(and receive the largest part of their noise ex-
posures) as flight instructors, and so they prop-
erly are classed as instructors.

It was impractical to test one group of people
who are regularly exposed to the noises inside
airplanes; there is no sensible way to select repre-
sentative traveling salesmen and_ professional
travelers. The group was excluded from this
series of studies, Still, although the amount of
exposure-per-week may be fairly high for trav-
elers, they generally try to “get off the road”
when they have the chance, and their long-term
noise exposure is probably fairly small.

Like salesmen, stewardesses have frequent ex-
posures to noise while they fly, but until very
recently, most had only a few years of flight
experience. On the average, these girls have kept.
flying for only two or three years. However,
even before the regulations changed, some con-
tinued to fly for 15 or 20 years, and today there
is a good chance that longer periods of exposure
will become common. Thus, the stewardess data,
will be much more important in the future.

FAA flight inspectors form a special group.
Their log books show only a little recent flight
time—certainly not much more than most private
pilots have. Yet these are, without exception,
people with a great deal of past flying ex-
perience, and many of them still spend a large
part of their time in the air. These hours are
not usually logged, though, because the time is
spent in testing prospective licensees, and in
traveling to accidents, to inspection sites, or to
testing sites; additional flight time is spent in
making route checks on air-taxi services, in wit-
nessing or performing flight checks and type-
rating tests, and in observing various other air-
craft operations. Only occasionally does the in-
spector serve as pilot-in-command. Thus, despite
the appearance of little flight time, inspectors
are among the fliers who are most exposed to
noise,

For these series of tests, then, audiometric
examinations were performed on aerial-applica-
tion pilots, flight instructors (including com-
mercial pilots), stewardesses, and FAA flight
inspectors. Additionally, a group of older-than-
average people with private-pilot licenses was
tested.

II. Method.

Experimental subjects were selected from those
available to this laboratory. They included 12
aerial-application pilots, ranging in age from 22
to 58 years (mean 39.3), 15 flight instructors
ranging from 23 to 58 years (mean 349), 12
FAA flight inspectors ranging from 36 to 56
years (mean 44.0), and 16 private pilots ranging
from 40 to 58 years of age (mean 48.9). (The
mean age for all private pilots is 35.8 years.)
Two groups of stewardesses were tested: one
group of 10 girls ranging in age from 26 to 39
years (mean 29.4), with experience ranging be-
tween 6 and 15 years, was tested in the same way
that the pilots were; another group of 106 stew-
ardesses ranging in age from 21 to 44 years
(mean 26.7) was tested by a somewhat different
method. Additional subjects included control
groups (matched for age) for all pilots and for
the 10-stewardess group. For the group of 106
stewardesses, each girl served as her own control.

Data collection for each subject started with a
short history including age, exposure to aircraft
noise in hours-per-week, total years of flying ex-
perience, and, where it was pertinent (as, for ex-
ample, with FAA inspectors), total number of
hours in the air. Questions were also asked
about other kinds of noise exposure, such as gun-
fire, rock music, snowmobile and motorcycle rid-
ing, and so on. Information was also requested
about known past history of ear disease.

Each subject was tested on a Békésy audio-
meter (Grason-Stadler model E-800 or model
1701). This device automatically produces a con-
tinuous-frequency audiogram.

Audiograms were inspected for indications of
noise-induced hearing loss or threshold shift.
This inspection was based on an objective cri-
terion. High-frequency threshold shifts are not
necessarily evidence of noise damage, but a dip
in the threshold curve (the dip or tonal gap ap-
pears as a depressed threshold in a restricted
frequency region) somewhere between 2000 and
6000 Hz is generally accepted as indicating acous-
tic trauma. A simple measurement of threshold
at 4000 Hz is not an adequate test. To make a
meaningful test, first 2 smoothed threshold curve
is produced by joining the center points of each
excursion of the audiometer’s marking pen (Fig-
ure 2). Then, in the event that an apparent dip
in acuity is noted anywhere between 2000 and





perience levels to try to find whether a measur-
able change arises at some particular point.
Divisions were made at 1000-hours experience
(approximately one year of work), 2000 hours,
3000, 4000, 5000, 6000, 7000, 8000, 9000, and 10000
hours. In each case, comparisons were made be-
tween those with more than the criterion-number
of hours, and those with the criterion-number or
less.

