VENTS OPEN
am HS
a Y
2
3 105, yy
5
a
g 95
3
8
85 120 Min
© LL —
S 480 min
£75
3 GY,
5 6s Uj
°
3
3 te chads ate nto rane 00 ste one
2 g/t i et
H a bana Taboo *
Frequency in Hz
VENTS CLOSED
— 115
8 y l
ao LAT Nason
g 108 inst
sorplug
2 | |\7 ‘
g 95
8
3
© bs vom 120 ial
0 —
8 420 Min
£ 75
.
3
+ 65
°
2
2 os re Sy Ga se20e]
Frequency in Hz
MEANS
----- 2000'
+ 6000°
+ 10000!

Friaure 2, Effects of altitude on cockpit noise. 2a shows
the mean values for each altitude when the vents were
open. 2b shows the values when the vents were closed.
The solid lines represent DRO curves.

narrow-band or single-frequency components, and
in order to approximate certain critical-band
values (the human ear’s analysis of complex
sounds is similar to that of a third-octave filter
through most of the audible range).

IIL Results.

In order to illustrate the range of results in the
various planes, and in order to protect against
misinterpretation of high or low points that oc-
curred in a given plane but might not be totally

representative of that model, data from all the
analyses were combined. In each figure, then,
the hatched area represents the whole range of
noise levels (in third-octave bands) measured
under tho specified conditions for all the air-
craft tested.

Thus, all the data collected are presented in
Fig. 2. It shows the effect of altitude on cock-
pit noise. Figure 2a shows all the data taken
with open vents; Fig. 2b shows all the data taken
with closed vents. The mean values for each of
the three altitudes are plotted as dashed and
dotted lines. In each case, the level appears just
slightly decreased with each increase in altitude;
the maximum changes occur at frequencies above
250 Hz. Each 4000-foot increment produces at
most a change of 2 dB—somewhat less at most
frequencies.

The data are replotted to produce Fig. 3. It
shows the effects of ventilation on cockpit noise.
Figure 3a shows all the data recorded at 2000
feet, MSL; 3b shows the data at 6000 feet; and
3c shows the data at 10,000 feet. The mean values
for each of the two vent conditions are plotted
as dashed and dotted lines. In each case, the
level is somewhat increased when the vents are
opened, and again, the maximum changes occur
at frequencies above 250 Hz, The maximum
change is about 4 dB; the change is somewhat
less at most frequencies.

The effect of the type of landing gear is also
small. A comparison of fixed- and retractable-
gear planes shows the retractable to be quieter—
as one would predict—but by only 4 dB at the
most. No consistent difference at all can be
sensibly attributed to the propeller type (fixed-
or variable-pitch).

Through all of these data, the major peaks of
intensity remain at about the same frequencies.
(There is no evidence of hazardous single-
frequency components. however.) This low-
frequency similarity results from the fact that
the two major noise sources (the engine and the
exhaust resonance) have their maximum effects
in the same general range: from 50-250 Hz.
Changes in air movement, either én the cockpit
or around it are relatively small compared to the
constant engine and exhaust noises.

Finally, it is useful to know that the noise
levels were similar from plane to plane. No
single tested plane could be called quietest or
noisiest. In fact, the high limit of noise shown



a

. Kryrer. K,

REFERENCES

. Kryrer, K, D, Effects of ear protective devices on the

intelligibility of speech in noise. J. acoust. Soc. Am,
18, 418-417, 1946,

D.. Warp, W.D.
Exprever, D. H. Hazardous
and steady-state noise,
464, 1966,

Sronz, R. B. Cockpit noise levels of airline aircraft.
In 1967 Annuat Scientific Mecting: Preprints of
Scientific Meeting. Pp, 49-50, Washington, D.C.:
Aerospace Medical Association, 1967.

Mire, TD, &
posure to intermittent
J. acoust. Soe Am, 39, 451~

4

ot

Topas, J. V. Cockpit noise intensity: three aerial-
application (crop-dusting) aircraft. J. Speech and
Hearing Res., 11, 611-615, 1968.

. Wernick, J. S. & Tosras, J. V. Central factor in pure-

tone auditory fatigue. J. acoust. Soc. Am., 35, 1967-
1971, 1963,

. Wick, R. L., Roserrs, L. B., & Asne, W. P. Light air-

craft noise problems,
1968,

Aerospace Med., 34, 1188-1137,


«4131 BEK
CONDENSER MICROPHONE

B&K SOUND &
LEVEL METER

ANALYZING SYSTEM

© cee

NAGRA I TAPE RECORDER

AMPEX PR-IO TAPE-LOOP
PLAY BACK

. RECORDING SYSTEM

NAGRA IT TAPE RECORDER

B&K 1/3 OCTAVE BAND ANALYZER
WITH B&K GRAPHIC LEVEL RECORDER

Figure 1. Diagrams of the recording and analyzing systems used.

