep
deprivation has more adverse effects on "poor" than on "good" sleepers
(Williams and Williams, 1966).
D. Noise, Sleep Disturbance, Health,and the Quality of Life
Brief sounds of sufficient intensity and fluctuating noise levels
definitely can alter the normal sleep pattern. These changes in sleep
pattern are in the direction of lighter sleep. The effects of noises
are to produce sleep patterns that are more like those of "poor sleepers"
than "good sleepers" (Luce, 1966, p. 105-108; Williams and Williams, 1966).
Whether such sleep disturbance constitutes a health hazard is
debatable. While good sleep is necessary for physical and mental health,
normal persons who lose sleep compensate by spending more time in deep
sleep, by becoming less responsive to external stimuli, and by napping.
Thus, it may be very difficult to deprive a normal person of sufficient
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sleep to produce adverse health effects.
On the other hand, the data presented here amply support the notion
that people exposed to sufficient noise will complain of sleep loss.
Everyday experience strongly supports the notion that a "good" sleep is
important to one's feeling of well-being.
All factors considered, one must tentatively assume that sleep dis-
turbance by excessive noise will reduce one's feelings of well-being.
Furthermore, when noise conditions are so severe as to disturb sleep on
a regular, unrelenting basis, then such sleep disturbance may constitute a
hazard to one's physical and mental health.
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Section 4. LOUDNESS, PERCEIVED NOISINESS, AND UNACCEPTABILITY
Introduction
To be annoyed, irritated, distracted, or disturbed by sound is
commonplace. Often the annoyance, irritation, distraction, or dis-
turbance can be traced to particular situational factors. If a conversa-
tion is interrupted by the noise of a neighbor's power mower, the
annoyance may be traced to masking and speech interference. If one inter-
prets a sonic boom as the explosion of a water heater, the annoyance may
be attributed to fear. If the noise of a motorcycle awakens one from sleep,
perhaps the annoyance can be traced to the disturbance of sleep. A sudden
noise, which may produce an unnecessary startle or fear reaction, may be
annoying because of the startle and fear reaction. Thus, a great many
instances of annoyance produced by sound may be due to the masking effects
of sound, to particular responses to the message content of the sound, or
to physiological responses to the sound.
If all instances of annoyance from noise were purely idiosyncratic,
then the possibility of dealing with the relations between the physical
properties of sound and the frequency and intensity of annoyance would be
hopeless. This is not the case. In spite of wide variations among members
of a community with regard to the intensity of their reactions and the
specific noises that they find objectionable, vrell-defined trends have
emerged. On the average« there are relations between the physical charac-
teristics of noises and the amount of annoyance, irritation, distraction,
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and disturbance.
Annoyance per se is not to be the topic of this section. Rather,
the dimensions of auditory experience which can be used to predict some of
the annoyance produced by sound are discussed.
One of these dimensions is the judged loudness of a sound. Loudness
is clearly an attribute of auditory experience. Another dimension is called
the perceived noisiness of a sound. Some argue that perceived noisiness
is a basic attribute of auditory experience, while others argue that it is
a response to auditory experience. There is no doubt, however, that perceived
noisiness is closely tied to the physical characteristics of the sounds
themselves. The third dimension is the unacceptability of a sound and it
is probably the same as perceived noisiness, or nearly so. These terms
are often used interchangeably.
Knowledge of the loudness and perceived noisiness and their relations
to the physical characteristics of sounds provide part of the foundation
for the description of annoyance and community response (Section 5).
A. Measurement of Auditory Dimensions
Field studies. One approach to the relations between the physical
properties of sounds and judgments of loudness, noisiness, or unacceptabil-
ity is the field study and questionnaire. By this technique, people are
asked either directly or indirectly to what degree they judge various sounds
to be loud, noisy, or unacceptable. The characteristics of the sounds are
then measured, and one attempts to find the relations between the characteristics
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of the sounds and the responses to them. These methods and their results
will be discussed in the next section, Section 5, because they are most
often applied to the annoyance and disturbance of activities produced by
noise.
Laboratory methods. Another approach is to bring subjects to the
laboratory and ask them to judge a variety of sounds. The sounds are often
artificial sounds with well-specified properties. In this way the researcher
tries to ferret out the underlying relations between the properties of
sounds and their judged loudness, noisiness, or unacceptability.
Three techniques are often used. Category scaling is a very simple
procedure. One asks the subject to place a sound into one of several
categories that seem to fall along a single dimension. For example, a
subject may be asked to categorize each of a series of sounds as not noisy,
slightly noisy, moderately noisy, very noisy, or intolerably noisy.
Category scaling is the familiar everyday process of judgment that we all
use many times in many different situations. It has the advantage of sim-
plicity. Among its disadvantages are the following: people tend to use
the middle categories, the way people categorize one stimulus strongly depends
on the other stimuli included in the set being judged, and people are often
strongly influenced by seemingly irrelevant aspects of the stimuli or the
judgmental situations (for example, judgments of the loudness of sounds may
be influenced by their esthetic quality).
Another method is that of magnitude scaling. People given a series of
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stimuli can and will judge the relative magnitude of the stimuli along some
dimension. Thus, people can estimate whether a sound seems twice as bad as
another, or ten times as noisy as another, or one-half as beautiful as
another. Magnitude judgments allow a measure of the apparent "somethingness"
of a stimulus in quantitative but subjective terms.
A third method is that of paired comparisons. People can be presented
with a pair of stimuli. They are then asked to judge which is louder, more
pleasant, noisier, and so on. By many such comparisons the stimuli can be
ordered along a so-called "psychological dimension."
"Psychological dimensions" measured by "psychological instruments"
seem formidable to the uninitiated. They are not! Psychological dimensions
as measured by psychological instruments are simply orderly descriptions of
the judgments we all make in our everyday experience.
There are some differences between what the psychologist does and
what we all do in our everyday judgments of the events of the day. The
psychologist tries to standardize the conditions under which the judgments
are made and the methods by which these judgments are summarized. The
psychologist may select, control, and measure the events that are to be
judged. And in order to be able to communicate accurately the conditions,
he may invent terminology which refers to the specific conditions and
judgments. Unfortunately, the terminology which is invented for preciseness
and clarity sometimes confuses the audience or consumer of the knowledge.
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B. Loudness, Perceived Noisiness, and the Physical
Characteristics of Sounds
Loudness. Loudness is an attribute of auditory experience. As a
rule of thumb, people agree that when a single component sound such as
a tone or a band of noise is raised in intensity by about 10 decibels, it
sounds twice as loud. While this basic and simple rule is of great impor-
tance, the complete story of loudness is much more complicated. Loudness
depends on the frequency (pitch) of a sound as well as its intensity level.
At moderate levels, low-frequency sounds (those below 900 hertz) are judged
to be less loud than high-frenquency sounds (those between about 900 and
5000 hertz) when both sounds are of equal physical intensity (sound pressure
level). The sound-level meter is so designed that tones or narrow bands
of noise will all sound equally loud if their A-weighted sound levels are
about 40 decibels. These relations change with intensity, however.
If a complex sound is made by simultaneous presentation of components
that are widely spaced in frequency (pitch) and about equally loud, then
the total loudness of the complex sound is the sum of the loudnesses of the
individual components. When the components are not widely spaced or are
greatly unequal in loudness, then there is mutual inhibition and interference
resulting in the total loudness being less than the sum of the loudnesses
of the components. Fortunately, methods are available to measure the loud-
ness of combinations of sounds (Stevens, 1961; Zwicker and Scharf, 1965).
The growth of loudness near the threshold of detectftbility is more
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rapid than the growth of loudness implied by the general rule that a change
of 10 decibels of intensity level equals double the loudness. Indeed, as
a sound emerges from inaudibility, a 10 decibel change of intensity level
may increase the judged loudness by a factor of ten instead of two. Also,
rapid growth of loudness may occur as sounds become audible over a masking
noise. Thus, masking sometimes may be an ineffective way of reducing the
loudness of unwanted sounds. Once audible, the unwanted sounds may seem
nearly as loud as without the masking noise.
Perceived noisiness (unacceptability). If one assumes that people
don't like loud noise, it would seem that the goal of acoustical engineers
should be to reduce the loudness of noise. If this were the case, design
objectives could be specified in terms of loudness and the appropriate
measurements of noise would then be measurements of loudness.
It has been proposed that there is yet another dimension of human
response to noise that is similar to, but distinct from, loudness. This
dimension is called perceived noisiness. The notion is that people can
judge their impression of the unwantedness of a sound. These judgments
are made of sounds that are expected and that do not provoke pain or fear.
Dr. Karl D. Kryter of the Stanford Research Institute, who developed the
idea that people can judge the "noisiness" of a sound as opposed to its
loudness, explains the concept as follows (Kryter, 1970, p. 270-277).