III. Results.

A. FAA Flight Inspectors. Every Federal
Aviation Administration flight inspector tested
shows some degree of threshold shift attributable
to noise exposure. On the average, the loss of
high-frequency hearing would be classed as mod-
erate (the worst thresholds are in the range from
40-60 dB HTL, ISO). Statistically, the fact that
the entire test. sample evidences audiometric dips
is significant. However, flight inspection is not
likely the major source of these threshold shifts.
Rather, it is the previous flight experience, pri-
marily in commercial or military aviation, that
has decreased the hearing acuity. Each one of
the subjects had many thousands of hours of
flight time logged before he was tested. Thus,
so far as their histories of noise exposure are
concerned, they are really displaced commercial
and instructor pilots.

B. Flight Instructors and Commercial Pilots.
Those who fly light planes for a living, either
as charter pilots or as instructors (or as both)
show measurable high-frequency threshold shifts
ranging from mild to moderate (the worst
thresholds are in the range from 25-60 dB HTL,
ISO) in about 85 per cent of the cases (13 out of
15). Statistically, these shifts are significantly
more frequent than those found in the control
group (p<.005).

C. Agriculture Pilots, Aerial-application pi-
pilots, among all the people who make their living
inside airplanes, receive the longest exposures
to the most noise. Every pilot in this group had
some loss of hearing, with worst thresholds rang-
ing from 30-70 dB HTL, ISO. When compared
to the control group’s hearing loss, this 100 per
cent value turns out to be highly significant
(p<.001). Clearly, agricultural aviation is not
good for hearing. Many of these pilots wear
helmets or headsets, but, particularly for the
person who flies in an open-cockpit plane, such

circumaural protection does not keep out enough
of the predominating low-frequency components
of the noise. In fact, it is even unlikely that a
combination of a good cireumaural earmuff and
a good insert earplug would provide adequate
protection against the wind-blast that a pilot
vets whenever he puts his head over the side.

D. Air Transport Pilots. This laboratory has
not made a systematic collection of audiometric
tests on airline pilots and flight engineers, so
no body of data on this population can be in-
cluded in this study. Still, it is worth noting
that, of the few who have been seen and tested,
all have had threshold shifts large enough to be
included in the noise-affected group, according
to the criterion used here. However, among
70 or 80 thousand medical examinations involved,
only one or two air-transport people a year are
grounded because of their hearing. That means
that their threshold shifts, although they some-
times meet the laboratory criterion, are not ade-
quate to interfere with speech reception in the
noisy flight environment.

E. Stewardesses; Group I. Of the 10 stew-
ardesses in the first group (all selected to have
six years or more of flight experience), eight
were shown to have noise-attributable threshold
changes. Among the girls in the control group,
only two showed a loss similar to that seen in
the stewardesses. Of these, one had spent a
considerable period of time in target shooting.
The difference between groups is significant
(p<.001), which suggests that the noise exposure
of flight does produce a measurable degree of
threshold shift in those who have spent more
than a few years flying. However, despite the
fact that this shift occurs, no case of noise-
induced shift in either group of stewardesses
could be considered clinically important. The
very largest dip measured is a narrow tonal gap
(no worse than 30 dB HTL for any stewardess
tested, and averaging only 12 dB HTL) of the
sort that has little or no effect on the hearing
of speech. It must be considered extremely un-
likely that any of the stewardesses is consciously
aware of any debilitating problem with her
hearing.

F. Stewardesses; Group II. The 106 stew-
ardesses in the larger group were tested in order
to try to answer some other kinds of questions.
The attempt was only partially successful. For



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HEARING THRESHOLD LEVEL IN DECIBELS

Ficure 2. Threshold-dip graph. The lighter, oscillating curve is the one plotted by the Békésy audiometer.
The dashed line represents the interpolated “continuation
In this example, the depth of the high-frequency dip is about 45 dB, a relatively

heavier, solid line is the smoothed threshold curve.
line” described in the text.
large value for subjects used in the present study.