Nagra III tape recorder. The system was cali-
brated before and after each flight with a Bruel
& Kjaer pistonphone, model 4220. This calibra-
tion was used not only for the sound-level meter,
but also for the amplifiers in the tape recorder.
In addition, the tone produced by the pistonphone
was recorded as the first 30 seconds of tape so
that it was always possible to return to precisely
the value of the noise being recorded. Whenever
changes were made in the meter’s settings, the
operator reported the change into the microphone.
In that way, all the information necessary for
& complete reconstruction of the cockpit noise
environment was available once the tape was
returned to the laboratory.*

The analyzing system used the same Nagra IIT
tape recorder, but as a playback. The amplified
recording was first played into a Bruel & Kjaer
graphic level recorder, model 2305, for a con-

tinuous record of the overall level throughout
the tape. This chart was used in the selection of
representative segments for complete analysis.
These selected segments were rerecorded on an
Ampex PR-10 tape recorder that was modified
to handle loops. Ten-second tape loops were then
played from the PR-10 into a Bruel & Kjaer,
model 2111, third-octave-band analyzer, and from
there into the graphic recorder. Third-octave-
band analysis was used rather than octave-band
analysis in order to produce more accurate pro-
files of the noise levels, in order to show any

*For those interested in doing similar work, but who are
unfamiliar with the physical measurement of acoustic signals,
please note that any reliable, wide-range, stable-speed, portable
magnetic recorder will do. Even more important is the fact
that any sound-level meter that is built in accordance with the
American or the International standard is comparable in per-
formance to any other. ‘Thus, data collected with two meters
made by two manufacturers will differ no more than data col-
lected with two instruments made by the same company.


2000'

115

NS

KK. +20 aia
105 win vin! |

\
WS

: g
ig

65

Noise Level in 1/3 Octave Bands (dB)

55

Frequency

6000'
us

105

Wo

95

85 120 win _|

480 min

a
2
3
=
5
a
$
2
38
o
8

rs
6 Uy

os a SS

: Toe + 000 Laear eam

Frequency in Hz

10000°
5

480 win

\S

95

es 120 win_|

400 min

65 S

Noise Level in 1/3 Octove Bonds (dB)

ae

$5

Frequency in Hz
MEANS WITH VENTS

settee OPEN
— —- cLoseo

Fieure 3. Effects of ventilation on cockpit noise. 3a
shows the mean values for each condition of ventila-
tion at 2000 feet MSL; 3b shows the same information
at 6000 feet; and 3c shows it at 10,000 feet. The solid
lines represent DRC curves.

in the figures: represents measurements, not from
one, but from eight different models. Similarly,
the low limit represents measurements from six
different planes.

IV. Risk of Damage.

Audiological experience suggests that pilots
generally have hearing losses, but evidence to
this point is lacking. FAA medical records do
not furnish adequate information because so few
audiograms are included; a whisper test for hear-
ing is still accepted by the FAA for medical
certification. The conclusions drawn here, then,
are based in theory, and in the finding* that
every aerial-application pilot tested had some
degree of hearing loss.

In Figs. 2 and 3, each of the graphs includes,
in addition to the noise-measurement data, three
solid lines. They are “damage-risk criterion”
(DRC) curves? Each one shows the maximum
amount of steady, complex noise that a human
observer can listen to once a day, for the time
indicated, without incurring permanent damage
to his hearing. A noise that exceeds 2 DRC
curve at any point is considered damaging if
the listener is exposed for longer each day than
the time indicated on the curve.* The DRC

Note that the measured noise levels are exces-
sive. In fact, someone who exposes himself un-
protected to such noise intensities for more than
3 or + hours a week can expect to experience
some irreversible hearing loss in a few years.

The simplest and cheapest solution to this
problem is the use of well fitted earplugs. They
decrease the damaging effect of noise until it is
almost negligible. And earplugs have no adverse
effect on the speech-to-noise ratio in the cockpit—
not for face-to-face communication, not for loud-
speaker communication, and not for communica-
curves are one for noise exposures of eight hours
per day, one for exposures of two hours per day,
and one for eight-hour exposures when the
listener is wearing standard earplugs.**
tion under a headset. Indeed, at the noise levels

*Exposure to a sound whose intensity corresponds anywhere
with a given DRC curve will produce some permanent hearing
loss, but the loss will not have a significant effect on the per-
ception of speech—which is the eriterion for determining
whether a loss of hearing is “damaging.”