Perceived Noisines_s. The subjective impression of the
unwantedness of a not unexpected, nonpain or fear-provoking
sound as part of one's environment is defined as the attribute
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of perceived noisiness. The measurement or estimation of this
subjective attribute or quantity is of central importance to
the evaluation of environmental sounds or noises with regard
to its physical content. For this reason, this topic will be
discussed in considerable detail.
Confusion sometimes results in the use of the word noise as
a name for unwanted sound because there are two general classes
of "unwantedness." The first category is that in which the sound
signifies or carries information about the source of the sound that the
listener has learned, to associate with some unpleasantness not due to the
sound per se, but due to some other attribute of the source
In these cases it is not the sound that is unwanted (although for
other reasons it may also be unwanted) but the information it
conveys to the listener that is unwanted. This information is
strongly influenced by the past experiences of each individual;
because these effects cannot be quantitatively related to the
physical characteristics of the sounds, they are rejected from
the concept of perceived noisiness. After all, the engineer,
attempting to control the noise from a given source, must shape
the characteristics of the noise in as effective a way as possible
for the majority of the people and the most typical of circum-
stances; those legislating or adjudicating the amounts of noise
to be considered tolerable must also have a quantitative yard-
stick that is relatable to groups of people and typical circumstances.
Psychological judgment tests have demonstrated that people
will fairly consistently judge among themselves the "unwantedness,"
"unacceptableness," "objectionableness," or "noisiness" of sounds
that vary in their spectral and temporal nature provided that
the sounds do not differ significantly in their emotional mean-
ing and are equally expected. Presumably this consistency is
present because men learn through normal experience the relations
between the characteristics of sounds and their basic perceptual
effects; masking, loudness, noisiness, and, for impulses, startle.
This is a basic premise of the concept of perceived noisiness and
of the word noise as unwanted sound
Psychological-sociological factors can usually be recon-
ciled with the general attribute of sound called perceived
noisiness. ...[Some have].-.found that propaganda, stress-
ing the importance of military aviation to the people and the
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plans of the government to control and lessen the noise, reduce
the willingness of citizens near military airports to complain
about the aviation noise; the reduction was equivalent to the
effect that would have been obtained by lowering the noise
levels by 6 dB or so. At the same time, the concept of perceived
noisiness would maintain that reduction of the actual noise level
should further reduce the willingness of the average person to
complain about the noise, regardless of his particular absolute
willingness at a given moment, and that this amount of average
reduction in complaints would be a function of how cleverly, and
compatible, to the attribute of perceived noisiness, the noise
spectrum and its duration were tailored. ... It has been ...
proposed to obtain the quantitative relations between some of
these psychological, sociological, and attitudinal factors and
noise exposure. According to this concept, one could apply
correlations or adjustments during the calculation or measure-
ment of noise exposures to take these factors into account.
Although the evaluation of the relative contribution of the
physical aspects of sounds to their perceived noisiness should
in no way interfere with or diminish the manipulation of psycho-
logical and sociological factors in the control of environmental
noise, basic aspects of perceived noisiness probably set certain
fundamental limits, as will be discussed later, on the tolerability
of noise.
Loudness versus Noisiness. Loudness of sounds is often assumed
to be an adequate indicator of the unwantedness, for general noise
control purposes, of sounds. Experiments have shown, however,
that for many sounds there are differences between some physical
aspects of sounds, and judgments of loudness compared to judgments
of perceived noisiness. The difference between loudness and
perceived noisiness in terms of spectral content per se (the equal
loudness vs. equal noisiness contours) is insignificantly small
for broadband sounds, On the other hand, the differential
effects of duration and spectral complexity upon these two attributes,
are rather large.
The fact that loudness is apparently not influenced by duration
and spectral complexity features of a sound would seem to dis-
qualify loudness as an appropriate attribute for the estimation
of the unacceptability of environmental noises. Although loud-
ness and perceived noisiness differ in some respects, an assump-
tion of the concept of the perceived noisiness of non-impulsive
noises is that, as the intensity of a noise changes, keeping other
factors constant, the subjective magnitude of loudness and noisiness
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change to a like degree; e.g., a 10 dB increase in the physical
intensity of nonirapulsive sounds causes a doubling of the subjective
magnitude of its loudness and its noisiness. There is some experimental
proof of this common relation between this subjective scale of noisi-
ness and loudness, but, as with loudness, the scale found is somewhat
dependent on the experimental methods used and sounds judged.
Instructions to Subjects. The words used in the instructions to the
subjects for judgment tests of the acceptability of sounds have some
influence upon their rating of sounds, It is difficult and
probably academic to fathom what is the basis for the range of differ-
ences [usually small] , such as whether the words used really mean
different things to different people. In any event, there is no apparent
reason why listeners should not be. asked to rate directly sounds in terms
of their unwantedness, unacceptability, annoyance, or noisiness, as
synonyms, rather than to rate their loudness in the expectation that the
latter is an indirect clue to the noisiness or unwantedness of the
sounds.
Following are parts of the instructions that have been given to subjects
who were asked to make subjective judgment tests of the noisiness of
sounds. "Instructions. Method .of Paired-Comparison. for Judgments o£
Noisiness. You will hear one sound followed immediately by a second
sound. You are to judge which of the two sounds you think would be the
most disturbing or unacceptable if heard regularly, as a matter of course
20 to 30 times per day in your home. Hemember, your job is to judge
the second of each pair of sounds with respect to the first sound of
that pair. You may think that neither of the two sounds is objection-
able or that both are objectionable; what we would like you to do is
judge whether the second sound would be more disturbing or less disturb-
ing than the first sound if heard in your home periodically 20 to 30
times during the day and night." The purpose of including in the in-
structions to the listeners a number of terms in rating the noisiness
or unwantedness of expected sounds is to try to reduce possible differ-
ences in how different subjects might interpret the purpose or intent
of the judgments when only one term such as "disturbing" or "annoyance"
is used.
• * • » *
Five Physical Aspects. So much for the general concept of the perceived
noisiness of individual sounds. For practical purposes the measurable
physical aspects of a sound that are most likely to control its per-
ceived noisiness must be determined. To date, five significant features
have been identified or suggested —(l) spectrum content and level;
(2) spectrum complexity (concentration of energy in pure-tone or narrow
frequency bands within a broadband spectrum); (3) duration of the total
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sound; (4) duration of the increase in level prior to the maxi-
mum level of nonimpulsive sounds; and (5) the increase in level,
within an interval of 0.5 sec, of impulsive sounds. Some physical
aspects that might seem important—for example Doppler shift (the
change in the frequency and sometimes noted pitch of a sound as
a sound source moves towards and away from the listener) and modu-
lation of pure tones—appear to be very secondary in their effects
on people compared to the five physical characteristics mentioned
above.
The five physical factors mentioned by Kryter operate approximately
as follows: (l) Intensity, and frequency content—noisiness increases with
sound level approximately as does loudness, that is a ten-decibel increase
results in a doubling of judged noisiness. Sounds with energy concen-
trations between 2000 hertz and 8000 hertz are judged to be more noisy than
sounds of equal sound pressure level outside this range. This effect can
be equivalent to 10-20 decibels or a factor of 2-4 in judged noisiness.
(2) A concentration of energy or spectrum complexity—this may have an
effect which increases the noisiness by 2-3 times or 10-15 decibels over
that noisiness that would be predicted by the sound pressure level. (3)
Duration—the noisiness of a sound increases with its duration. The rela-
tion is logarithmic, and over a range from a few seconds to a few minutes,
an increase in duration by a factor of ten results in a change that is
roughly equivalent to ten decibels. In other words, this means an increase
in noisiness by a factor of two. Detailed study indicates that the relation
between noisiness and duration is more pronounced in the range from 1-4
seconds and less pronounced beyond 15 seconds than indicated by the general
rule just stated. (4) Duration of the period of rising sound pressure
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level—sounds that are increasing in level are judged to be of greater
noisiness than those decreasing in level. A sound that takes ten seconds
to reach a raaxiniura level may be judged more noisy than one that reaches
its maximum level in three seconds. This difference can be the equivalent
of about three decibels or a factor of 1.5 in noisiness. (5) Sudden
increases in level—in contrast, impulsive sounds that reach a high peak
very abruptly, say in less than 0.5-1.0 second, may be judged to be very
noisy. While this effect depends on the magnitude of the impulse, it can
be very large. People judge impulsive sounds to be very noisy even when
these sounds are familiar and expected.
Physical measurements of sounds can be weighted in such a manner as
to enable one to predict judgments of noisiness. The resulting decibel
values are said to be perceived noisiness levels (PNLs) and they are expres-
sed as PNdB.
There has been great debate among students of loudness and noisiness
concerning (l) whether these two attributes are the same or different; (2)
the relative importance of the various temporal and spectral attributes of
sound for loudness and noisiness; and (3) the relative merits of various
schemes for predicting loudness and noisiness from physical measurements
of sound. These debates are of some importance to the practical problems
of noise control. Mainly it is important to be able to predict the loud-
ness or perceived noisiness of a potential source of sound, such as a new
machineyWhile it is being designed.
