A Comparison of 

Baseline Hearing 

Thresholds Between 

Pilots and Non-Pilots 

and the Effects of 

Engine Noise

Dennis B. Beringer 

Howard C. Harris, Jr.

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

June 2005

Final Report

DOT/FAA/AM-05/12

Office of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
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iii

ACKNOWLEDGMENTS

The authors extend their appreciation to Kurt Joseph for assistance with the initial statistical evaluations 

of the data and to the CAMI Clinic (AAM-700) for providing the sound-attenuating booth and hearing 

assessment equipment used in the study.

1

A COMPARISON OF BASELINE HEARING THRESHOLDS BETWEEN PILOTS

AND NON-PILOTS AND THE EFFECTS OF ENGINE NOISE

BACKGROUND

Several events of recent years have motivated a reexamination of noise levels in general aviation (GA) aircraft 

cockpits. First, the failure of the noise-canceling system 

onboard Voyager early during its record-setting non-stop, 

around-the-world flight, which resulted in significant 

hearing impairment to both pilots, suggested that noise 

was still very much an issue in piston-powered aircraft. 

Second, the continuing development and use of noisecanceling systems, both as contained in headsets and as 

installed in aircraft cockpits, indicated that noise was 

indeed of interest to the GA community. Third, we observed, in simulator studies, that some of the older pilots 

in our samples had difficulty hearing certain auditory 

warnings during our autopilot failure research (Beringer 

& Harris, 1997). Thus, a program was initiated to examine both the current hearing capabilities of pilots and 

non-pilots, and developments in auditory warning design 

that might serve to provide more distinct and detectable 

alerts for the pilot.

Tobias (1968a, 1968b) examined noise levels in 15 

single-engine GA aircraft, which were representative of the 

models then being manufactured, and 11 light twin-engine aircraft. Findings were similar in both cases, with the 

two major sources of cockpit noise — engine sounds and 

exhaust resonance — producing the majority of the noise 

in the range of 50 to 250 Hz. There was no evidence of 

hazardous single-frequency components. However, there 

were places in the frequency distribution that exceeded 

the OSHA-specified two- and eight-hour exposure limits. 

In particular, two-hour limits were exceeded at various 

points between 100 and 1.6 kHz, with a 112 dB peak 

around 80 to 100 Hz. Eight-hour exposure limits were 

exceeded at various points from 100 Hz to 2 kHz. The 

upper end of this distribution is well into the area of 

speech sounds and thus could be problematic. 

Tobias (1972) further examined the effects of noise 

on aircrew personnel to determine what effects, if any, 

this exposure was having on their hearing thresholds. 

Of all the groups measured (aerial applications pilots, 

flight instructors, older private pilots - 40 to 58 years of 

age - FAA flight inspectors, and flight attendants) only 

the private pilots failed to evidence significantly elevated 

thresholds or localized “dips” of 15 dB or more in their 

threshold curves. For most of the samples, then, the predicted effects of exposure to ambient cockpit noise levels 

were confirmed in elevated thresholds at various points 

along the frequency continuum.

A two-phase study was designed (1) to obtain current 

threshold data and determine the relationship between 

the pilot and non-pilot populations and (2) to examine 

the effects of cockpit (engine) noise on the hearing 

thresholds of pilots.

PHASE 1

Method

Phase 1 research was designed to assess the present 

state of hearing capabilities in samples of both pilots and 

non-pilots. Toward this end, 150 pilots and 150 nonpilots, stratified by 10 age groups with 15 participants 

per group, were tested to determine their hearing abilities by measuring their detection thresholds at a series 

of selected frequencies (0.5, 1, 2, 3, 4, 6, and 8 kHz). A 

standard, computer-based testing paradigm for audiometric assessment was used to obtain the data. Participants 

were initially tested in a sound-attenuating booth, and 

the test tones were presented through headphones. This 

process produced left- and right-ear threshold curves for 

each participant.

Results

Prior to analysis, all threshold measures were examined to validate accuracy of data entry, check for outliers, check for missing values, and assess the fit between 

distributions and the assumptions of analysis of variance 

(ANOVA). There were no missing values or outliers, nor 

were there any violations of the homogeneity of variance 

assumption.

