Psychological tests of human responses to aircraft noise level

Johnson, P. J.

Kryter, K. D.

Young, J. R.

English

NASA Technical Reports Server

37. JUDGMENT TESTS OF AIRCRAFT NOISE 
By Karl D. Kryter,  Paul J. Johnson, 
and James R. Young 
Stanford Research Institute 
SUMMARY 
Psychological tests were conducted to determine the ability of various physical units 
to predict the judged perceived noisiness of the sounds from a variety of aircraft operating 
under landing and take-off power. The best agreement between objective measures and 
subjective judgments of the noisiness or unacceptability of aircraft noise of all types is 
generally found by calculating the tone-corrected effective perceived noise level in- 
EPNdBt from 1/3-octave band spectra taken every 1/2 second during the noise cycle. 
When low-frequency aircraft noise is compared with high-frequency aircraft noise, a 
systematic overestimation of the perceived noisiness of the lower frequency noise is 
obtained by the various PNdB units. 
INTRODUCTION 
Over the past 20 to 30 years,  research has led to the development of a number of 
ways of measuring noise that purport to be correlated with the measurement or  response 
of man's auditory system to noise. These methods run the gamut from simple peak sound- 
level meter readings to weightings of 1/3-octave band spectra taken every 1/2 second 
during the duration of a noise occurrence. Although the measurement procedures vary, 
the purpose and goal of making these measurements is to estimate or predict how 
unwanted, unacceptable, or noisy the sounds being measured are perceived to be by people. 
Most of the psychological tests concerned with establishing the relationships between 
perceived noisiness and the physical aspects of aircraft noise have been conducted in the 
laboratory wi€h recordings of the noise; relatively small groups of subjects and often a 
restricted variety of aircraft noises were used. The present tests were designed to per- 
mit as valid an examination as possible of the ability to predict from physical measure- 
ments the judged acceptability to people of a wide variety of actual aircraft noise when 
heard in or  outside typical homes. In addition, a number of ancillary questions related to 
the generation and propagation of the noise and the reaction of house structures to the noise 
were to be investigated. These tests were recently conducted at the NASA Wallops Station. 
The present report is concerned primarily with the relation between the psychologi- 
cal judgment of people outdoors and physical measurements made from recordings of the 
noise from microphones located outdoors. Measurements of house vibrations at Wallops 
587 
https://ntrs.nasa.gov/search.jsp?R=19690002246 2020-03-12T06:02:11+00:00Z
Station are reported in reference 1. Additional technical reports by the National Aero- 
nautics and Space Administration and Stanford Research Institute on various aspects of 
the studies will be prepared later as further analyses of the data a re  completed. 
PROCEDURE 
A frame house, a brick-veneer house, and a large yard near one of the houses, all 
located in a residential area of Wallops Station, were chosen as the test sites. The sub- 
jects were adults, primarily housewives, selected from communities in the local area. 
Figure 1 is a photograph of some of the subjects as seated for the tests. The subjects 
were  tested with an audiometer and all were found to have normal hearing (*15 dB from 
audiometric zero). The subjects were paid and given careful instructions prior to and 
during the tests as to the importance of the tests and the nature of the task they were to 
perform . 
The fundamental task of the subjects was to mark on an answer sheet which of two 
aircraft sounds presented to them in a brief period of time they considered to be the least 
acceptable, assuming these noises were heard in or  near their home 20 to 30 times per 
day. The subjects also rated each noise on a scale ranging from completely acceptable 
to completely unacceptable. 
Because of the very large number of aircraft tested, it was not possible to pair, for 
the judgments, each aircraft noise with every other aircraft noise. Instead, two of the 
aircraft were chosen to provide a reference, or standard, noise, and this noise was paired 
with the noise from each of the other aircraft when operating under landing and take-off 