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No doubt these debates will continue and as a result our knowledge
will become more refined. In spite of the apparent conflict and confusion,
numerous reports indicate that many of the major variables have been iden-
tified and their effects are known, at least qualitatively.
C. Verbal Descriptions of Sound and Auditory Experience
Auditory experience has a richness and variety that far exceeds those
aspects represented by loudness or noisiness. Even sustained pure tones
have the attributes of loudness, pitch, and volume. Tones appear to be of
low or high loudness, low or high pitch, and of small or large volume
(Stevens and Davis, 193&). Volume refers to the fact that some tones seem
to be large and diffuse, while other tones seem to be thin and compact.
Complex tones, being mixtures of pure tones, vary in quality or timbre and
seem to have at least three qualities in addition to loudness, pitch, and
volume. These are brightness, roughness, and fullness (Lichte, 1940).
Everyday sounds and music grow in dimensionality and variety as they are
extended in time. The full richness of sound only emerges when sounds form
a sequence spread over time. While an extremely rich visual scene can be
"taken in" at a glance, the auditory scene must be "taken in" over a period
of time. Psychologists have only begun to study the richness and variety
of auditory experience. A few studies (Solomon, 1958, 1959a, 1959b) have
been done. Even though only limited sets of sounds have been used, the
results suggest that people can meaningfully evaluate sounds on a magnitude
dimension (heavy-light); on an esthetic-evaluative dimension (good-bad,
beautiful-ugly); a clarity dimension (clear-hazy); a security dimension
(gentle-violent, safe-dangerous); a relaxation dimension (relaxed-tense);
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a familiarity dimension (familiar-strange); and a mood dimension (colorful-
colorless) . These dimensions relate to the overall spectral patterns of
the sounds, their temporal pattern of spectral changes, and their rhythmic
structure. These examples of possible dimensions are not meant to be taken
as the dimensions of auditory experience. Rather, these results are men-
tioned only to suggest the diversity of auditory experience and its des-
cription.
An approach to the verbal description of objects, events, and percep-
tion has been developed by Charles E. Osgood of the University of Illinois
(Osgood, 1952). Subjects are allowed to rate objects, events, or stimuli
along many dimensions as defined by pairs of adjectives in opposition.
After statistical treatment, it is found that many of these dimensions are
highly correlated. In general, an intensity dimension (weak-strong), an
activity dimension (active-inactive), and an evaluative dimension (good-
bad) emerge whether people are judging pictures, sounds, political ideals,
or whatever. In addition, several special dimensions are usually isolated
that are specific to the situation and the set of stimuli being judged.
Loudness and perceived noisiness are similar, but probably distinct,
attributes of auditory experience. These dimensions in turn are correlated
with many adverse effects of excess and unwanted sound. Indeed, loud-
ness and noisiness are probably the most important dimensions of auditory
experience in this regard. Other
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variables will undoubtedly be uncovered that are also of importance—the
apparent extent in space may be an example.
But if we are to reach a stage where we wish to speak of an optimal
acoustical environmentt as opposed to a damaging or intolerable environ-
ment, we shall have to learn much more about the dimensions of auditory
experience. Perhaps the techniques of Osgood and Solomon will lead to a
better understanding of auditory experience and allow improved acoustical
design. For example, it may be possible to design a vacuum cleaner that
sounds "busy" and "active" without excessive loudness.
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Section 5. ANNOYANCE AND COMMUNITY RESPONSE
Introduction
Annoyance by noise is a response to auditory experience. Annoyance
has its base in the unpleasant nature of some sounds, in the activities
that are disturbed or disrupted by noise, in the physiological reactions
to noise, and in the responses to the meaning or "messages" carried by the
noise.
The degree of annoyance and whether that annoyance leads to complaints,
product rejection, or action against an existing or anticipated noise source
are dependent upon many factors. Some of these factors have been identified
and their relative importance has been assessed. Responses to aircraft
noise have received the greatest attention. There is less information
available concerning responses to other noises such as those of surface
transportation and industry and those from recreational activities. None-
theless, the principal factors controlling annoyance appear to be understood.
Action by individuals or communities against noise sources or those respon-
sible for the regulation of noise is not as well understood; but even in
this difficult area there seem to be sufficient data to allow prediction
of major trends.
A. How Annoyance and Community Response to Noise Are Studied
Case histories. Case history data are usually collected when there
are complaints about particular noise sources. Often an acoustical consult-
ant analyzes the problem. The consultant usually obtains the following
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kinds of information: (l) He measures the sound and tries to analyze how
it is being generated by the source. (2) He interviews the involved
people. (3) He establishes hypotheses concerning the "noise problem."
(4) He suggests corrective action. If the corrective action is taken and
the "problem" is eliminated or significantly reduced, he feels that the
hypotheses were probably correct. Such case history data have contributed
greatly to our understanding of the problem of noise.
Social surveys. The social survey is a more elaborate version of the
case history. There are two kinds of social surveys. One can either study
areas that are experiencing high levels of noise, or one can deliberately
introduce a new source of noise, such as a sonic boom, and evaluate its
effects on the community.
The tools of the field study are: (l) instruments for the measurement
of the noise; (2) interviews and questionnaires; (3) records of complaints;
and (4) statistical description of the measurements, whether they be of
the noise or of the responses to it.
The appropriateness of social surveys have been discussed elsewhere
(Borsky, 1970), and there are many difficulties. The mere presence of
observers in a community as well as the way in which they present them-
selves can influence the response. The exact method of an interview and
the construction of a questionnaire are also important. The measurement
of the irregularly fluctuating noise levels within a community is also
difficult. The measurement of both the noise and the responses to it
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require careful sampling methods and adequate statistical treatment of
the resulting data.
In spite of all of these difficulties, the results of case histories
and more formal social surveys appear to be in overall agreement concern-
ing the major facts of annoyance and community response to noise.
B. Acoustical and Situational Factors
Acoustical factors. Annoyance from sound depends, in part, on the
properties of the acoustical environment, and some of these properties
were discussed in the previous section on Loudness, Perceived Noisiness,
and Unacceptability. Included among these are: the intensity level and
frequency content of the noise, the concentrations of energy in narrow
regions of frequency (pitch), the duration of a noise, the period of
initial rising intensity level, and the presence of impulses (such as
those associated with gunfire, automobile backfires, hammering, and so
on). These variables have been isolated in laboratory studies of judg-
ments of single noise events in relatively controlled and quiet environments.
Other variables become obvious in social surveys or case histories
where attention is usually focused on one kind of noise such as aircraft
noise, and other noises are considered as part of the background noise.
The definitions of the terms "noise" and "background noise" shift with the
intent of the discussion. For example, if interest is focused on aircraft
noise, then the noises of flyovers will be called "intruding noise" or
"the noise" while other noises, such as those of surface transportation,
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household devices, and so on, would be grouped together as "background
noise." It is interesting that when the "background noise" is great,
then the annoyance attributed to a particular "intruding noise" may be
less than when the same intruding noise appears against a lesser back-
ground noise. Field studies of annoyance and community responses to par-
ticular types of noises must include, therefore, direct or indirect meas-
ures of the number of repetitions of the "intruding noise," the level of
the "background noise" from all other sources, and in one way or another
the variability in the noise exposure from the combination of "intruding
noises" and "background noises."
Further complications arise in field studies because the exposure
that each individual receives is not measured. Rather, the noise is
usually measured at some rationally selected monitoring point. For this
reason, there are two other sets of acoustical variables that are crucial
for an individual's response to sound. One set concerns the transmission
path between the point where the sound is measured and the location of the
exposed person. The other set of acoustical variables has to do with the
acoustical characteristics of the exposed person's immediate environment.
Propagation of sound along a transmission path depends on many factors.
The nature of the terrain, such as the sound-absorbing properties of its
surface and whether it includes barriers which produce "sound shadows,"
are important. Weather conditions such as wind and thermal layering also
influence the transmission of sound. Thus, an individual's exposure can
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only be predicted on a statistical basis from noise monitoring stations.
An individual's exposure will depend on whether there is a building between
him and the sound source, whether he is outside or inside, in which part
of his dwelling he spends most of his time, whether windows are open or
closed, the construction of his dwelling, and so on.
