S41 87th Meeting: Acoustical Society of America S41

higher frequencies was so different may indicate that these findings are probably due to different processes.

3:00

T7. Effects of acoustical stimulation on equilibrium. Sharon L. Vanderhei, John D. Repko, and Michel Loeb (Department of Psychology and Performance Research Laboratory, Uni- versity of Lousiville, LouiSville, Kentucky 40208

Effects of acoustical stimulation on human equilibrium were measured on a rail test developed by Graybiel and Fregly [Acta Otolaryngol. 61, 292--312 (1966)]. Two tasks were used--standing with eyes open on rails 1¬ in. and • in. wide and standing with eyes closed on rails 1• in. and 1• in. wide. Five noise conditions were used--bilateral and unilateral

presentations of impact (135 dB) and continuous (110 dB), and a bilateral control (75 dB). Analyses revealed no significant effects of types of noise on rail performance. These findings appear somewhat at variance with earlier results [H. C. Som- mer and C.S. Harris, Aerospace Med. Res. Lab. Rep. 70- 26, Wright-Patterson AFB, Ohio (1970); C.W. Nixon, C.S. Harris, and H. E. von Gierke, Aerospace Med. Res. Lab. Rep. TR-66-85, Wright-Patterson AFB, Ohio (1966)] indicat- ing-that asymmetrical, free-field noise stimulation produced significant impairment on a similar test, but their asymmetri- cal conditions involved unequal protection of the ears, result- ing in a much greater interaural intensity difference.

3:15

T8. Effect of noise exposure during primary flight training on the conventional and high-frequency hearing of Naval Aviation Officer Candidates. R.M. Robertson and C.E. Williams

(Naval Aerospace Medical Research Laboratory, Pensacola, Florida 32512)

This investigation was part of a larger study conducted by Memphis State University. This laboratory's portion of the study focused on administering conventional audiomerry (manual and self-recording), high-frequency audiomerry (4-- 18 kHz), and a speech intelligibility test in noise to 108 Naval Aviation Officer Candidates prior to and following primary flight training in T-34 aircraft. Hearing protection consisted of either the APH-6C or APH-6D flight helmet. Cockpit noise levels in the T-34 range from 96 to 115 dBA; during cruise the noise level is approximately 100 dBA. Results indicate no significant change in hearing sensitivity or speech discrimina- tion that could be attributed to noise exposure during primary flight training. Pre- and post-primary hearing levels obtained for the high frequencies compare favorably with high-frequency hearing levels obtained by Northern et al. (1968) for males in the age range 20--29 years. Questionnaire data indicated that a considerable number of the subjects had been exposed to potentially hazardous noise before entry into military service.

3:30

T9. Assessment of hazard posed by training to military personnel wearing ear protection and its relationship to questionnaire responses. Michel Loeb (Department of Psy- chology, University of Louisville, Louisville, Kentucky 40208), Paul D. Cameron (St. Mary College, St. Marys, Maryland 20686), George Luz and Shelden Luz (U.S. Army Medical Re- search Laboratory, Ft. Knox, Kentucky 40121), and Sharon Vanderhei (Department of Psychology, University of Louis- ville, Louisville, Kentucky 40208)

Temporary hearing loss following small arms fire was monitored for a group of trainees at a U.S. Army post. Audiograms were also taken for another group, and question- naire responses were elicited regarding such matters as perceived quality of hearing, difficulty in hearing and com- municating in noisy situations, and use of protective devices. Generally it appeared that trainees' hearing levels were within acceptable limits and that the temporary shifts after firing, while significant, were not excessive. There was a small but statistically significant hearing loss within the first eight weeks of training, and hearing levels and loss in training were related to responses on certain items on the questionnaire.

3:45

T10. On the noise level of ears and microphones. Mead C. Killion (Industrial Research Products, Inc., A Knowles Company, Elk Grove Village, Illinois 60007)

Although it is well known that the self-noise of the human ear is less than that of many microphones, little has been published directly comparing the noise level of ears and microphones. When a microphone is to be used in a hearing aid, however, it .is sometimes of interest to known how soft a sound the wearer can hear if he turns the gain up far enough. If the noise spectrum of the microphone is known, the "aided pure-tone threshold" which can be achieved with that micro- phone can be calculated by utilizing the extensive literature on the masking of pure tones by noise. Following French and Steinberg [J. Acoust. Soc. Am. 19, 90--119 (1947)], one can turn the tables and calculate the apparent noise spectrum of the ear considered as a microphone. Such a calculation indi- cates that an acute young ear has an apparent A-weighted noise level of 20 dB SPL. Experimental verification has been ob- tained on a new hearing aid microphone designed to be quieter than the human ear. A review of the experimental technique again calls attention to one pitfall in the routine use of the dif- ference between aided and unaided thresholds as a measure of

hearing aid gain. 4:00

Tll. On the influence of the mechanical support on the hearing aid frequency response. Igor V. Nab•l•k (Department of Audiology and Speech Pathology, University of Tennessee, Knoxville, Tennessee 37916)

Hearing aid frequency response measured in an unechoic chamber and in a hearing aid test box may differ appreciably. The usual explanation is that the difference in equipment and the sound field configuration cause the discrepancy. Often very small attention is paid to the effects of mechanical sup- port used for fixing the aid in the desired position. The sup- port is satisfactory if it does not disturb the distribution of the sound field and if it does not transmit mechanical vibra-

tions. While the first requirement is usually fulfilled, the assumption that the second one is fulfilled also is often not justified. During the reported study, large alternations in hearing aid frequency responses caused by such vibrations were observed when measured, namely, in hearing aid test boxes. Even in commercial facilities considerable influence of

mechanical resonances of supports was found in the low-fre- quency range (between lowest frequencies and approximately 1000 Hz). Some simple methods for improvement of the re- liability of frequency response measurement are discussed.

4:15

T12. Electronic subiect for audiometric practice. James H. Borsford (Howard Engineering Company, P.O. Box 3164, Bethlehem, Pennsylvania 18017)

A worrisome problem in the training of audiometric tech- nicians is the unavailability of subjects for practice testing. The usual procedure is to instruct one trainee to test several of the other trainees. This procedure is not entirely satisfac- tory, because it is tiring to undergo repeated hearing tests. Also, only a limited number of audiograms, mostly normal, are available which quickly become known. To overcome this difficulty, an electronic subject to be used for practice in hearing testing has been developed. It has a statistical thresh- old which mimics human response. The threshold response can be made normal, or sharp as in the case of sensorineural hearing loss. It can also be made vague, as in the case of a subject who is unattentive or does not understand the test pro- cedure. All types of audiograms can be simulated. For train- ing large groups, several electronic subjects with different kudiograms can be used serially by the trainees. The audio- gram of each device is known to the instructor, so trainee performance can be graded. For individual or small group practice, a single instrument which can be siritched through several different audiograms may be used. The operating characteristics and applications of this device are described.

J. Acoust. Soc. Am.,,Vol. 55, Supplement

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