6000 Hz, the curve is smoothed as if the dip
were not there. That is, a smooth continuation
line is inserted, in the gap, to join the threshold
curve at frequencies below the dip to the thresh-
old curve at frequencies above the dip. The
depth of the dip is then taken as the maximum
vertical distance between the inserted line and
the threshold line. Separations deeper than 15
dB were classified as significant. Any time a
dip was more than 15 dB for a subject, he was
tallied as having a threshold shift that might be
attributable to noise. It was necessary to use
this kind of criterion because of the huge varia-
tions in frequency at which tonal gaps appear.
An average of all the audiograms in a group
would wash out most of the information. Meas-
uring the hearing loss or threshold shift only at
4000 Hz might or might not reflect the true state
of the subject’s hearing, particularly in the rela-

750 1000 '500 2999 3000 goqq 6000 go99
FREQUENCY IN HERTZ

The

tively unexposed groups such as stewardesses.
Also, the statistical treatment was made rela-
tively easy by this dichotomizing procedure.

Comparisons between experimental and con-
trol groups were made using Fisher’s Exact-
Probabilities Test. For these tests, the differ-
ences between experimental and control groups
are large enough so that no more than 10 sub-
jects per group were required. In some circum-
stances, more were tested—when extra subjects
were available, they were used.

Controls for the group of 106 stewardesses
were the less-exposed members. The 106 were
separated into several series of two groups; in
each series, the people with less flight time were
compared to those who had more experience.
Both the proportion with dips in each group and
the size of the average threshold shift were
studied. The groups were divided at several ex-




TECHNICAL REPORT STANDARD TITLE PAGE

1. Report No. 2% Government Accession No. 3. Recipient’s Catalog No. |
FAA-AM- 72-32
@. Title and Subtitle 3. Report Date
November 1972
AUDITORY EFFECTS OF NOISE ON AIR-CREW PERSONNEL 6. Performing Organization Code
7. Author(s) 8. Performing Organization Report No.

Jerry V. Tobias, Ph.D.

9. Performing Organization Name and Address 10. Work Unit No.
FAA Civil Aeromedical Institute
P. 0. Box 25082 T1. Contract or Grant No.

Oklahoma City, Oklahoma 73125

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Addres*
Office of Aviation Medicine

Federal Aviation Administration
800 Independence Avenue, S. W.
Washington, D. C. 20591

15. Supplementary Notes

This research was performed under Tasks AM-B-69-PSY-8, AM-B-70-PSY-8, AM-A-71-PSY-8,
AM-A-72-PSY-8, and AM-A-72-PSY-37.

OAM Report

14. Sponsoring Agency Code

16. Abstract

Hearing-threshold tests were made on flight personnel of several sorts, including
aerial-application pilots, flight instructors, private pilots, stewardesses, and
FAA flight inspectors. Excluding those people whose flight experience is of
short duration, each group shows some measurable degree of threshold shift,
although this shift is frequently not enough to be regarded as a clinically
significant entity. Data on the sorts of noise exposures each group commonly
receives are presented, and some cautions are offered regarding interpretation
of the data.

17. Key Words 18. Distribution Statement

Air Crews Pilots Availability is unlimited. Document may
Cockpit Noise Threshold Shift be released to the National Technical
Hearing Information Service, Springfield, Virginia
Hearing Protection 22151, for sale to the public.
Noise

T9. Security Classif. (of this report) 20. Security Classif. (of this page) i. No. of Pages | 22. Price
Unclassified Unclassified 7 $3.00

Form DOT F 1700.7 (8-69)



example, it turned out not to be possible to spec-
ify the effects of noise from various types of
aircraft. For one thing, short-term stewardesses
(who might have flown only one type) show
relatively little threshold shift, making compari-
sons useless; longer-term girls invariably have
flown several types. The interactions and con-
founding factors are too great to permit the
desired measurements to be made.