The acoustical properties of an individual's immediate environment
are also important. In the exposed person's immediate environment, it is
the intensity level of the background noise and the reverberant character-
istics of the space that are crucial. For example, background noise can
mask an intruding noise. The reverberant characteristics of the space have
to do with its acoustical liveliness. For example, a room with heavy
carpeting on the floor, cloth drapes, furniture covered with fabric, and
walls and ceiling that absorb sound (either because of their construction
or treatment with acoustical materials) is acoustically dead. Such a room
is not reverberant. If the interior surface of a room is hard and acous-
tically reflective, sound within the room will "bounce around" for a long
time. This is a reverberant room. Notice that the transmission loss from
the point of measurement of the sound, the background noise, and the acous-
tical liveliness of the exposed person's immediate environment can operate
separately and in different "directions." A "dead" room with many open
windows provides little loss in the transmission path. A "live" room with
thick concrete walls and no windows may provide a large attenuation in the
transmission path, but sound that does penetrate the space will be
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frequently reflected within the room. Internally generated sound can
vary the level of background noise, and so on.
It is not surprising, therefore, that measures of acoustical variables
at the monitoring points are not successful in predicting each exposed
individual's degree of annoyance or disturbance. However, as will be
shown, measurements from monitoring points have been successful in pre-
dicting average levels of annoyance and disturbance among persons located
near the point where the measurements are made.
Relations between situational and acoustical variables. It has been
found that evaluation of intruding noises should include situational vari-
ables if annoyance, disturbance, and community responses are to be predicted.
For example, the type of neighborhood makes a difference. For a fixed
exposure, instances of annoyance, disturbance, and complaint will be great-
est in number for rural areas, followed by suburban, urban, residential,
commercial, and industrial areas, in decreasing order. Similarly, a given
noise usually will be more disturbing at night than during the day. Sea-
sonal variations have also been noted; noise is more disturbing in summer
than in winter.
Some of the situational factors that are correlated with annoyance
by noise may be related to the attitudes and activities of people in these
various locations and at different times of the day or year. But it is
also plausible that these situational variables directly influence the
noise exposures that people actually receive. Background noise levels
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vary in an appropriate manner with type of neighborhood and with the time
of day; that is, it is generally quieter in a rural than in an industrial
area, and it is often quieter at night than during the day. Also, there
are fewer acoustical barriers in rural than urban areas. There are fewer
acoustical barriers between the people and the point of raearsurement in
summer than in winter. This is true because in summer more often than in
winter people are likely to be outdoors or, when indoors, to have their
windows open.
Physical measurements of noise exposure. From the previous discus-
sion, it should be obvious that annoyance, disturbance, and complaints
cannot be predicted simply by measurement of the sound emitted by a single
source. Furthermore, it should be obvious that measurements from noise
monitoring stations cannot be expected to predict the responses of partic-
ular individuals.
A variety of methods have been proposed for the measurement of commu-
nity noise or noise due to particular sources, such as aircraft, traffic,
and so on. The array of methods and their names, usually given by initials,
is bewildering to the uninitiated and the experienced specialist alike.
There are CNR, UNI, NfiF, TNI, NPL, CN3L, and even more. However, in gen-
eral, these measurement schemes are more alike than they are different.
Each includes several of the following factors: (l) a scheme for the identi-
fication of single noise "events;" (2) allowance for the intensity levels
and durations of the noise events; (3) allowance for the number of noise
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events; (4) allowance, either direct or indirect, for the intensity levels
of the background noise; (5) allowance for the variability of the intensity
levels of the noises; and (6) allowance for one or more special factors
related to the loudness or perceived noisiness of the noises. As previously
discussed, situational factors such as season, time of day, and type of
area often are included as corrections on the acoustical measurements.
The measure of community noise exposure suggested by Dr. D. W. Robinson
of England (Robinson, 1971) may have special merit. This measure, called
the noise pollution level (NPL), is conceptually simple. Furthermore, it
seems to incorporate some of the same basic features as does the adaptation —
level theory developed by Helson (1964). Adaptation—level theory deals
with human reactions to and judgments of stimuli. Helson supposes that
responses to stimuli are controlled by the focal properties of the stimuli
and their variation from an adaptation level. The adaptation level is
determined by the background levels of stimulation and the residual effects
of previous and other incidental stimulation. When a variable such as per-
ceived noise level is used in conjunction with Robinson's noise pollution
level this measure seems to take into account the focal properties of the
stimulus as well as the difference between the stimulus and the adaptation
level as established by the background stimuli.
Since Robinson's noise pollution level was so recently proposed, there
has not been sufficient time to evaluate its effectiveness as a predictor
of human responses to noise.
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C. Annoyance, Attitudes, and Disruption of Activities
Annoyance and noise. Annoyance as measured in field studies is dis-
tinct from judgments of loudness, perceived noisiness, and unacceptability.
Annoyance, as described at the beginning of this section (Section 6), is a
response to noise rather than a dimension of auditory experience. A variety
of techniques have been used to measure the annoyance that results from
noise. The most direct is simply to ask a person to categorize his degree
of annoyance. "Rate your annoyance from one to seven where one is 'no
annoyance1 and seven is 'extremely annoyed1." In general, direct ratings
have been found to be subject to a great many biasing influences especially
in studies of attitudes. In the case of annoyance by noise, however, such
direct ratings correlate very highly with more subtle and indirect measures,
and the complicated procedures developed for the study of general attitudes
may not be necessary for investigations of annoyance by noise (McKennel,
1970).
Indirect measures are obtained by asking a person about the kinds of
activities that are disturbed by noise and about the degree of the disturb-
ance. Total annoyance is calculated from a combination of the number of
activities disturbed and the degree to which they are disturbed (Tracer
Staff, 1971). For example, persons may be asked to rate the degree of
disturbance by noise for: TV/radio reception, conversation, telephone use,
relaxing outside, relaxing inside, listening to records or tapes, sleeping,
reading, and eating. The degree of annoyance might then be taken as the
sum of the ratings of the degree of disturbance (Tracer Staff, 1971).
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Annoyance by noise depends in part on the characteristics of the
noise itself, and typical results are illustrated in Figures IS and 19.
These graphs support the contention that the average degree of annoyance among
people in an area can be predicted from the characteristics of the noise
measured at an appropriately selected monitoring point. Nonetheless,
reported annoyance also depends on other attitudinal-psychological factors.
Annoyance and attitudes. There are several attitudinal-psychological
factors which correlate with the degree of scaled annoyance. These can be
classified under; (l) general attitudes toward noise including differences
among individuals in their sensitivity to noise; (2) attitudes of the
exposed person toward the source of noise, such as whether they consider
the noise-producing activity to be important for their social and economic
well-being and whether they believe that the noise is a necessary by-product
of the activity that produces it; (3) whether they believe that those
persons responsible for the operation and regulation of the noise-producing
activity are concerned about their (the exposed population's) welfare; and
(4) factors specific to particular noise sources, such as fear of aircraft
crashes or the belief that sonic booms cause property damage.
For example, highly annoyed persons are likely to believe that those
'i
responsible for the noise are not concerned about those being exposed to
the noise, and they are also likely to believe that the source of noise
is not of great importance to the economic and social success of the com-
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83 86 89 92 95 98 101 103 i
Average peak loriMss (pNdB)
Figure 18. Average scores on an annoyance scale for persons exposed to
various levels of aircraft noise are shown. The dashed lines include
two-thirds of the persons interviewed. (From McKennel, 1970, with the
permission of the author, editor, and the University of Washington Press.)
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c
CO
CL>
e
_o
w
c
o
u
c
c
c
c
36 40 44 48 52 56 60 64 68 72 76
Exposure index dB(A)
Figure 19. Average annoyance scores for persons exposed to various
levels of traffic noise are shown. Notice that the scales on Figures
18 and 19 cannot be compared for absolute magnitudes. (From Kajland,
1970, with the permission of the author, editor, and the University of
Washington Press.)
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nwnity. In addition, highly annoyed persons are: likely to have negative
attitudes toward many kinds of noise j likely to be generally sensitive to
irritation produced by noisef likely to believe that their neighbors share
their annoyance} likely to say that they would be unwilling to accept further
increases in noise levels; and likely to believe that noise is a health
hazard. People highly annoyed by the noise of sub-sonic aircraft are likely
to express a fear of a crash in their neighborhood, whereas people highly
annoyed by sonic booms are likely to believe that these booms cause property
damage. Such statements are based on statistical relations and many highly
annoyed people do not conform to the profile given above.
The examples of characteristics of highly annoyed persons were
abstracted from several sources (Borsky, 1970; Kryter, 1970; McKennel, 1970;
Tracer Staff, 1971; and references cited therein). Unfortunately, when one
tries to compare social surveys, he finds that the exact attitudinal-
psychological variables that emerge as most prominent vary from study to
study. Such variation is to be expected because of differences in the
methods, the sampled populations, and the noises.
