available once the tape was returned to the
laboratory.*

The analyzing system used the same Nagra III
tape recorder, but as a playback. The amplified
recording was first played into a Bruel & Kjaer
graphic level recorder, model 2305, for a contin-
uous 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
profiles of the noise levels, in order to show any
narrow-band or single-frequency components,
and in order to approximate human 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).

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
occurred 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 the specified conditions for all the aircraft
tested.

One presentation of the data appears as Fig. 2.
It shows the effect of altitude on cockpit 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. Unlike the data taken in single-engine
planes*, where the level appears just slightly
decreased with each increase in altitude, no sig-
nificant altitude effects can be seen.

*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
magnetie 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 performance to any other. Thus, data collected with two
meters made by two manufacturers will differ no more than
data collected with two instruments made by the same company.

VENTS OPEN
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Frequency in Hz
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£ 75
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' T to? TT 00 TT i000
Frequency in Hz
MEANS
----- 2000'
-—-—- 6000'
seeese+ 10000!
Fictre 2. Effects of altitude on cockpit noise. 2a

shows the mean values for each altitude when the
vents were open (in the 8 aircraft so equipped).
2b shows the values when the vents were closed.
‘The solid lines represent DRC curves.

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; 8b shows the data at 6000 feet; and
3c shows the data at 10000 feet. The mean values
for each of the three vent conditions are plotted
as dashed and dotted lines. In each case, the
level is highest for opened vents, and lowest for
central systems. The maximum changes occur




long sample when the two engines’ noises are
partially cancelling each other, thus giving an
underestimate; and simultaneously to avoid Jong
samples with the engines’ noises reinforcing

acoustical solution requires that the engines be
purposely placed out of synchrony in order to
produce an average level of noise during any
reasonably long sample. For the tests reported

each other, thus giving an overestimate. The here, the engines were run 100 rpm apart.
x—~4131 B&K
CONDENSER MICROPHONE
—< RECORDING SYSTEM

B&K SOUND
LEVEL METER

ANALYZING SYSTEM °

NAGRA IT TAPE RECORDER

NAGRA II TAPE RECORDER

Figure 1,

Apparatus.

The recording and analyzing systems used are
illustrated in Fig. 1. A Bruel & Kjaer sound-
level meter, model 2203, was used both for the
specification of levels and as an input trans-
ducer. 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 Nagra ILI tape recorder. The
tem was calibrated before and after each flight
with a Bruel & Kjaer pistonphone, model 4220,

AMPEX PR-IO TAPE-LOOP
PLAY BACK

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

Diagrams of the recording and analyzing systems used.

This calibration 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 a change in the meter’s
settings was required, the operator reported the
change into the microphone. In that way, all
the information necessary for a complete recon-
struction of the cockpit noise environment was



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=
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2
5
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5 88
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Frequency in Hz
6000'
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z
105
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Frequency in Hz
10000°
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Frequency in Hz
MEANS
. +++ OPEN } Aircraft with vents only
‘—-—> CLOSED (n=8)
—--- = Aircroft with no vents (n= 3)
Fievre 3. Effects of ventilation on cockpit noise. 3a

shows the mean values for each con
lation at 2000 feet MSL; 8b show
mation at 6000 feet: and 3¢ shows i
The solid lines represent DRC curves.

dition of venti-
the same infor-
it at 10000 feet.

at frequencies above 500 or 1000 Hz. The maxi-
mum change between open and closed vents is
about 5 dB; between closed vents and central
systems, it is about 8 dB; the changes are some-
what less at most frequencies.

Through all of these data, the major peaks of
intensity remain at about the same frequencies.
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 in the
cockpit or around it, are relatively small com-
pared 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
in the figures represents measurements, not. from
one, but from five different models. Similarly,
the low limit represents measurements from
seven different planes,

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
hearing is still accepted for most American
pilots. 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 a 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
curves include one for noise exposures of eight
hours per day, one for exposures of two hours

posure 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
perception of speech—which is the criterion for determining
whether a Joss of hearing is “damaging.”