Separate ANOVAs, which were executed using the 

SPSS General Linear Models procedure, were used to 

analyze the data for left and right ears. Each ANOVA 

consisted of a three-way, mixed-factor design. Age and 

Pilot Status were treated as between-subjects variables, 

and Test Frequency was treated as a within-subjects variable. The levels of these independent variables were as 

follows:
2 

• Age. 10 levels: (22 and younger, 23-27, 28-32, 33-

37, 38-42, 43-47, 48-52, 53-57, 58-62, and 63 and 

older)

• Pilot Status. 2 levels: (pilot, non-pilot)

• Test Frequency. 7 levels: (0.5, 1, 2, 3, 4, 6, and 8 

kHz)

Figure 1 presents data, adapted from the results of 

research by Peterson and Gross (1972), that were collected 

at approximately the same time as the data on air crew 

personnel referenced earlier (Tobias, 1972). The Peterson, 

et al., data appear to resemble that from the present nonpilot sample, whose data are presented in Figures 2 and 

3. Both figures illustrate the expected threshold shifts in 

the higher frequencies associated with age and, of course, 

longer periods of environmental exposure.

The first set of statistical evaluations centered on the 

mean detection threshold curves and a comparison of 

non-pilots and pilots. Mean threshold values for the pilots 

are shown in Figures 4 and 5. The ANOVA for the left ear 

yielded a statistically significant three-way interaction of 

pilot status by test frequency by age, F(54, 1680)=1.35, 

p < .05. The two-way interactions of pilot status (pilot/

non-pilot) by test frequency, F(6, 1680)=4.48, p<.001, 

and age by test frequency, F(54, 1680)=11.8, p<.001, also 

were statistically significant, as was the main effect of test 

frequency, F(6, 1680)=265.17, p<.001. The ANOVA for 

the right ear produced statistically significant two-way 

interactions for both pilot status by test frequency, F(6, 

1680)=2.85, p<.01, and age by test frequency, F(54, 

1680)=9.52, p<.001. There was also a main effect for 

test frequency, F(6, 1680) =176.65, p<.001. The higher 

detection thresholds for the left ear were not surprising 

as the exposure to noise of this ear is greater by reason 

of seating arrangement and other cockpit variables, particularly in twin-engine aircraft.

One can compare equivalent age groups of pilots and 

non-pilots by specific frequencies through examination 

of the paired figures (2, 4) and (3,5). Closer examination 

of these average threshold curves indicated that differences between the pilots and non-pilots were not revealed 

until age group 43-47, where the thresholds were slightly 

though not significantly elevated for the pilots. Much 

larger differences in mean thresholds were seen at 3, 4, 

6, and 8 kHz test frequencies for the 48-52 age groups. 

The differences at the 4, 6, and 8 kHz test frequencies 

were even greater for the 58-62 age groups. Interestingly, 

pilots in the 48-52 age group have an average threshold 

curve that looks remarkably similar to that of the 43-47 

age group of non-pilots. The two oldest non-pilot age 

groups are clearly differentiable in Figure 2, whereas in 

Figure 4 the average threshold curves for the two oldest 

pilot age groups are not that different. 

Pearson correlation coefficients ( r) were calculated to 

determine the relationship between total hours of flight 

time for pilots and detection thresholds at the various 

test frequencies. Total hours of flight time correlated 

significantly with detection thresholds for the 3 (r=.27, 

r=.21), 4 (r=.32, r=.23), 6 (r=.34, r=.24), and 8 (r=.34, 

r=.29) kHz test frequencies in both the left and right ears, 

respectively. Additionally, a significant correlation was 

revealed for the left ear at 2 kHz (r=.25). Correlation coefficients were also calculated to determine the relationship 

between detection thresholds for the different age groups 

and total hours of flight time. The highest correlation 

between detection thresholds and total hours of flight 

time was obtained for the 48-52 age group (r=.34).

The second form of evaluation involved an examination 

of individual pilot threshold curves. Of particular interest 

was a comparison of our data with Tobias’ (1972) data, 

which estimated the percentage of the pilot population 

that could be categorized as exhibiting a significant shift, 

or “dip,” in their threshold curve. Tobias (1972) found 

that 20% of his sample, or 3 of 15 private pilots from 

the ages of 40-58, demonstrated a significant threshold 

shift, which he defined as a localized 15-dB shift from 

the smoothed/extended threshold curve. In our sample, 

89% of the private pilots (25 of 28) in the same age 

range demonstrated a significant threshold shift by Tobias’ definition.