power. The reference, or standard, aircraft chosen were the turbojet 880 and the 1049G 
(Super Constellation), a propeller-driven aircraft with reciprocating engines (recip-prop). 
The aircraft were flown so that about 5 minutes elapsed between pairs of noises, 
and usually 1 minute elapsed between each member of a pair. The altitude for the oper- 
ational conditions for each aircraft was carefully monitored and controlled for all flights. 
An attempt was made to operate the reference aircraft at an altitude that would be reason- 
able for that aircraft at about 2 to 3 miles from an airport. Figure 2 is a schematic 
illustration of the flight paths followed by aircraft for the tests. Table I summarizes the 
aircraft tested and number of overflights made per aircraft. 
Physical Measurements 
Tape recordings made of the aircraft noises reaching the ground were played through 
1/3-octave band filters. Each filter output was passed through an envelope detector, 
smoothed, and sampled every 1/2 second. These samples were digitized and stored on 
magnetic tape and were used for subsequent calculation of the following units: 
588 
(1) Max dB(A), dB(B), dB(C), dB(N) 
(2) Max phons (Stevens), PNdB, PNdBtl, and PNdBt2 
(3) Peak phons (Stevens), PNdB, PNdBtl, and PNdBt2 
(4) E (effective) dB(A), dB(N), PNdB, PNdQ1, and PNdBtZ 
(5) EE (estimated effective) PNdB, PNdBtl, and PNdBt 
References 2 and 3 give a detailed description of these units. These units of noise mea- 
surement appear at the present time to be the best available units for the evaluation of 
the perceived noisiness of the sound from aircraft. An additional method is that of cal- 
culating loudness levels, in phons, by the 1/3-octave band method of Zwicker (ref. 4). It 
is planned to obtain max phons (Zwicker) for these aircraft noises for their inclusion in 
the final report of the tests conducted at Wallops Station. 
2 
One-third-octave band spectra present outdoors when the overall sound pressure 
level (SPL) was at max dB(C) a re  shown in figure 3 for representative flights for each of 
the aircraft tested. 
Psycho logic a1 Measur e ment s 
The paired-comparison tests which are believed to provide the most essential 
psychological data from this study are scored and interpreted as follows: 
Step 1.- The percent of listeners in a group (for this report a group of 33 people) 
who preferred the reference aircraft noise when it appeared first in a given pair and when 
it appeared second in the same pair are averaged. 
Step 2.- The percent obtained in step 1 is plotted against the level, measured by a 
given physical unit, of the comparison aircraft noise. Inasmuch as the level of the com- 
parison noise was systematically varied, the percent of people, in general, who preferred 
the reference noise increased as the level of the comparison noise increased. An attempt 
was made to have the comparison noise vary over a range that caused the percent of peo- 
ple preferring the reference aircraft noise to change from near zero percent to near 
100 percent. Sample plots of the data are shown in figure 4. 
Step 3.- On each function, such as those shown in figure 4, a perpendicular is 
dropped to the abscissa from the point where the 50-percent line crosses the curve drawn 
through the data points. 
Step 4.- The value obtained in step 3 is taken as the level, for the given unit of mea- 
surement, required for the comparison noise if and when it is to be perceived as equal to 
the reference aircraft noise in unacceptability o r  noisiness. 
589 
Step 5.- The difference, if any, between the reference and comparison noises when 
judged to be equal is taken as the index of the ability of each of the physical units to prop- 
erly measure or indicate the perceived noisiness of each pair of sounds. 
RESULTS mD DISCUSSION 
Table 11 presents a summary of averaged data obtained when the noise from jet air- 
craft was judged against that from other jet aircraft or from turboprop aircraft, recip- 
prop aircraft, or helicopters. Two equally important indicators that show the accuracy 
with which an objective measure predicts the subjective judgment data are shown in 
table 11 and are described as follows: 
(1) The average of the differences between an objective measure for the reference 