COCKPIT NOISE INTENSITY: ELEVEN TWIN-ENGINE
LIGHT AIRCRAFT

Jerry V. Tobias

Approved by Released by

Cimr, Civin
Inst

October 1968

Department of Transportation

FEDERAL AVIATION ADMINISTRATION
Office of Aviation Medicine

PARIS


per day, and one for eight-hour exposures when
the listener is wearing standard earplugs.**
Note that the measured noise levels are exces-
sive. In fact, someone who exposed himself un-
protected to such noise intensities for more than
6 or 8 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 ad-
verse effect on the speech-to-noise ratio in the
cockpit, either for face-to-face communication,

**The V-SIR 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.

i ____.

for loudspeaker communication, or even for com-
munication under a headset. Indeed, at the noise
levels found in this study, the use of earplugs is
likely to produce an improvement in speech in-
telligibility And they may lessen the suscepti-
bility 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 sus-
ceptible to noise than these DRC curves predict.
So there is still another reason to protect fliers.
The noise emergency never ceases, but earplugs
protect against it continuously.

REFERENCES

1. Kryter, K, D. Effects of ear protective devices on
the intelligibility of speech in noise. J. acoust. Soc.
Am., 18, 413-417, 1946,

2. Kryver, K, D., Warp, W. D., Mrtter, J. D., & Exp-
ripce, D. H. Hazardous exposure to intermittent
and steady-state noise. J. acoust. Soc. Am., 39,
451-464, 1966.

3. Stong, R. B. Cockpit noise levels of airline aircraft.
In 1967 Annual Scientific Meeting: Preprints of Sci-
entific Meeting. Pp. 49-50. Washington, D.C.: Aero-
space Medical Association, 1967.

: three aerial
J. acoust. Soc.

4, Tosras, J. V. Cockpit noise intensit
application (crop-dusting) aireraft.
Am., 43, 1968.

. Tosras, J. ¥. Cockpit noise intensity: fifteen single-
engine light aircraft. (In press.)

6. Wernick, J. S. & Topras, J. V. Central factor in
pure-tone auditory fatigue. J. acoust. Soc. Am., 35,
1967-1971, 1963.

. Wick, R. L., Ronerts, L. B. & AsHe, W. F. Light
aircraft noise problems. Aerospace Med., 34, 1133-
1187, 1963.

Pa

1


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 requesters 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: ELEVEN TWIN-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 mili-
tary 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 three
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;
one’ discussed single-engine aircraft in the same
way that this paper discusses light twins; the
other dealt. only with the specialized kinds of
civil aircraft that are used for aerial-application
(crop-dusting) work. Because of the vast num-
bers of light planes presently in use and the
large number of people flying in them—there
are now nearly two-thirds of a million valid
licenses for civilian pilots—a program of careful
measurement and analysis of cockpit noise was
undertaken.

In cooperation with the major manufacturers
of light, general-aviation aircraft, measurements
were taken of the noise levels in the cockpits of
eleven representative models of the most popular,
currently manufactured, twin-engine light air-
planes in the country. The planes tested were
essentially similar in their interior configura-
tions—except for two eight-place planes, all were
fitted for six passengers. All had retractable
landing gears and constant-speed (variable-
pitch) propellers. One was pressurized. Two
had muffled central ventilation systems.

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
country in which the tests were made, 2000 feet
MSL (mean sea level) was the lowest indicated
minimum legal altitude. Therefore, measure-
ments were made at 2000, 6000, and 10000 feet
MSL. Most non-pressurized twin-engine planes
such as were involved here, although they can
operate at much higher altitudes, usually do not
because of the oxygen requirement for passengers.

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.

In the eight planes without central ventilation,
air rushing through the ventilating system made
a meaningful subjective difference. So record-
ings were made at each altitude with all vents
open, and also with all vents closed.

In addition to the recordings made at each of
the three altitudes (2000, 6000, and 10000 feet
MSL), recordings were made at takeoff and
landing. However, because the object 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 noting, though, that
the takeoff and landing measurements indicate
only slight changes in spectrum, and almost no
change in overall level.

Measurements in any aircraft are variable, but
the variations are usually easy to minimize.
Multi-engine aircraft add a special source of
variation—the “beating” tone that sometimes
occurs when the engines are not quite synchro-
nized. It is better, in such measurements, to
avoid the possibility of recording a relatively