Discussion

That the present sample showed a much larger proportion of private pilots with significant localized shifts in 

their hearing thresholds than was found by Tobias (1972) 

is of some concern. The explanation of this difference is 

complicated by history variables, including individual 

exposure and its moderators. The statistically significant 

effects underscore the reliability of the detected trends, 

and none are counter to what one would normally expect 

as a function of aging and exposure. Interestingly, there is 

little direct evidence of impairment in the lower frequencies given that the primary components of aircraft cockpit 

noise are between 50 and 250 Hz. However, generalization 

of the present results to lower frequencies (i.e., 50-250 

Hz) may be limited because the lowest test frequency 

measured thresholds at 500 Hz. Losses of approximately 

45 dB at 3 kHz for the oldest pilot group reinforce the 

subjective and verbal-report data from previous simulator studies where warning tones between 2.5 and 3 kHz 

could not be heard. 

It should also be noted that the 58-62 age group in 

the nonpilot sample exhibited an average shift of about 

40 dB at 8 kHz, whereas Peterson’s data indicated a shift 

of about 30 dB for those aged 60. One should also consider, then, the possibility of an increase in general noise 
3

exposure over the intervening 26 years as a contributor 

to these additional threshold shifts. Whatever the contributing factors to the underlying thresholds, the use 

of pure-tone warnings must take into account the highfrequency loss that appears to be exacerbated by exposure 

to the GA cockpit environment. A second assessment was 

conducted with a limited sample to determine to what 

extent the introduction of engine noise further elevated 

hearing thresholds.

PHASE 2 

Method

Phase 2 experimentation was designed to evaluate 

threshold shifts present during engine noise and when 

wearing headphones. Data collection was conducted 

in the medical clinic’s sound booth where the Phase 1 

measurement of hearing thresholds was performed. The 

procedure was the same with the exception that engine 

noise (digitized Piper Malibu engine sound from cruise 

flight) was presented at a level comparable to that present in GA cockpits (Tobias, 1968a,b). Specifications of 

the headphones worn by the participants indicated an 

attenuation effect of 21 decibels. The participants were 

15 pilots from Phase 1.

Results

Pilots who returned for the noise-environment assessment fell into three age groups; 19-22 (n=4), 28-32 (n=5), 

and 38-42 (n=6). Figure 6 shows the average threshold 

shift as measured in the left ear, by frequency, for each 

of these three groups. The values that are shown are 

the means of the differences between each participant’s 

baseline thresholds and the thresholds as measured in the 

noise environment. Averaged across groups, the highest 

positive shifts in the left ear appear from 2KHz down, 

which is consistent with Tobias’ data showing that the 

preponderance of engine sounds falls into the range of 

50 to 250 Hz with some components as high as 2 kHz. 

The shifts at 500 Hz, the value closest to the majority of 

engine sounds, can be seen to range from 10 dB for the 

38 to 42 age group to as high as almost 19 dB for the 22 

and younger group. It is consistent with expectations that 

the two older groups show a more-or-less steady decline 

in threshold shift as the test points move farther away 

from the noise range.

Figure 7 presents the data for the right ear, and the data 

are not dissimilar to those in Figure 6. The one feature 

of particular interest may be that the smallest threshold 

shifts in the youngest age group occur at 2 and 3 kHz, 

with elevated shifts to either side of those frequencies. 

This is unlike the data seen in the other two age groups, 

but should be approached with some caution due to the 

small sample size.

If we think of engine noise acting somewhat like a 

filter through masking, then we need to look at the range 

of frequencies key to the interpretation of speech (1 to 3 

KHz). It is worth noting that the threshold for the lower 

end of what is considered the “voice” spectrum, around 

500 Hz, exhibits a considerable shift, with a comparable 

shift seen at 1 KHz. The effect is somewhat diminished 

by the time we look at 2 KHz, and the threshold shift 

is minimal at 3 KHz. These data suggest that there is 

considerable opportunity for interference with speech 

comprehension.

SUMMARY

Auditory warning design is influenced not only by the 

ambient noise environment but also by the signal-detection capabilities of the system operator. The data obtained 

in this research suggest that we should proceed in several 

areas to improve the utility of auditory warnings and to 

conserve hearing ability in GA pilots. The data clearly 

suggest that continued promotion and proper use of hearing protection devices are in order given the threshold 

shifts revealed in the present data, both to prevent further 

hearing loss in the pilot population and to promote the 

comprehension of speech and other aurally presented 

alerts and warnings. It is also clear that the selection of 

auditory warning signals should take into consideration 

the specific points along the frequency spectrum where 

large threshold shifts can be seen. Present recommendations include:

• Avoid the use of pure-tone warnings at or near 3 

KHz

• Promote use of clearly differentiable warning signals

• Promote use of signals easily detected by older pilots 

without overwhelming other auditory information

• Promote use of hearing protection devices, passive 

and/or active
4 

REFERENCES

Beringer, D.B. and Harris, H.C., Jr. (1997). Automation 

in General Aviation: Two studies of pilot responses 

to autopilot malfunctions. Washington, DC: FAA 

Office of Aviation Medicine, report number DOT/

FAA/AM-97/24.1

Edworthy, J., Loxley, S. and Dennis, I. (1991). Improving auditory warning design: Relationship between 

warning sound parameters and perceived urgency. 