However, it was possible to plot the way in
which patterns of threshold shift change with
added years of flying experience. For the first
few years, according to this sample, the worst.
threshold for each girl averages about 10 dB
(ISO).* (This value is not necessarily part of a
dipping region.) Then, after about seven years
of experience, the worst-threshold average jumps
to something more than 20 dB (Table 1). In

‘Taste 1. Worst-Threshold Averages for 106 Stewardesses

Hours of N Worst-threshold
fight experience average

1-1000 15 10
1001-2000 i 12
2001-3000 18 10
3001-4000 15 7
4001-5000 9 14
5001-6000 10 14
6001-7000 6 9
7001-8000 5 18
8001-9000 2 82
9001-10000 4 19
10001+ Bae 21

years ago were much noisier than those that are
around today. But more likely, these shifts
are the result of relatively long-term cumulative
exposures, without regard for the types of air-
craft involved.

An additional attempt was made to clarify the
critical period for noise exposures in stewardesses.
In this instance, short-term girls were used as
controls for long-term girls. This approach is
imperfect in two or three regards, but it does
permit a classification of sorts. Chi-square con-
tingency tables were built for various cutoff
durations of experience: those with 1000 hours
or less of flight time were compared with those
with more than 1000 hours; another comparison
was made at a cutoff of 2000 hours, and still
others were run for 3000, 4000, 5000, 6000, 7000,
8000, 9000, and 10000 hours (Table 2). The re-
sult is that, when stewardesses with fewer than
6000 hours are included in the “more-experienced”
sample, there is no useful significant difference
between the more-experienced and the less-ex-
perienced groups. But once the cutoff reaches
7000 hours, the groups are consistently different
(p<.05). The finding suggests that it takes the
first seven years or so for the noise effects to
reach our criterion values. Again, it must be

Taste 2, Influence of Flight Experience on Dips in
Hearing Thresholds for 106 Stewardesses

no case is the aircraft-noise-induced threshold
shift large enough to be noticed except on a test
as sensitive as this one, but the change is indica-
tive of the level of the sound fields in which
these people work. There is, of course, no cer-
tainty that the shifts measured are a result of
the time spent in the flight environment. It
might be possible to interpret this change as
showing that the planes that were used several

*One stewardess with less than two years of experience
had a moderate low-frequency loss of hearing. ‘The
decrement was clearly not related to noise exposure.
She had no history of infections or blockages, and was
referred to an audiological service in her home town.
She was found to be otosclerotic. Her data are not
included in the “worst-threshold” average. Another
stewardess with a 9-year history of working in the pits
at hydroplane races was also excluded from this
average.

Number Ohi-square
Hours Number without (listed only
of flight with dips dips where signif-
experienco 215 dB >15 AB cant: p<.05)
1000 or less 10 5
More than 1000 58 38
2000 or less 19 7
More than 2000 47 33
8000 or less 27 17
More than 3000 38 24
4000 or less 33 26
More than 4000 30 17
5000 or less 38 30
More than 5000 26 12
6000 or less 42 36
More than 6000 19 9
7000 or less 44 40 507
More than 7000 18 4 °
8000 or less 46 43 6.38
More than 8000 15 2 "
9000 or less aT 44 5.08
More than 9000 13 2 °
10000 or less 50 45
More than 10000 10 1 4405



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Fieve 1, Noise spectra in the cockpits of several types of aircraft. (a) An open-cockpit aerlal-application plane
with a small windscreen, (b) a light, single-engine plane, (c) a light, twin-engine plane, (d) a twin-engine
plane of a type formerly used in air transport, (e) a two-engine turbo-prop in alr-transport use, (£) a four-en-
gine turboprop in air-transport use, (g) an air-transport jet with wing-mounted engines, (h) an air-transport
jet with rear-mounted engines, (i) a business jet with rear-mounted engines, Superimposed on each spectrum
are three damage-risk criterion (DRC) curves (Kryter et al., 1966): the lowest curve represents the maximum
acceptable sound level at any frequency (a spectral line that rises above a DRC curve at any point represents
a noise that can be considered hazardous) for noise exposures of eight or more hours per day; the next DRC
curve represents the maximum acceptable level for exposures of two or more hours per day; the top DRC
curve shows the maximum acceptable exposure for eight-hour-per-day exposures for a listener wearing prop-
erly fitting earplugs. Spectra (g), (h), and (i) lack this last DRC curve,

tive; for the most part, they fly in the quietest | somewhat longer periods of exposure than air-

planes, and they do so for relatively few hours
per week. The same statements apply to flight
engineers.