**The V-51R is only one of many earplugs available; it is
used in this illustration only because data on it are more
readily available than on the other popular varieties,


AM 68-21

COCKPIT NOISE INTENSITY: FIFTEEN SINGLE-ENGINE
LIGHT AIRCRAFT

Jerry V. Tobias, Ph.D.

Approved by Released by
é
J. R. Diz, M.D. P. V. Smcer, M.D.
Cuter, Crvin AEROMEDICAL Feperay Air Surckon
InsrrruTe

September 1968

Department of Transportation

FEDERAL AVIATION ADMINISTRATION

Office of Aviation Medicine
23827



found in this study, the use of earplugs is likely
to produce an improvement in speech intelligi-
bility And they may lessen the susceptibility
to fatigue from other sources.

Wernick and Tobias’ found a positive correla-
tion between the amount of auditory fatigue one
experiences, and the intensity of mental activity

going on during noise exposure. It is possible,
then, that pilots, because of the kind of mental
effort that flying requires, are even more suscep-
tible to noise than these DRC curves predict. So
there is still another reason to protect fliers. The
noise emergency never ceases, but earplugs pro-
tect against it continuously.



Acknowledgments

The work reported here could not have been accomplished without the aid
of P. Bergsneider; R. L. Christie; H. L. Gibbons; A. H. Hasbrook; J. C.
Morgan; and W. E. Morris; or of the manufacturers, all of whom showed,
through their cooperation, an intense interest in the welfare of their customers.

Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific
and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151


COCKPIT NOISE INTENSITY: FIFTEEN SINGLE-ENGINE LIGHT
AIRCRAFT

In the history of measurements of aircraft
noise, nearly all of the published work has been
devoted to noise under the flight path of large
planes, to the exposures received by flight-line
personnel, and to the noise in and around military
aircraft. Almost nothing has been done about
recording noise levels inside civil aircraft. The
few reports on airline pilots are equivocal,
although the one study* that compared cockpit-
noise exposures in air transport planes with the
latest DRC (damage-risk criterion) curves of
Kryter, Ward, Miller, and Eldredge? found
levels that are potentially damaging. Only two
papers have been written on the noise problems
in light aircraft (aircraft weighing less than
12,500 pounds). One of these® covered some of
the same territory that the present paper will;
the other* dealt only with the specialized kinds
of civil aircraft that are used for aerial-applica-
tion (crop-dusting) work. Because of the vast
numbers of light planes presently in use and the
large number of people flying in them—there
are now nearly two-thirds of a million active
civilian pilots—a program of careful measure-
ment and analysis of cockpit noise was under-
taken.

In cooperation with the major manufacturers
of light, general-aviation aircraft, measurements
were taken of the noise levels in the cockpits of
fifteen representative models of the most popular,
currently manufactured, single-engine light air-
planes in the country. The planes tested were
essentially similar in their interior configura-
tions—except for one six-place plane, all had
four seats. Some had fixed landing gears; some
had retractable. Some had fixed-pitch propel-
Jers; some had constant-speed (variable-pitch)
propellers.

I. Method.

Preliminary measurements suggested that the
measured noise in the cockpits of these planes
might change as a function of the cruising alti-

tude. Meaningful variations were expected to
appear for altitude increments of 4000 feet; if
smaller values were selected, time would be
wasted, and if larger values were selected, some-
thing might be missed. For the part of the coun-
try in which the tests were made, 2000 feet MSL
(mean sea level) was the lowest indicated alti-
tude that would still keep the planes out of trees
and houses. Therefore, measurements were made
at 2000, 6000, and 10,000 feet MSL. Most single-
engine planes such as were tested here, although
they can operate at higher altitudes, usually do
not because of the need for supplemental oxygen.

‘At each of the three altitudes, noise recordings
were made at normal cruise power, or if the
engine was incapable of maintaining that
power—usually 75%—at the higher altitudes,
then the highest possible power setting was used.

Because air rushing through the ventilating
systems made a meaningful subjective difference,
recordings were made at each altitude with all
vents open, and also with all vents closed.

In addition to the two recordings made at each
of the three altitudes (2000, 6000, and 10,000 feet
MSL), recordings were made at takeoff and
landing. However, because the objective of this
report is to indicate the kinds of continuing noise
exposure that the pilots and passengers in light
aircraft receive, the levels measured during these
short-duration parts of the flight are not reported
here. It is worth nothing, though, that the take-
off and landing measurements indicate only slight
changes in spectrum, and almost no change in
overall level.

II. Apparatus.

The recording and analyzing systems used are
illustrated in Figure 1. A Bruel & Kjaer sound-
level meter, model 2203, was used both for the
specification of levels and as an input transducer.
The meter’s microphone was held at the height of
the pilot's ear, but a minimum of 6 inches from
his head. The output of the meter was led to a