Human Factors, 33(2), 205-31.

Edworthy, J. (1994). The design and implementation of 

non-verbal auditory warnings. Applied Ergonomics, 

25(4), 202-10.

Hellier, E.J., Edworthy, J., and Dennis, I. (1993). Improving auditory warning design: Quantifying 

and predicting the effects of different warning 

parameters on perceived urgency. Human Factors, 

35(4), 693-706.

1

This publication and all Office of Aerospace Medicine technical 

reports are available in full-text from the Civil Aerospace Medical 

Institute’s publications Web site: http://www.cami.jccbi.gov/aam400A/index.html

Patterson, R.D. (1990). Auditory warning sounds in the 

work environment. Philosophical Transactions of the 

Royal Society of London, Series B, Biological Sciences, 

327, 485-92.

Peterson, A.P.G. and E.E. Gross, Jr. (1972). Handbook 

of Noise Measurement, 7th ed. New Concord, MA: 

General Radio Co.

Tobias, J.V. (1968a). Cockpit noise intensity: Fifteen 

single-engine light aircraft. Washington, DC: 

FAA Office of Aviation Medicine, report number 

DOT/FAA/AM-68/21. 1

Tobias, J.V. (1968b). Cockpit noise intensity: Eleven 

twin-engine light aircraft. Washington, DC: FAA 

Office of Aviation Medicine, report number DOT/

FAA/AM-68/25. 1

Tobias, J.V. (1972). Auditory effects of noise on air-crew 

personnel. Washington, DC: FAA Office of Aviation 

Medicine, report number DOT/FAA/AM-72/32. 1
5

FIGURES

Figure 1. Non-pilot threshold shifts for male subjects from the general population by age group and 

frequency (adapted from data in Peterson & Gross, 1972). 

Adapted from Peterson & Gross, 1972

0

10

20

30

40

50

60

70

80

250 1000 2000 3000 4000 6000 8000

Frequency (Hz)

Threshold Shift (dB)

20

30

40

50

60

70

80

Age (years)
6 

Figure 2. Mean non-pilot left-ear hearing-threshold shifts by age group and frequency. 

Nonpilots, Left Ear

0

10

20

30

40

50

60

70

0.5K 1K 2K 3K 4K 6K 8K

Frequency (Hz)

Threshold Shift (dB)

<23 years

23-27

28-32

33-37

38-42

43-47

48-52

53-57

58-62

>62

Figure 3. Mean non-pilot right-ear hearing-threshold shifts by age group and frequency. 

Nonpilots, Right Ear

0

10

20

30

40

50

60

70

0.5K 1K 2K 3K 4K 6K 8K

Frequency (Hz)

Threshold Shift (dB) 

<23 years

23-27

28-32

33-37

38-42

43-47

48-52

53-57

58-62

>62
7

Pilots, Left Ear

0

10

20

30

40

50

60

70

0.5K 1K 2K 3K 4K 6K 8K

Frequency (Hz)

Mean Threshold Shift (dB)

<23 years

23-27

28-32

33-37

38-42

43-47

48-52

53-57

58-62

>62

Figure 4. Mean pilot left-ear hearing-threshold shifts by age group and frequency. 
8 

Figure 5. Mean pilot right-ear hearing-threshold shifts by age group and frequency 

Pilots, Right Ear

0

10

20

30

40

50

60

70

0.5K 1K 2K 3K 4K 6K 8K

Frequency (Hz)

Threshold Shift (dB)

<23 years

23-27

28-32

33-37

38-42

43-47

48-52

53-57

58-62

>62
9

Figure 6. Mean left-ear pilot hearing threshold shifts for three age groups of pilots by frequency as 

measured in a simulated engine-noise environment. 

-5

0

5

10

15

20

0.5 1 2 3 4 6 8

Frequency (KHz)

Threshold Shift (dB)

Age 19-22 (n=4)

28-32 (n=5)

38-42 (n=6)

-5

0

5

10

15

0.5 1 2 3 4 6 8

Frequency (KHz)

Threshold Shift (dB)

Age 19-22 (n=4)

28-32 (n=5)

38-42 (n=6)

Figure 7. Mean right-ear pilot hearing threshold shifts for three age groups of pilots by frequency as 

measured in a simulated engine-noise environment.