Methods of measuring the composite acoustic environment of helicopters in order to quantify the effective acoustic environment of the crewmen and to assess the real acoustic hazards of the personnel are examined. It is indicated that the attenuation characteristics of the helmets and hearing protectors and the variables of the physiology of the human ear be accounted for in determining the effective acoustic environment of Army helicopter crewmen as well as the acoustic hazards of voice communications systems noise

Camp, R. T., Jr.

Mozo, B. T.

English

NASA Technical Reports Server

26 
THE EFFECTIVE ACOUSTIC ENVIRONMENT OF HELICOPTER CREWMEN 
Robert T. Camp, Jr., and Ben T. Mozo 
Bioacoustics Di vi si on 
US Army Aeromedical Research Laboratory 
SUMMARY 
Internal and external noise levels of helicopters are usually measured 
to determine the acoustic environment of the crewmen. These types of measure- 
ments alone are inadequate for assessing the real acoustic hazards of 
personnel. The attenuation characteristics of helmets and hearing protectors 
and the variables of the physiology of the human ear must be taken into 
account in determining the effective acoustic environment of Army he1 icopter 
crewmen. Also, the acoustic hazards of voice communications systems noise 
may influence the overall acoustic environment of the personnel. The compo- 
site acoustic environment can be determined only with complex acoustic 
measurements that are necessary to quantify the effective acoustic environment 
of the crewmen. 
I NTRODUCT I ON 
Noise characterdstics of he1 icopters should be given consideration in 
the design and purchase of aircraft for several reasons. The high sound 
pressure levels associated with the operat4on of military aircraft are hazard- 
ous to hearing by most damage risk criteria. 
interfere with communications and operational efficiency of crewmen. 
Also, the high level sounds may 
The traditional approach to the task of ascertaining the acoustic 
environment that may affect the personnel i s  to measure sound pressure levels 
at various crewmen's positions under various operational conditions. In dis- 
cussions of helicopter noise problems in popular and scientific articles, it is 
often assumed that the sound pressure levels within the aircraft are the 
actual pressure level values that impinge on the ears of the personnel who 
operate the aircraft. 
are the same as the effective acoustical environment of the crewmen. The 
effective acoustical environment is defined as the actual acoustic energy 
that is received by the hearing system. 
It is assumed that the ambient sound pressure levels 
Measuring the effective acoustic environment of he1 icopter crewmen is a 
complex process. The purpose of this paper is to discuss the differences 
between the ambient acoustic level environment of the aircraft and the actual 
acoustic levels that affect the aircraft crewmen's ears. 
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https://ntrs.nasa.gov/search.jsp?R=19790002679 2020-03-22T02:10:12+00:00Z
HUMAN AUDITORY SYSTEM EFFECTS 
The human ear is divided into three sections: the external, middle, and 
inner ear. The inner ear contains a complex system of membranes, nerves, and 
hair cells that may be damaged by noise. The amount of noise-induced hearing 
loss is determined by the effective acoustic environment or the actual energy 
transmitted into the inner ear. 
protective mechanisms that affect the acoustic input. 
that limit the input to the inner ear. 
The middle ear is a pathway to the inner ear and is equipped with certain 
It contains two muscles 
The external canal is shaped as an irregular tube. There are various 
forms and sizes. The variations of sizes and shapes may produce various 
amounts of protection, especially in the high frequencies. 
limiting characteristics of the middle ear plus the variations of sizes and 
shapes of the external ear canal make it difficult to measure the actual 
acoustic stimuli traveling through the system to the inner ear. 
Unpredictable 
HEARING PROTECTOR AND VOICE COMMUNICATIONS SYSTEMS EFFECTS 
The human auditory system just discussed is not the only source of vari- 
ables that may affect the actual sound transmitted to the ear. Hearing 
protective devices , earplugs , headsets, and helmets contribute to the trans- 
formation of the acoustic environment to sound spectrum characteristics and 
sound pressure levels that may be vastly different from the values obtained 
using free field measurements in the cockpit. 
protectors, both insert and circumaural , attenuate high frequencies much more 
efficiently than they attenuate low frequencies. 
In general, all types of hearing 
Let us take, for example, the case of a typical pilot flying an Artqy 
CH-47 helicopter wearing the standard SPH-4 helmet. 
change of sound characteristics beneath the helmet when fitted on the head of 
the aircrewman would be the original sound spectrum on the outside of the 
helmet minus the attenuation characteristics of the helmet. 
flying with no voice communication system or warning signals activated, the 
resultant spectrum at the external ear would be approximately the values as 
determined by this method. 
sealed tightly over the ears of the crewmen and contain earphones that gener- 
ate a sound source beneath the helmet. One normally does not associate very 
high sound pressure levels with an earphone. 
the canal of the human ear, earphones are capable of generating up to 120 dB 
sound pressure level Our investiqations of sound sources from communication 
systems, warning signals and navigation signals, have revealed that a very signi- 
ficantly high sound pressure level may be transmitted from the earphone. These 
levels are much greater than the ambient noise that is transmitted through the 
helmet. 
One would expect that the 
I f  the pilot were 
One must remember that the helmet earcups are 
Coupled to small volumes such as 
514 
Another source of noise that may enter in the total effective acoustic 
environment is the distortion created by the design of voice comnunications 
electronic systems. 
are designed with distortion. 
intelligibility and specify certain various amounts o f  peak clipping which 
yield harmful distortion harmonics. The effects of peak clipping have been 
thoroughly discussed in some of my previous presentations to this group. Peak 
clipping may cause a decrement of intelligibility and also creates unnecessary 