A recently published survey of responses to the noise of sub-sonic
aircraft (Tracer Staff, 1971) reports that an individual's level of annoy-
ance as measured by interview-questionnaire techniques can be fairly
accurately predicted if one knows the noise exposure (measured at a
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community monitoring point) and the weights to assign to seven attitudinal-
psychological factors. These factors, ranked in order of predictive power,
are: (l) the fear of aircraft crashes; (2) the susceptibility of the indi-
viduals to other noises, such as banging doors, dripping water, and so on;
(3) the distance from the airport; (4) the willingness of the individual to
accept additional increases in noise exposure from aircraft; (5) city of
residence4 (6) the extent to which residents of the community believe that
they are being treated unfairly; and (?) the attitudes of the residents
with respect to the importance of the airport and air transportation. Most
of these factors can be placed into the four general classes described at
the beginning of this subsection. However, exactly why "distance from the
airport" and "city of residence" should be important is unexplained.
It is also interesting to contrast responses to sonic booms with
responses to the noise of sub-sonic aircraft. There is some indication
that annoyance from sonic booms may be most related to the physiological
and psychological responses to the suddenness of the booms, whereas annoy-
ance from the noise of sub-sonic aircraft may be more strongly related to
the activities disturbed by the noise (Tracer Staff, 1971)• The major
attitudinal factor that contributes to annoyance by sonic booms is the
belief held by many people that booms cause property damage, while the
major attitudinal factor that contributes to annoyance by the noise of
sub-sonic aircraft is the fear of aircraft crashes.
All of the above is convincing evidence that people's responses to
noise depend on their values, beliefs, and attitudes. This is not
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surprising, for the definition of noise as an unwanted sound is a state-
ment of an attitude and a value judgment. Some researchers have gone so
far as to state that the attitudinal-psychological factors are more important
for predicting annoyance (by noise) than are the properties of the noise
itself. But individuals' exposures to noise, as opposed to community
exposure measured at a monitoring point, have never been measured in these
social surveys. Also, if the noise were not present, then the attitudinal-
psychological factors could not operate. Thus, one must return to the
sound itself as the fundamental stimulus for the annoyance from noise.
Examples of activities disturbed by aircraft and traffic noises as
measured in social surveys. Two recent studies (Tracor Staff, 1971;
Griffiths and Langdon, 1968) report activities disturbed by sub-sonic-
aircraft and traffic noise. In the Tracor study (Phase I) people were
interviewed in an area with a radius of 12 miles and within an angle of
40° to the right and left of the end of a runway. These runways were in
the major airports near Chicago, Dallas, Denver, and Los Angeles. Of 4»153
persons interviewed, 98.6$ reported one or more disturbances of daily
activities by aircraft noise, and, correspondingly, at least some degree of
bother. The percentage who rated an activity as extremely disturbed were
as follows: TV/radio reception, 21$; conversation, 15$; telephone use,
14$; relaxing outside, 13$; relaxing inside, 11$; listening to records or
tapes, 9$; sleep, 8$; reading, 6$; and eating, 4$. In the study of Griffiths
and Langdon, people indicated that traffic noise disturbed sleep, conversation
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with visitors, conversation at mealtimes, TV and radio reception, and
increased the time for children to fall asleep.
Of course, these examples of disturbed activities are based on
responses to interviews and questionnaires. These lists are neither com-
plete nor do they necessarily reflect the true ranking of activities dis-
turbed by noise. For example, the interference with formal education in
schools, mentioned in Section 2, does not appear on these lists probably
because only adult residents of an area were interviewed. Also, the person
being interviewed is not necessarily aware of all the ways in which noise
may disturb his activities.
Adaptation to noise. There is little evidence that annoyance due to
community noise decreases with continued exposure. Rather, under some
circumstances annoyance may increase the longer one is exposed to it
(Borsky, 1970).
D. Community Response
There are ample data to show that community responses to aircraft
noise are related to measures of the exposures. Typical results are shown
on Figures 20 and 21. These graphs speak for themselves and clearly show
that community response can vary from indifference and mild annoyance to
highly organized community action. Complaints such as letters or telephone
108
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RESPONSE
VIGOROUS
LEGAL ACTION
THREATS OF
LEGAL ACTION
STRONG
COMPLAINTS
MILD
COMPLAINTS
MILD
ANNOYANCE
NO ANNOYANCE
D E
NOISE RATING
Figure 20. The relation between community response and noise exposure
is shown. The noise exposure increases from A to I. (From Rosenblith
et al., 1953.)
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REACTIONS TO
NOISE
I
1 FGAI ACTION -
LCpVr^l_ rl v 1 t^/l«
GROUP APPEALS
TO STOP NOISE
SOME COMPLAINTS
TO AUTHORITIES
NO COMPLAINTS
TO AUTHORITIES
THRESHOLD OF
1 ' 1 '
ESTIMATED PROBABLE
• nin DAMPC
OF REACTIONS OF
TYPICAL PERSONS TO
1
AVERAGE
^^^
>^ xx
A GIVEN NOISE * x \
ENVIRONMENT X*S,. \
x'' ^
X
x"
xx
x-
>$/
.X
./^
,' •*. y. j^t. -''
'' 1 /I .^
ANNOYANCE
NEF 0 10
CNR 65 70 75 80 85
X X
x .X
^
/
s
,''
x
^
1
20
90
AVERAGE
PERCENT
OF PEOPLE
RATING NOISE
ENVIRONMENT
UNACCEPTABLE
• i x" i^
^/' / X
lX .x*^ x
x v • • •• ^x^ •••• ••• ^ 60
xx \ -x*"^ x''
XX \ x xX^ ^
x>^
. ••••••••••• •••••• • -"25
x'x\. ^xx
\. xx
Vx
xx X AND • REPRESENT ~ 10
x' ACTUAL "CASE HISTORY"
x FINDINGS FOR
COMMUNITIES
« C
— 3
.1.1. 0
30 40 50
95 100 105 110 115 120
INCREASING NOISE EXPOSURE
Figure 21. Relations between community noise levels (measured in CNR
or NEF), judgments of unacceptability, and community responses are shown.
(After Kryter et al., 1971.)
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calls to relevant officials and community action are determined by much
more complicated sets of circumstances than are annoyance and disturbance
of activities. It has been shown, however, that those who complain are
not necessarily highly annoyed by noise. In spite of intensive efforts
(one study included as many as 1? sociological variables such as age,
socio-economic status, and so on), there has been little success in iden-
tifying and characterizing those who complain as opposed to those who do
not.
It is clear, however, that only a small percentage of those who are
highly annoyed or disturbed actually register a formal complaint to some
authority in the form of a letter or a telephone call. For example, it
was found (Tracer Staff, 1971) that in an area with high noise levels, the
number of highly annoyed households per thousand (h) can be predicted from
the number of complaints per thousand (c) in the area by the simple
equation,
h » 196 + 2c.
By simple calculation, if there are 200 persons who complain in a
tract of 1,000 households, there will be nearly 600 highly annoyed house-
holds! This equation, however, probably holds only for a given set of
sociological and political circumstances. Annoyance is probably a good
measure of the potential for complaint and action. Whether complaints or
anti-noise actions actually develop will depend on social and political
factors such as the presence of anti-noise leadership, attitudes toward
the source of noise or regulatory agents, and so on. These last-mentioned
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factors are only a few of the factors which make up the whole of community
dynamics. These community dynamics, which are poorly understood, very
strongly control anti-noise actions.
Examples of the complexities of community response are given by the
following two case histories.
Case History A. The noise source was a positive displacement
blower within three hundred feet of an apartment house. The
sound levels were sufficiently high to cause the pure tone gen-
erated by the blower to be heard clearly along the face of the
three-story apartment house during the day, and to be well above
the ambient sound level at night. The industrial plant and the
apartment house were over one thousand feet from a main truck
route through the suburb of a major eastern city. The noise
produced by the blower was inaudible halfway to the highway
because of the shielding of buildings and distance. However,
the neighbors of the plant and those living along the highway
had joined together as a neighborhood association to fight the
industrial plant on the noise issue. The residents within five
hundred feet of the highway, all owners of private dwellings,
were unable to hear the plant noise, but appeared to have joined
with the other members of the community, the apartment dwellers,
to fight the "noise problem" even though they could not hear the
noise at their homes. Investigation of the situation showed that
a traffic hazard problem existed for the community along the high-
way and that a death had occurred because of the hazardous condi-
tions. The investigation further showed that the identifiability
of the owner of the noise source and the focus of the community
effort on the noise problem served as an outlet for their frustra-
tions. This in turn caused a community response out of proportion
to the actual number of people exposed.
Case History B. The community was complaining to local officials
and to the industrial plant of mechanical vibrations shaking their
homes. The vibrations ostensibly originated in a new railroad-
car-shaker building in which heavy duty vibrators were attached
to railroad cars in order to shake the contents loose during their
transfer from the hopper car to a plant conveyor. The neighbors
were located 300 to 500 feet from the car shaker. Measurement of
the vibrations in the earth at various distances from the car-
shaker facility indicated that at distances beyond 50 feet, no
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vibrations were detectable, indicating levels well below O.Olg.