and comparison aircraft noise when they a re  judged to be equally noisy. lf the physical 
units were perfectly correlated with the psychological data, the average of the differences 
would be zero for each pair; that is, when the aircraft noises were  judged to be equal, they 
would be measured physically as being equal. 
(2) The range of the differences between an objective measure for the reference and 
comparison aircraft noise when they are judged to be equally noisy. It is important that 
an objective measure be reasonably accurate with respect to the most common or  most 
prevalent aircraft noises evaluated. This accuracy is reflected in the average of the dif- 
ferences between the objective measures of any two noises judged to be subjectively equal. 
Two particular conclusions can be drawn from table II. First, the average differ- 
ences for the better units are significantly smaller when the comparison and reference 
noises are both high-frequency jet noises than when one of the noises is a low-frequency 
propeller-aircraft noise. These differences are about 1.5 dB when the comparison and 
reference noises are both high-frequency jet noises and about 3.0 dB when one is a low- 
frequency propeller-aircraft noise. Appropriate modifications to procedures for mea- 
suring noise in the lower frequency bands should reduce the differences between the phys- 
ical measures and judged noisiness and thereby provide an effective perceived noise level 
that would predict, with an average accuracy of about 1.0 dB, judgments of the noisiness 
or unacceptability of aircraft noise regardless of its source, spectral complexity, and, 
within limits, duration. Second, when all types of aircraft noises and both the average 
differences and the total range of differences between the values for the references and 
comparison noises are considered, the most accurate units of measurement are usually 
EPNdBt. 
The relative accuracy of the various objective measures is illustrated in table 111, 
which shows how each unit ranks with regard to the average of the average differences and 
the range of differences. A unit with the rank of one would have the best agreement with 
the subjective judgments. 
590 
Two other recent studies (see refs. 5 and 6) have been made of subjective judgments 
of aircraft noise in which all, o r  nearly all, of the objective measures used in the present 
study were evaluated. Table IV shows that EPNdB has the highest rank in each of the 
studies. It should be pointed out, however , that the practical significance between some 
of the differences in these measures, an amount equivalent to less than 1.0 dB in level 
in some cases, is perhaps questionable. 
CONCLUSIONS 
The following conclusions a re  based on the results from judgment tests of aircraft 
noise : 
1. The best agreement between objective measures and subjective judgments of the 
noisiness or  unacceptability of aircraft noise of all types is generally found by calculating 
the tone-corrected effective perceived noise level in EPNdl3t from 1/3-octave band spec- 
tra taken every 1/2 second during the noise cycle. 
2. When aircraft noise containing its energy predominately in the lower frequencies 
is compared with predominately high-f requency aircraft noise, a systematic overestima- 
tion by about 3.0 dB of the perceived noisiness of the lower frequency noise is obtained by 
the various PNdB units. 
591 
REFERENCES 
1. Mayes, William H.; Findley, Donald S.; and Carden, Huey D.: House Vibrations Signifi- 
cant for Indoor Subjective Response. Conference on Progress of NASA Research 
Relating to Noise Alleviation of Large Subsonic Jet Aircraft, NASA SP-189, 1968. 
(Paper No. 39 herein.) 
2. Kryter, K. D.: Concepts of Perceived Noisiness, Their Implementation and Application. 
J. Acoust. SOC. Amer., vol. 43, no. 2, Feb. 1968, pp. 344-361. 
3. Stevens, S. S.: Procedure for Calculating Loudness: Mark VI. J. Acoust. SOC. Amer., 
vol. 33, no. 11, Nov. 1961, pp. 1577-1585. 
4. Zwicker, E.: A Graphic Method of Determining Loudness and Loudness Level From 
the t*Third-Octave't Level Diagram. Frequenz, vol. 13, no. 8, Aug. 1959, 
pp. 234-238. 
5. Hecker, Michael H. L.; and Kryter, Kar l  D.: Comparisons Between Subjective Ratings 
of Aircraft Noise and Various Objective Measures. Tech. Rep. NO-68-33, FAA, 
Apr. 1968. 
6. Kryter, K. D.; Johnson, P. J.; and Young, J. R.: Psychological Experiments on Sonic 