harmonics that add to the total excessive energy that is transmitted to the 
ears and thereby the contributor to hearing loss. Added to these harmonics, 
often we have found inverter power-line noise that yields very high acoustic 
signals through the earphones and therefore is a significant source of acoustic 
hazard. 
It is well-known that military voice communication systems 
Some military specifications require only 70% 
TRANSDUCER CHARACTERISTICS EFFECTS 
Another aspect of the problem that one should note is the characteristics 
Ideally, microphones and earphones should have flat response of transducers. 
within the audio range. 
peaks, the crewmen may set gains of voice communications and other signals 
emitted through the earphones that are unnecessarily high level. 
evaluations the M-87 microphone has been considered one of the best military 
type noise cancel1 ing microphones available. The response characteristics 
contain a very large peak in the 4000 Hz range. 
transmitted through this microphone, there is an unnecessarily high emphasis 
of the 4000 Hz range. With our present-day knowledge of hearing loss causes, 
it is well established that this is the most vulnerable portion of the audio 
spectrum for hearing damage. 
noise conditions and if the reception were in quiet environments, there would 
probably be no significant amounts of hearing loss. However, in its applica- 
tion in high noise environments where gain settings are very high, the net 
result is that the ear is subjected to large quantities of energy which, for 
long periods of time, may cause significant hearing loss for some personnel. 
If either the microphone or the earphones have 
By some 
So when one receives messages 
If this microphone were used in low ambient 
Earphones should also have flat response. If they produce peak 
one would expect the same potential hearing damaging spectra in high 
noise for much the same reason that peak microphones cause problems. 
recent trend to change to lighter earphones should be watched carefu 
avoid regression of headphone response. Some of the small earphones 
produce high distortion'when driven at the levels required in the mi 
noize environment. 
response , 
1 eve1 
The 
ly to 
may 
i tary 
With the excess energy caused by variables of transducer response plus 
the many other signals such as voice communication messages, warning signals, 
navigation signals, and special instrumentation, there are significantly high 
acoustic levels generated beneath the helmet earcups. 
5 15 
SPECIAL PROBLEMS OF LOW FREQUENCY NOISE 
One other aspect of the helicopter noise problem that may affect the 
effective acoustic environment of the crewmen is extremely low rotary blade 
passing frequency which is usually below the octave bands traditionally given i r  
the analyses of survey data. In this region of the audio spectrum, measurement 
is seldom made of the aircraft noise spectra. The attenuation characteristics 
of the hearing protectors that are also worn in helicopters are not usually 
reported. The questions arise: Are these low frequencies passing through the 
helmets? Do they cause high frequency harmonic distribution that contributes 
to the total noise in the effective acoustic environment? 
Our laboratory is presently engaged in the investigation of the effects 
of low frequencies on animal ears. We have found significant temporary 
threshold shift in the high frequencies when the animals were exposed to long 
periods of low frequency band centered at 63 Hz. 
CONCLUDING REMARKS 
In summary, the purpose of our presentation is to call attention to the 
fact that the ambient sound pressure level measurements often made in helicop- 
ter cockpits with sound level meters will not yield essential information 
about the noise characteristics that truly affect the he1 icopter crewmen. 
should be aware of the difference between these levels and the actual effective 
acoustic environment that must be determined by other means. We have shown 
how the ambient acoustics level transmitted through the helmets is transformed 
by the attenuation characteristics of the helmet or other hearing protective 
devices that the crewmen may be wearing. In addition to these transformations, 
there are added signals caused bv the desian of microphones, earphones, hearing 
protective devices, and electronic systems. In a word, the actual acoustic 
energy at the eardrums of the crewmen is usually totally different from the 
acoustic environment measured around the various positions inside the aircraft. 
We have also discussed the variables of the middle ear that limit and modify 
the acoustic spectrum that is produced at the eardrum. All of these variables 
and unknowns make it very difficult to assess the real energy that reaches the 
inner ear, the'locale of hearing damage. We have methods by which we measure 
the output o f  earphones and the magnitudes of various warnings and navigation 
signals necessary in the operation of he1 icopters. 
We 
Precise narrow band measurements are necessary, and in addition, standard 
real -ear attenuation characteristics of helmets and hearing protective devices 
must be accomplished. A better estimate of the total acoustic input to the 
crewmen is made by the insertion of a tiny microphone in the ear with small 
5 16 
vires that will not affect the attenuation characteristics of the helmets. 
\lso, we have developed a portable cement chamber that is very useful in 
istimating the output o f  earphones as used in the he1 icopter operations. 
iuditory warning systems, and other special instrumentation with auditory 
signals be designed with consideration for the total systems output so as to 
ninimize the acoustic hazards that presently exist. This approach is necessary 
For realizing the most efficient operation of the crewmen. The evaluation of 
this low frequency problem can be done only after sufficient research has been 
xcomplished. 
9oise spectra cast doubt about the present universal application of dBA for 
reporting helicopter noise measurements. It is also recommended that as more 
sophisticated instrumentation is obtained for the measurement of helicopter 
noise, mo-re attention be given to the extremely low frequency of the rotary 
bl ade. 
It is recommended that future he1 icopters voice communication systems, 
The recent findings about the significance o f  low frequency 
517 


The effective acoustic environment of helicopter crewmen

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