The audiometer produced by Rudmose Associates utilizes the Bekésy principle (fig1) and is commercially available. Twelve of these instruments are currently in use at Lackland AFB.
[Compare and contrast the program of today to the program of the past in this historical presentation. Links are to large-sized images and figures. Press the Back button in your web browser to alternate between images and the text.]
Ralph N Kraus, LtCol, USAF, MC
Department of Ear, Nose, and Throat
Review 3-58
Air University
School of Aviation Medicine, USAF
Randolph AFB, TX
September 1957
Up to an including World War II, crewmen of Air Force aircraft were subject to serious damage to hearing. The necessity for acute hearing was recognized as evidenced by the requirement of an audiogram as part of the routine physical examination for aviation cadets and as part of the annual physical examination for flying personnel. Certain audiometric criteria were established and fliers whose audiograms did not meet these criteria were disqualified. The number of personnel requiring examination was ordinarily not excessive at any one facility. Thus, one concept of the current Flying Safety Program has been operative for some time.
With the advent of jet aircraft most flying personnel were no longer exposed to high-intensity noise. A pressurized cabin was used on the jet aircraft and the seal that provides for maintenance of the pressure is also effective in keeping out noise. The engine is located behind the pilot and crew, allowing the noise to be carried away from the cabin. The jet helmet is fitted with cushion-type radio receivers which also protect the ears. Acoustic trauma is no longer a major problem for flying personnel. However, a new and greater problem has arisen among the jet mechanics and other personnel whose duties are on the flight line. As in civilian industry, larger and heavier machinery in the shops has created noise which is potentially harmful to hearing. A much larger number of personnel than ever before are, therefore, possible victims.
The Surgeon General of the USAF recognized this problem and appointed an Ad Hoc Committee on Hearing, which met for the first time in February 1953. As a result of the deliberations of this committee a requirement for a Hearing Conservation Program was established. Any Hearing Conservation Program consists of the following aspects: 1) hearing tests, 2) protection, and 3) indoctrination or education. This paper will describe how the USAF has implemented this program. In this work, the Committee on Hearing and Bioacoustics of the National Research Council has given invaluable assistance and cooperation.
The purposes of hearing tests in the Air Force vary little from the purpose of hearing tests in civilian life. The tests attain the following objectives:
The type of hearing test that would best fulfill the above requrements was considered. It was obvious that some type of screening test would be good for selection but not for assignment, evaluation, and disposition or research. Only a pure-tone threshold audiogram would be useful for the five purposes of hearing tests as outlined above.
It was felt that the logical place to obtain initial audiograms as part of the physical examination was at the Basic Training Center which may process as many as 500 airmen in one day.
The next consideration was personnel. It was estimated that one technician can do six audiograms manually per hour and that with ten audiometers, ten operators can do 60 tests per hour or a total of 480 tests in an 8-hour period. Because of the fatigue factor, however, it is necessary to have a relief technician for every technician. Thus, it was estimated that 20 people would be involved in administering the tests besides the maintenance and administrative personnel to keep the facility operating.
It became essential to develop an automatic audiometer which could administer and record the test. Three different types of instruments are now being evaluated.
The audiometer produced by Rudmose Associates utilizes the Bekésy principle (fig1) and is commercially available. Twelve of these instruments are currently in use at Lackland AFB.
The second type under evaluation was developed by Dr. Scott Reger of the State University of Iowa under contract with the School of Aviation Medicine, USAF. It utilizes the pulse-tone principle and records the results on an IMB card.
The third type of instrument was developed by Frank A. Brogan of the School of Aviation Medicine, USAF. This instrument utilizes the pure-tone and simulates manual audiometry very closely. The results may be recorded by one of three methods:
Every airman and officer entering the USAF at Lackland AFB now receives a pure-tone audiogram and otologic examination as part of the routine physical examination. Since this is the only basic training center for the Air Force, 400-500 individuals are processed daily. Those who are assigned to noise hazard areas receive an audiogram at the time of assignment, another three months later, and then annually. Persons who show a progression of hearing loss will be referred to a Diagnostic Hearing Center for evaluation and dispositions. While it may seem expensive to transfer people to another base for evaluation, it would be much more expensive to needlessly reassign an individual whose training costs more than $10,000 and whose experience is essential to the mission of the Air Force.
The program to develop suitable protective devices was instituted by the armed services under contract with the Psycho-Acoustic Laboratory at Harvard during World War II. The V-51R ear defender was developed there and has been adopted as the standard item (of issue) ear defender. Continuing efforts at evaluation of new types are constantly being carried out at the Bio-Acoustic Laboratory, WADC. A proper seal between the ear defender and the ear canal is essential for it to be effective. The advantages of this defender are that it is cheap, can be manufactured in large quantities, causes relatively few allergic reactions, and is small, light, and easy to clean. The disadvantages are: it is not easy to obtain a proper fig; it is lost easily since it is small and light; it hardens with age and becomes difficult to remove and unusable after about six months. The V-51R ear defender is available to anyone working in noise levels over 90 decibels. The attenuation of the V-51R is shown in figure 8.
Since the maximum protection that can be afforded is not obtainable with an insert-type ear defender, a search has been conducted for an over-the-ear protector, commonly referred to as a muff. The most effective muff that has been evaluated to date is the David Clark Straightaway, No. 372-8-A. The attenuation curve of this muff is shown in figure 10.
For individuals who are exposed to noise levels of 120 db or more the ear defenders and ear muffs are recommended. This combination affords approximately 40 db attenuation in the 500 to 2000 cps range.
Another important method of protection is the reduction of noise at the source. Two ways for doing this are to use concrete test cells for engine tests in place of open test stands and to locate engine run-up areas at an isolated area on the flight line rather than near the hangars where many airmen are working.
All three aspect of the Hearing Conservation Program are important, and the weakness of one may disrupt the whole program. This is especially true of "indoctrination." The necessity of ear protection must be "sold" to the individual. He must be convinced that it is to his benefit to wear ear protection. The reason for ear protection is presented to the airmen during the Air Force Information Program, in base and service newspapers, and in service magazines. Bulletin boards and posters also are used for this purpose.
The Air Force Hearing Conservation Program can be summed up as follows: all individuals are given an audiogram and otologic examination upon enlistment or commission, and at periodic intervals. Ear protection is available in the form of the V-51R ear defender and a suitable ear muff when indicated. Noise sources are isolated by test cells and distance. Individuals are educated in the need for ear protection.