Measurement of the airborne sound levels at the neighboring
residences showed levels of 64 dB(A), and in the octave band
centered at 31.5 Hz of 80 decibels. This results in mechanical
forces at the face of the residences sufficiently large to cause
the walls of the houses to vibrate with amplitudes of 0.1 inch.
The low-frequency noise in the air is inaudible to all but a
well-trained listener. However, the result of the shaking wall
is a rattling sound, and this higher frequency sound is readily
associated by the residences with the plant operation. It
should be pointed out that the neighbors are exposed to broad-
band noise levels from the plant in the neighborhood of 58 dB,
which have in the past produced no complaints. Elimination of
the low-frequency radiation from the shaker building eliminated
complaints.
(These case histories were provided by Goodfriend-Ostergaard
Associates of Cedar Knolls, New Jersey.)
3. Concluding Statement
Community noise exposure can be measured and summarized. There are
a variety of competing methods that take into account at least some of the
following, not necessarily independent, factors: (l) a scheme for identi-
fication of noises; (2) the intensity levels and durations of identifiable
noise events; (3) number of occurrences of the noise events; (4) the back-
ground noise level; (5) the variability of the noise levels; (6) one or
more special factors related to the perceived noisiness or loudness of the
sounds; and (?) the time of day and type of area, whether urban, suburban,
rural, and so on. While efforts to standardize and refine these measure-
ments will and should continue, many of the important variables have been
identified and methods for the measurement have been developed. Of course,
these methods cannot accurately measure any single person's exposure to
noise.
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The degree of annoyance averaged over a large number of individuals
near a noise monitoring station can be predicted, in a statistical sense,
from the physical characteristics of the noise. Each individual's degree
of annoyance cannot be as accurately predicted as can the average annoy-
ance. This is true because individuals differ considerably in the exact
noise exposure they receive (due to variations in environmental acoustics),
because individuals differ in their sensitivity to disturbance by noise,
and because individuals differ in other relevant psychological and social
attitudes.
Community responses to noise can range from indifference and mild
annoyance to highly-organized group action. Those who complain about air-
craft noise cannot be identified as having a special set of psychological
and sociological characteristics. Those who complain about aircraft noise,
contrary to the beliefs of some, are not highly sensitive to noise
they do not seem to be, in general, unusual citizens. Nevertheless, total
numbers of complaints and community anti-noise action are correlated with
measures of the severity of the noise exposure.
While community responses to aircraft noises have been more thoroughly
studied than the responses to other noises, such as those of traffic and
construction, case histories reveal that people become annoyed and they
complain about a wide variety of noises. In addition to noise exposure,
psychological and sociological considerations modify the extent of the
annoyance and the inclination to complain. Case histories also reveal
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that, regardless of whether the noise is produced by traffic, aircraft,
construction and so on, the probability of overt action against the noise
producers or regulators can be estimated from knowledge of the noise expo-
sure. However, these estimates are fallible and numerous exceptions can
be cited.
Two speculations about possible future community actions may be
worthy of note. Right or wrong, these speculations serve to illustrate
how attitudes and beliefs might combine with actual exposure to noise to
influence anti-noise actions.
In a recent survey, members of a sample of about 8,200 people who
live within 12 miles of airports in seven major cities of the United States
were asked whether they would be able to accept increases in noise exposure
from aircraft operations. Fifty-four percent replied that they could not
(Tracer Staff, 1971). This, coupled with the fact that fear of aircraft
crashes strongly enhances the annoyance produced by aircraft noise, leads
to the speculation that substantial increases in aircraft traffic along
with a few crashes in populated areas could result in vigorous community
action against aircraft operation and those responsible for its regulation.
A second speculation is this. If members of a community believe that
noise is necessary to an approved activity and if they believe that people
are free to move away from the noise, then they will be less likely to
institute or support action against the source of noise than if they dis-
approve of the activity or believe that there is no freedom to move so as
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to escape the noise. If this speculation is correct, then perhaps an
increase in the total area or number of persons exposed to annoying levels
of noise would result in an increase in support for anti-noise actions.
One fact about the relations among perceived noisiness, annoyance
from noise, disturbance of activities by noise, complaints about noise,
and community actions against noise is especially significant. It is that
noisiness, annoyance, and disturbance of activities are more closely tied
to the physical characteristics of the noises than are the rates of for-
mally placed complaints or the probabilities of group anti-noise action.
Thus, whether or not one files a formal complaint or participates in group
anti-noise action, the quality of one's life is influenced by unwanted
sound.
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Section 6. OTHER POSSIBLE PSYCHOLOGICAL AND SOCIOLOGICAL EFFECTS
An Old Story
Scene. It is a courtroom in a rural area. There is a
judge on the bench and a defendant before him.
The defendant is about 50 years old, poor, and
uneducated, but well-knomand well-liked in the
community.
Jud^e. "You are charged with stealing chickens from Brown's
coop. There is a strong case against you. I advise
you to plead guilty, and we'll try to make it as easy
for you as possible. How do you plead?"
Defendant. "Not guilty, Judge."
Judge. "Aw, don't do that. It'll just cause us all a lot
of trouble. The prosecutor has ten witnesses who
saw you stealing those chickens."
Defendant. "That's nuthin1 Judge, I have twenty witnesses who
didn't."
Introduction
There have been numerous claims about many deleterious psychological
and sociological effects of noise on man. Many of these are difficult to
evaluate because of conflicting information (ten people saw them and twenty
didn't)—or because of lack of information (nobody looked). In many cases,
firm conclusions cannot be drawn and one must rely on one's experience,
intuition, and judgment, as well as upon published data in order to reach
a tentative conclusion.
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Even the selection of the claims to be discussed requires a consider-
able degree of arbitrary judgment. The areas discussed in this section
were selected on the basis of the amount of available information and on
the basis of judgments concerning plausibility, importance, and interest.
No more could be done.
A. Noise and Performance
The action of noise on the performance of tasks has been studied
extensively in the laboratory and in actual work situations. Excellent
summaries and reviews of these studies are available (Broadbent, 1957;
Burns, 1968; Cohen, 1969; Kryter, 1970; Kryter et al., 1971).
When a task requires the use of auditory signals, speech or nonspeech,
then noise at any intensity level sufficient to mask or interfere with the
perception of these signals will interfere with the performance of the
task.
When mental or motor tasks do not involve auditory signals, the effects
of noise on their performance have been difficult to assess. Human behavior
is complicated and it has been difficult to discover exactly how different
kinds of noises might influence different kinds of people doing different
kinds of tasks. Nonetheless, certain general conclusions have emerged.
(l) Steady noises without special meaning do not seem to interfere with
human performance unless the A-weighted noise level exceeds about 90 deci-
bels. (2) Irregular bursts of noise are more disruptive than steady noises.
Even when the A-weighted sound levels of irregular bursts are below 90
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decibels, they may sometimes interfere with performance of a task. (3)
High-frequency components of noise, above about 1000-2000 hertz, may pro-
duce more interference with performance than low-frequency components of
noise. (4) Noise does not seem to influence the overall rate of work, but
high levels of noise may increase the variability of the rate of work.
There may be "noise pauses" followed by compensating increases in work
rate. (5) Noise is more likely to reduce the accuracy of work than to
reduce the total quantity of work. (6) Complex tasks are more likely to
be adversely influenced by noise than are simple tasks.
It has been and will continue to be difficult to assess the effects
of noise on human performance. Laboratory studies are usually of short
duration and the subjects are usually well-motivated young adults. These
subjects may be able to perform without decrement in noises that might
influence performance under more "everyday" conditions. Studies of the
effects of noise in actual work conditions are difficult because factors
other than the noise itself are difficult to control.
Sven when a person maintains high performance in noise as opposed to
quiet, there may be a cost. This cost might include reduced psychological
or physiological capacity to react to additional demands and increased
fatigue after completion of the task (Finkelman and Glass, 1970; Glass et al.,
1969? Glass et al., In press).
The effects of noise on human performance are often conceptualized
in terms of three classes of effects: (l) arousal; (2) distraction; and
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(3) specific effects. Arousal of bodily systems including the musculature
can result in either detrimental or beneficial effects on human performance.
The direction of the effect will depend on the nature of the task and on
the person's state prior to exposure. For example, noise might induce
muscular tension that could interfere with delicate movements. On the
other hand, a sleepy person might be aroused by noise and, therefore, may
perform more effectively in noise than in quiet. Distraction can be thought
of as a lapse in attention or a diversion of attention from the task at
hand. Often distraction is due to the aversive or annoying characteristics
of the noise. Distraction can sometimes be related to the physiological
responses to noise or to the responses to messages carried by the noise.
Also, if the noise is sufficiently intense, it may somehow "overload" the
mental capacities and result in a momentary lapse in attention or "mental
blink." Specific effects include auditory masking, muscular activation
such as startle responses to brief intense noises (sonic booms, backfires,
etc.) and the like.
