Project # 5 - CHEW FATIGUE RESEARCH 
Sub-Project ft 5 - 8 Estimation of the Practical 
Significance of Tank Noise 
Subject: - affect of Exposure to Tank Noise Upon Hearing 
Acuity of Tank Crews 
Project No. 5>-8 
January 20, 1943 ARMORED FORCE MEDICAL RESEARCH LABORATORY 
Fort Knox, Kentucky 
Project No. 5-8 
File Mo. 741.22 
January 20, 1943 
EFFECT OF EXPOSURE TO TANK NOISE UPON HEARING 
ACUITY OF TANK CREWS 
1. PROJECT: Estimation of the Practical Significance of Tank Noise. 
a, Authority - Letter of Commanding General, Headquarters Armored 
Force, Fort Knox, Kentucky, 100,112/6 GNOHD, dated September 21, 1912, 
b. Purpose - To determine if tank crew members suffer any hearing 
loss from the noise in tanks. 
2. DISCUSSION: 
si. Under certain conditions prolonged exposure to high noise levels 
is known to produce permanent impairment of hearing, the damage frequently 
being greater at certain specific sound frequencies than at others. Shorter 
exposures to somewhat lower noise levels may result in temporary hearing loss 
or so-called fatigue deafness. Since the noise in tanks may reach relatively 
high levels it appeared desirable to determine whether or not the hearing of 
tank crews suffered impairment as a result of fairly long periods of oper- 
ation. An opportunity to investigate the problem was presented in connection 
with extensive road and field tests of a group of M-4 A-3 medium tanks. 
b. Methods - 
1. Twenty-three (23) members of tank crews were tested 
throughout the audible band at intervals over a period of forty-one (41) 
days. Readings were taken approximately twelve (12) hours after exposure 
to avoid any effects of temporary fatigue deafness. Data from each man 
were plotted chronologically in order to demonstrate whether or not 
changes in hearing acuity occurred with time. 
‘2, In addition, three men were studied for a period of several 
hours immediately following exposure for the purpose of observing fatigue 
deafness. 
3. CONCLUSIONS; 
a. No measurable hearing loss at any frequency was observed among 
the subjects during the test period (see Figs. 1 and 2 in the Appendix) 
b. In general, no fatigue deafness was found in the' three subjects 
who were tested immediately following a period of 5 hours exposure to tank 
noise, (see Fig, 3 in the Appendix) 
1 c. On the basis of these tests, the likelihood of permanent 
damage to hearing from exposure to the noise in M-4 A--3 medium tanks 
appears to be remote if indeed existent. 
Prepared by: 
Major Theodore F. Hatch 
1st Lt. Robert H, Walpole 
nsfateC CeS 
WILLARD MACHLE 
Lieut, Col., Medical Corps 
Comnianding APPENDIX 
I, Permanent changes in hearing - 
a. Hearing acuity tests were made with the Maico Audiometer at 
the following frequencies; 128, 256, 512, 1021, 2018, 2896, 1090, 5792, 
8192 and 11,581. 
b# Subjects were tested in each ear separately, approximately 12 
hours after tank exposure. Tests were conducted in a relatively/ quiet room, 
c. Hearing loss (from the instrument "normal") to the nearest 5 db. 
was recorded at each frequency for each ear. 
d. Typical curves are shown in Figs. 1 and 2. In no subject was 
there any consistent evidence of change in hearing at any tone frequency 
throughout the test period which extended over 11 days and included up to 
8 hours of exposure to tank noise each day, 
II. Fatigue deafness - 
a. Hearing tests were conducted as above, except that tests were 
started approximately 'j>0 minutes following a period of 5 hours continuous 
tank driving. Testing was repeated at intervals for 5 hours. 
b. Curves for the three subjects are shown in Fig. S and, for 
comparison, the averages of readings (at each frequency) obtained in the 
previous tests are also shown. 
ci It will be observed that no evidence of fatigue deafness was 
found except at frequencies 512 and 2896 for subject //I and frequencies 
5792 for subjects fl:2 and '}• In these cases the departure from the subjects 
normal did not exceed 15 db, which is not striking, 
d. Subjectively, the only symptom reported was slight ringing 
in the ears. 
1 CHANGES IN HEARING ACUITY OF TYPICAL SUBJECT DURING 5 WKS. TANK OPERATtrJ 
(SUBJECT NO I) I 
FIG. I 
HEARING LOSS IN DECIBELS 
RIGHT EAR 
LEFT EAR CHANGES IN HEARING ACUITY OF TYPICAL SUBJECT DURING 5 WKS. TANK OPERATION 
(SUBJECT NO; 2) 
FIG. 2 
HEARING LOSS IN DEC\BELS 
t 
RIGHT EAR 
LEFT EAR TEMPORARY HEARING LOSS IMMEDIATELY AFTER 5 HOURS TANK OPERATin 
(THREE SUBJECTS) ~~ 
(FIG. 3) 
HEARING LOSS IN DECIBELS 
RIGHT EAR 
LEFT EAR