Booms. Annex B of Sonic Boom Experiments at Edwards Air Force Base, 
NSBEO-1-67 (Contract AF 49(638)-1758), CFST1, U.S. Dep. Com., July 28, 1967. 
592 
TABLE 1.- NUMBER OF OVERFLIGHTS FOR JUDGMENT TESTS 
p o t a l  of 189 pair4 
Reference aircraft: 
Turbojet. 880 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  132 
Recip.prop. 1049G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  80 
Comparison aircraft: 
Turbofan (front). 727 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22 
Turbofan (front). C-141A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22 
Turbofan (aft). 990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28 
Turbojet. 720 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20 
Turbojet. 1329 Jet Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18 
Turbojet (afterburner). F-106 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12 
Turboprop (STOL). CV-7A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16 
Turboshaft (helicopter). 204B . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20 
Turboshaft (helicopter). CH-47 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8 
593 
TABLE II.- COMPARISON OF OBJECTIVE MEASURES AND SUBJECTIVE JUDGMENTS 
Max dB(B) 
2.0 
5.5 
7.0- 
20.0 
Aircraft 
noise 
Jet vs  jet 
Max dB(C) 
2.1 
7.5 
7.9 
25.0 
Jet vs turboprop, 
reeip-prop, o r  
Grand average 
for  all aircraft  
helicopter 
Average difference 
Range of differences 
Indicator 
of 
accuracy 
Average difference 
Range of differences 
Average difference 
Range of differences 
Best units 
2.9 
11.0 
5.6 
17.0 
Indicator 
of 
accuracy 
Aircraft 
noise 
594 
Best units 
EPNdB 1 EPNdBtl /EEPNIB 
EdB(N) EPNdBt2 
10.3 11.0 
Worst units 
I I I 
Peak MaxdB(B) MaxdB(C) 
PNdBtll  1 1 
20.0 25.0 
TABLE ID.- RELATIVE ACCURACY WITH WHICH OBJECTIVE MEASURES 
PREDICT JUDGMENT DATA OBTAINED AT WALLOPS STATION 
BO units u s e d  
Unit name 
EPNdB 
EdB(N) 
EPNdBtl 
EPNdQ2 
EEPNdB 
EdB(A) 
EEPNdBtl 
EEPNdBt2 
Peak phons 
Peak PNdB 
Rank 
1 
1 
2 
2 
3 
4 
5 
6 
7 
8 
Unit name 
Max phons 
Max dB(A) 
Max PNdBt2 
Max PNdB 
Max dB(N) 
Peak PNdBt2 
Peak PNdBtl 
Max dB(B) 
Max dB(C) 
Max PNdBtl 
Rank 
8 
8 
9 
9 
9 
10 
11 
12 
13 
14 
I 
595 
596 
Predictors 
Best 
Worst 
TABLE 1V.- BEST AND WORST PREDICTORS OF 
JUDGMENTS OF AIRCRAFT NOISE 
Edwards AFB FAA 
field tests laboratory tests 
(ref. 5) (ref. 6) 
EPNdBtl EPNdBtZ 
EEPNdBtl EPNdB 
EEPNdB Max  dB(N) 
Max dB(A) Peak PNdBtl 
Max dB(B) Max dB(C) 
Max dB(C) Max PNdBt1 
NASA 
field tests 
EPNdB, EdB(N) 
EEPNdB 
Peak PNdBtl 
Max  dB(B) 
Max dB(C) 
EPNdBtl, EPNdBtZ 
Figure 1 L-68-8585 
- - AIRCRAFT FLIGHT PATH 
/-- 
/ - - - - - .  
) 
I- 
COMPARISON / 
/ / ---- AIRCRAFT -- 
.I 
-/----- 
/ 
- - / --- / 
-IL_ /----<- REFERENCE I 
1’ AIRCRAFT // O z r s G ) /  ’
<----- 
’ . . OUTDOOR SUBJECTS 
Figure 2 
597 
0 
-10 
-2 0 
- 30 
m, -40 - h
a -50 z 
(3 
5 
s o  
0 
U 
-I 
v) 
-I 
u 
W > 
0 -30 
a -10 
zi -20 
e 
G 
W -40 
L 
-1 -50 W a 
-I 
w e - I o  
- 20  
-30 
- 40 
-50 
59 8 
I/SOCTAVE BAND SPECTRA PRESENT OUTDOOR§ WHEN OVERALL SPL 
IC WEIGHTING) REACHED ITS MAXIMUM 
I /  3-OCTAVE BAND CENTER FREQUENCY, Hz 
l l l l l l l l l l l l l l l l l l l l l l l f  
- TURBOFAN (AFT), 
I I I I I I I I I I I I I I I I I I I I I I I I  
990 (CJ805-23) 
- 727 (JT8D-I) 
Figure 
TURBO FAN (FRONT!, 
C-141A tJT3D-8) 
I I I I I I I I I I I I I I I I I I I I I I I I I I  
0 
-10 
-20 
-30 
m 
-o -40 
n 
a -50 z 
I/3-OCTAVE BAND SPECTRA PRESENT OUTDOORS WHEN OVERALL SPL 
(c WEIGHTING) REACHED TO MAXIMUM 
II3-OCTAVE BAND CENTER FREQUENCY) HZ 
TURBO JET (AFTERBURNER), 
F-106 (J75-P-17) - 
I I I I I I I I I I I I I I I I I I I I  
g o  
V 
-I 
v) 
-I 
IL 
W > 
0 
I- 
W -40 
w 
a -10 
d -20 
0 -30 
f a 
1 - 50 
W 
IL 
-I 
W 
HELICOPTER,CH- 47 
- 
Figure 3(bl 
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I I I  
HELICOPTER,204B 
(T53  -L- 13) 
CRUISE, 1100' 
599 
600 
EXAMPLES OF PAIRED-COMPARISON JUDGMENTS OF SUBSONIC 
NOISE BY OUTDOOR LISTENERS AT WALLOPS STATION 
100 r n 
50 
40 
-1.0 EPNdB, 
1 
20 
REFERENCE 
AT 97 EPNdB 
60 
t 
o t 
0 
82 87 92 97 102 107 112 
EPNdB, OF COMPARISON (C-141A) 
1 
Figure 4 
- 
REFERENCE 
AT 92 MAX dB(C) 
- 1049G 
- 
-12.0 MAX dB(C) 
I I I I I 
77 82 87 92 97 102 107 
MAX dB(C) OF COMPARISON (C-141A) 


Judgment tests of aircraft noise

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