Many physiological and psychological responses to sound diminish or
disappear when noises are regular or predictable. Sometimes strategies
can be learned so that the detrimental effects of noise on performance can
be avoided. Under certain conditions noise may even result in better con-
centration due to auditory isolation provided by the noise's masking of
other sounds, greater activation and alertness of the worker, or pace
performance when the noise is regular or rhythmic. For these reasons,
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people sometimes achieve excellent performance or even exceed their normal
performance in spite of noise.
Noises, however, often are not regular and predictable, adaptation
of responses to noise is not always complete, and strategies to eliminate
the effects of noise are not always learned. Furthermore, the fact that
distraction or disturbance can be the result of the "message" carried by
the noise rather than a result of the noise, p_er se_, may not seem important
to the average person. An ideal acoustical environment is one that does
not disturb human performance either because of the properties of the noise
itself or because of irrelevant messages carried by the noise. The trick,
of course, is to eliminate disturbing noises while maximizing the chances
that important, relevant messages carried by sound will reach the appro-
priate party.
B. Acoustical Privacy
Without opportunity for privacy, either everyone must conform strictly
to an elaborate social code, or everyone must adopt highly permissive
attitudes. Opportunity for privacy avoids the necessity for either extreme.
In particular, without opportunity for acoustical privacy one may experience
all of the effects of noise previously described and, in addition, one is
constrained because his own activities may disturb others (Cohen, 1969).
Without acoustical privacy, sound, like a faulty telephone exchange, often
reaches the "wrong number."
It would be helpful for owner and renter and for seller and buyer if
standardized acoustical ratings were developed for dwellings. These ratings
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might include measures of acoustical privacy as well as other measures
of acoustical quality. Such ratings would be particularly useful because
the acoustical properties of a dwelling are not immediately obvious to
the nonspecialist. If such ratings were available, the parties involved
could balance the acoustical value of a dwelling in relation to those of
appearance, size, convenience, cost, and so on.
C. Time Judgments
Steady noise with an A-weighted sound level up to about 90 decibels
seems to expand the subjective time scales; that is, less time has been
judged to pass than actually has (Hirsh et al., 1956).
Steady noise more intense than about 90 decibels seems to contract
subjective time; that is, more time is judged to pass than actually has
(Jerison and Arginteanu, 195^).
D. Effects on Other Senses
A variety of effects of auditory stimulation on the other senses have
been reported. These are called intersensory effects. Subtle intersensory
effects may occur as part of normal psychological and physiological func-
tion. At very high noise levels, more dramatic intersensory effects have
been reported. For example, there can be disturbances of equilibrium at
levels of about 130-150 decibels (Anticaglia, 1970; Kryter, 1970; and
von Gierke, 1965). Dramatic intersensory effects would not occur in
response to current levels of community noise.
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E. Mental Disorders
There is no definitive evidence that noise can induce either neurotic
or psychotic illness. There is evidence that the rate of admissions to
mental hospitals is higher from areas experiencing high levels of noise
from aircraft operations than in similar areas with lower levels of noise.
The type of person most affected appears to be the older woman who is not
living with her husband and who suffers from neurotic or organic mental
illness (Abey-Wickrama et al., 1969). These authors did not believe that
aircraft noise caused mental illness, but their tentative conclusion is
that such noise could be a factor that increases admissions to psychiatric
hospitals.
F. Anxiety and Distress
Nausea, headaches, instability, argumentativeness, sexual impotency,
changes in general mood, general anxiety, and other effects have all been
associated with exposure to noise (Andriukin, 1961; Cohen, 1969; Davis,
1958; Jansen, 1959; and Shatalov et al., 1962).
These effects are difficult to assess because intense noises are often
associated with situations that in and of themselves, even without noise,
might involve fear and stress. Whether the noise, purely as noise, con-
tributes significantly to the stress of life (see PART III) is difficult
to assess at this time. But all of the facts of speech interference,
hearing loss, noisiness, annoyance, and arousal and distraction previously
recited clearly support the contention that noises can act as a source of
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psychological distress, either because of responses directly to the noise
itself or because of responses to irrelevant "messages" carried by the
sound. Psychological distress in turn can contribute to the unpleasant
symptoms listed above.
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PART III. GENERAL PHYSIOLOGICAL EFFECTS
Preliminary Statement
There are three classes of transient general physiological responses
to sound: (l) the fast responses of the voluntary musculature mediated by
the somatic nervous system; (2) the slightly slower responses of the smooth
muscles and glands mediated by the visceral nervous system; and (3) the
even slower responses of the neuro-endocrine system.
It has been proposed that frequent repetition of these responses might
lead to persistent pathological changes in non-auditory bodily functions
(Jansen, 1959, 1969). Also, it has been proposed that frequent repetition
of these transient physiological responses might aggravate known disease
conditions. These proposals have not been verified, but evidence consistent
with them has been gathered (Kryter et, al., 1971; von Gierke, 1965). While
these claims of noise-induced pathology of non-auditory bodily function merit
further research and investigation, they are as of now unproven.
The transient physiological responses to sound, the possible persistent
physiological responses to sound, and the possible relation of noise to
stress theory are each discussed in the sections that follow.
Section ?. TRANSIENT AND POSSIBLE PERSISTENT
PHYSIOLOGICAL RESPONSES TO NOISE
A. Transient Physiological Responses to Noise
Responses of the voluntary musculature. Man is equipped with an
elaborate set of auditory-muscular reflexes. These serve the basic
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functions of orienting the head and eyes toward a sound source and of pre-
paring for action appropriate to an object whose presence is signalled by
a sound. These reflexes operate even at low levels of sound (Bickford
et al., 1964; Davis, 1950; Mast, 1965), and they can often be detected by
suitable electrical recording and averaging even after bodily movements
have habituated and are no longer detectable. Auditory-muscular reflexes
undoubtedly play a part in all muscular responses to sound. These may range
from rhythmic movement and dance to the body's startle response to impulsive
sounds such as those produced by gunshots or sonic booms.
These muscular responses to sound can be measured by direct observa-
tion of bodily movements (sometimes with the aid of amplifying levers or
high-speed motion pictures) or by measurements of the electrical activity
of the musculature.
The startle response has been studied in detail (Landis and Hunt,
1939)- It includes an eyeblink, a typical facial grimace, bending of the
knees, and, in general, flexion (inward and forward) as opposed to extension
of the bodily parts. The startle response to the sound of a nearby gunshot,
even when expected, may undergo various degrees of diminution with repeti-
tion of the sound. The amount of diminution of the response depends on the
individual, the rate of repetition, and the predictability of the impulse
sound. Some individuals show little diminution of the startle response
with repetition while others show a marked reduction of this response. The
eyeblink and head movement aspects of the startle response may never
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habituate completely. Even experienced marksmen exhibit these responses
each time they fire a gun.
All of the observations described in the preceding paragraph were
made with the aid of high-speed motion pictures. Using the electrical
devices available to them, Davis and Van Liere (1949) found that muscular
responses to the sound of a gunshot did not disappear with repetition.
An early response (the a-response with a latency of about 0.1 second)
showed little reduction with repetition of the sound. A later response
(the b-response with a latency of about 0.8 second) showed more reduction
with repetition.
A series of experiments, done by R. C. Davis and his colleagues at
Indiana University, demonstrated that the particular muscular responses
to sound and the way in which these responses will influence the performance
of a motor task depend in detail on (l) the pattern of muscular tension,
or posture, prior to the sound, (2) the movements required by the task, and
(3) the auditory-muscular reflexes (Davis, 1935, 1942, 1948a, 1948b, 1956a,
1956b, 1956c, 1956d, 1956ej and Fatten, 1953).
Among the important findings was that the magnitude of the muscle-
tension reflex in response to sound increased with increasing resting
tension in the muscle. (This generalization, of course, would not hold as
a muscle approaches its maximum level of tension.) Thus, if the subject
was required to make a movement that required flexion and if the subject's
posture heightened tension in the appropriate flexor muscle, then a burst
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of sound, which ordinarily produces the reflex action of flexion, would
speed the performance of the movement. Under other conditions, however,
the burst of sound could greatly interfere with the required movement.
For example, suppose that as before, the required movement was that of
flexion but that the subject's posture heightened the resting tension in
the opposing extensor. In this case, the burst of sound would result in a
greater response in the extensor (because of the higher resting tension)
than in the flexor, and consequently, the required flexion response would
be interfered with and delayed.
R. C. Davis (I956b, 1956d) also found that steady noise of 90 decibels
increased tension in all muscles and influenced the response time in a
simple choice task.
In summary, the ebb and flow of muscular activity is closely linked
to and influenced by the rise and fall of sound. The relations are com-
plicated. Gross bodily orientation toward an unexpected source of sound
will diminish as the sound becomes familiar and predictable. Some com-
ponents of the startle response to impulse sounds, for instance, will
diminish with the repetition of the stimulus. The exact amount of reduc-
tion, however, depends on the individual person, his state of muscular
tension as defined by posture or activity, and the characteristics of the
impulse sound. Subtle changes in the musculature in response to sound may
persist and their effects will depend in a complicated way on posture,
activity, and the characteristics of the sound.