Some commercial pilots, notably those who fly
“company planes” for industry, probably have

line pilots, and they may fly somewhat noisier
aircraft. Such aviators, though, are not repre-
sentative of the majority of commercial pilots.
Most of them handle charter flights as they come
along, but make the largest part of their salaries


AUDITORY EFFECTS OF NOISE ON AIR-CREW PERSONNEL

I. Introduction.

The public has become generally aware that
work around aircraft can be extremely noisy and
perhaps even hazardous to hearing. Thus, no
one is surprised that sound-attenuating earmuffs
are worn by ground crews when they are assigned
to ramps where engines are running. People
who live under flight paths often complain about
the external sounds created by airplanes. But
comparatively few people have expressed con-
cern for the noise inside the aircraft and for the
people who are exposed to it (however, there are
exceptions, such as Gasaway, 1970; Stone, 1969;
Tobias, 1968 a,b,c; and Wick et a/., 1963). Stud-
ies of cabin and cockpit noise show that many
planes are potential- producers of permanent
threshold shifts (Figure 1) for people who are
regularly exposed to the sound levels in the cock-
pit or the cabin (Tobias, 1968 a,b). Helicopters
and planes with open cockpits are the noisiest.
Then, more or less in order, are light single-
engine airplanes, light twin-engine planes, piston-
driven planes in air-transport use, turboprop
planes, planes with wing-mounted jet engines,
and planes with rear-mounted jet engines. Of
course, the rear-mounted jet-engine aircraft can
be quite noisy toward the rear of the cabin, but
toward the cockpit, they are generally quiet.
Since the concern of this paper is with the
hearing of members of flight crews, the effects of
noise at the rear of the cabin can be generally
ignored for everyone except the stewardess, who
rides there during takeoff and landing, and, as
the data will show, her problem with noise is
relatively minor.

The amount of hearing problem likely to be
engendered by a given amount of noise is a func-
tion of the susceptibility of the listener, and of
the amount of time he is exposed. A comprehen-
sive investigation would probably show that the
most exposed people are aerial-application (crop-
duster) pilots, flight instructors, helicopter pilots,
business and other commercial pilots, stew-

ardesses, airline pilots and flight engineers, and
Federal Aviation Administration flight inspec-
tors. What happens to the hearing of such
people? Systematic attempts to answer that
question are few. This study was devised to pro-
vide part of the data from which an answer can
be derived.

First, let us look at the amount of noise the
various groups receive. The most time in the
air probably accrues to aerial-application pilots
and to flight instructors; they commonly fly for
10 hours or so every day that weather permits,
and, in many parts of the country, that number
is increased to 14 hours a day or more. They
work as many days of the year as possible, and
normally they use aircraft whose noise levels
are among the highest. Many aviators involved
in agricultural work still use open-cockpit air-
planes in which the wind-blast noise is clearly
excessive for any duration of exposure (Tobias,
1968c). Helicopter pilots, too, work in ex-
tremely high intensities of noise, but the duration
is considerably shorter than it is for ag pilots.
For both of these groups, some sort of hearing
protection—a helmet, or headset cushions, for
example—is normally used. Although not en-
tirely satisfactory, these devices do afford some
attenuation. In a helicopter, the amount of pro-
tection can often be presumed to be somewhat
better than in an aerial-applications plane. How-
ever, it was not possible to locate an adequate
number of non-military helicopter pilots to test
(analyzing the hearing losses of military pilots
is complicated by their exposure to gunfire), and
so that group is not included here.

Airline pilots were also excluded because these ~
people are required to have semi-annual physical
examinations, including hearing tests, and only a
tiny minority fail because of hearing problems.
Although the precision of our laboratory tests
might have turned up some degree of noise-
attributable threshold shift, special tests of these
men would likely not be particularly informa-