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Responses ofthej jm^othL muscles and glands. In response to brief
sounds there is general constriction in the peripheral blood vessels with
a reduction in peripheral blood flow. There may be acceleration or
deceleration of heart rate, reduction in the resistance of the skin to
electrical current (an indication of activation of the peripheral visceral
nervous system), changes in breathing pattern, changes in the motility of
the gastro-intestinal tract, changes in the size of the pupils of the eyes,
and changes in the secretion of saliva and gastric secretions (Davis e£ al.,
1955; Jansen, 1969). These responses to brief sounds are obvious for
A-weighted sound levels over about 70 decibels. For sound levels below
70 decibels, it is doubtful whether the recording techniques have been
sufficiently sensitive to detect whether these responses occur. In any
case, they are either small or nonexistent.
Some aspects of these responses diminish and seem to disappear with
predictable repetition of the sounds. Others may not disappear (Davis
et al., 1955). Jansen (cited by von Gierke, 1965), for example, found
these responses persisted in industrial workers when they were exposed to
the same noises in which they had worked for many years.
Orienting and defense reflexes. Some of the responses of the smooth
muscles and glands to sound are part of a pattern of response known as the
orienting reflex. The orienting reflex is a "what is it" response, and
this reflex diminishes rapidly as a stimulus becomes familiar and pre-
dictable.
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Some of the responses of the smooth muscles and glands in response
to sound are part of a pattern of response known as the defense reflex.
A defense reflex prepares the organism to escape or accept injury and
discomfort. Responses that are part of a defense reflex disappear more
slowly with stimulus repetition than do those that are part of the orient-
ing reflex. Sometimes they may never completely disappear. Defense
reflexes occur in response to warnings of painful stimuli, to painful
stimuli themselves, or in response to very intense stimuli to any sense
organ. Informative discussions of the orienting and defense reflexes can
be found elsewhere (Sokolov, 1963a, 1963b; Voronin et al., 1965).
Neur o-endocr ine responses. Loud sounds as well as other intense
stimuli such as cold, forced immobilization, forced exercise, pain,
injuries, and so on can activate a complicated series of changes in the
endocrine system with resulting changes in hormone levels, blood composi-
tion, and a whole complex of other biochemical and functional physiological
changes (Lockett, 1970; Welch and Welch, 1970). Some of these changes and
their implications will be mentioned in the sections to follow.
B. Possible Persistent Physiological Responses to Noise
It has been claimed that steady noise of approximately 110 decibels
can cause some changes in the size of the visual field after years of
chronic exposure, but there is very little evidence to support this conten-
of
tion. Noise/about 130 decibels can cause nystagmus and vertigo. However,
these noise conditions are rarely encountered in the present environment
(Kryter et al., 1971)•
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Evidence from animals exposed to very high noise levels suggests that
exposure to these noises can interfere with sexual-reproductive functions,
can interfere with resistance to viral disease, and can also produce other
pathological effects (Kryter et al., 19?lj Welch and Welch, 1970). Among
these other effects are hypertrophy of the adrenal glands, developmental
abnormalities of the fetus, and brain injury (Welch and Welch, 1970). These
experiments often have not been well controlled; e.g., fear, handling, and
so on have not always been equated for noise-exposed animals and non-noise
exposed animals. Also, rodents have often been used as subjects, and these
animals are known to have special susceptibility to the effects of certain
sounds. Furthermore, the sound levels used in these experiments have usually
been well above those normally encountered in our present environment.
There is evidence that workers exposed to high levels of noise have
a higher incidence of cardiovascular disorders; ear, nose, and throat prob-
lems; and equilibrium disorders than do workers exposed to lower levels of
noise (Andriukin, 1961; Jansen, 1959, 1969; Kryter et al., 1971)- The
results of one of these studies are summarized on Figure 22.
The fact that those who work in high noise levels show greater evidence
of medical problems than those who work in lower noise levels is not con-
clusive evidence that noise is the crucial factor. In each case it is pos-
sible that the observed effects can be explained by other factors such as
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PERIPHERAL CIRCULATION PROBLEMS
N = 410 | VERY NOISY INDUSTRIES
N = 165 | LESS NOISY
HEART PROBLEMS
N = 161
VERY NOISY INDUSTRIES
N = 53 [ LESS NOISY
EQUILIBRIUM DISTURBANCE
N = 128 [ VERY NOISY INDUSTRIES
N = 51 LESS NOISY
I
20 40 60 80 TOO
PERCENT OF OCCURENCE
Figure 22. Differences between the percentages of physiological problems
of those who work in two different levels of noise. These data are from
1005 German industrial workers. Peripheral circulation problems include
pale and taut skin, mouth and pharynx symptoms, abnormal sensations in
the extremities, paleness of the mucous membranes, and other vascular
disturbances. (From Kryter erb al., 1971-)
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age, dust levels, occupational danger, life habits, and other non-noise
hazards. However, much more research of this type should be undertaken
with attempts to rule out the effects of non-noise factors.
From the facts presented about transient physiological responses to
noise, one can argue that chronic arousal by sound might lead to some of
the medical problems just described. These transient responses ordinarily
are useful to man because they help protect him from potentially harmful
events. It is also appropriate that these responses diminish when repeti-
tion of the stimulus signifies that particular noises do not represent a
threatening or harmful condition. The crux of the problem is whether man
is so designed to adapt to sufficiently loud or abrupt sounds or whether
the modern environment presents such ever-changing auditory stimulation
that arousal responses are chronically maintained.
Section 8. STRESS THEORY, HEALTH, AND NOISE
A. Stress Theory
The neuro-endocrine responses mentioned in Section 7 are similar to
the responses to stress. The response to stress is called the general
adaptation syndrome (Selye, 1956). It consists of three stages: an alarm
reaction, a stage of resistance, and a stage of exhaustion. If a stressor
is very severe and is maintained for prolonged periods of time, an organism
passes in succession through the stages of the alarm reaction, resistance,
and exhaustion. In the extreme case, the end result is a breakdown of
bodily function and death. In a less severe case, there may be a price to
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be paid in the stage of resistance. This price may include lowered resist-
ance to infection, and perhaps, specific diseases known as the diseases
of adaptation. These may include, among others, some types of gastro-
intestinal ulcers, some types of high blood pressure, and some types of
arthritis. Many medical authorities do not accept the theory that there
are diseases of adaptation. Rather, they theorize that each disease has
its own special set of causes.
Stress theory, even as presented by its strongest advocates, is com-
plicated. These advocates speak of complicated interactions between con-
ditioning factors that set the scene for disease, specific reactions to
particular stressors, and general reactions to non-specific stressors.
It is nearly certain that noise of extremely high level can act as a
stressor and, at least for some animals, can lead to some of the physiolog-
ical changes associated with the general adaptation syndrome. Also, it is
plausible that some of the more intense noises encountered in our present
environment can act as stressors for people. However, the details of hotf
such noises might act as stressors for people are unknown. The intensity
level of the noise, the amount of fear and annoyance produced by the noise,
and the susceptibility of the individual are probably examples of important
factors. While certain pathways in the central nervous system and the
hormonal system are probably important, these have not yet been established
for the case of noise. For example, it could be necessary for the noise
to produce ear damage, evoke annoyance and negative emotional reactions,
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or disturb sleep before elements of the general adaptation syndrome would
appear. The picture is further complicated by the fact that a mild amount
of stress at the right time of life may be beneficial. Therefore, while
it is plausible that noise can be a detrimental stressor for people, it
appears to be impossible to make firm statements about noise stress at
this time.
B. Noise and General Health
While physiological arousal in response to sound can be of great
benefit in the maintenance of response to possibly dangerous events,
unnecessary arousal to irrelevant sounds can provide a basis for annoyance
and for interference with performance of tasks. Chronic arousal from
noises of sufficiently high levels or from noises that are sufficiently
varied may, although it is unproven, contribute to the incidence of non-
auditory disease. However, the evidence does suggest that, if noise control
sufficient to protect persons from ear damage and hearing loss were insti-
tuted, then it is unlikely that the noise of lower level and duration
resulting from this effort could directly induce non-auditory disease.
Nevertheless, it is conceivable, though unlikely, that certain patterns
of exposures to irregular, brief sounds could produce non-auditory pathology
of greater significance than the noise-induced pathology of the inner ear.
As mentioned earlier (see end of Section 6), general psychological
distress produced by noise can add to the overall stress of life and in
this way may contribute to the incidence of non-^auditory disease. At this
135
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time, however, one cannot evaluate the contribution of noise-induced dis-
tress in relation to those other sources of stress we all encounter in
our daily activities.
136
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