Received 26 June 1969 4.6

Underwater Hearing Thresholds in Man as a Function of Water Depth*

JOHN F. BRANDT AND HARRY I-IOLLIEN

Communication Sciences Laboratory, Department of Speech, University of Florida, Gainesville, Florida 32601

Thresholds of human hearing were obtained underwater at depths of 35, 70, and 105 ft. Subjects were six divers experienced in taking underwater hearing-threshold tests by a modified B6k6sy technique. No sig- nificant effect resulting from the depth was noted. Threshold shifts (re air) for the three conditions of under- water hearing were consistent with those previously reported.

INTRODUCTION

Recently, Brandt and Hollien (1967) presented some data concerned with underwater hearing thresholds for pure tones. Part of those data included threshold sound- pressure levels (SPLs) from eight listeners at two water depths, 12 and 35 ft. While the difference was not statistically significant, the threshold SPL at 35 ft was greater than at 12 ft and increased with test frequency.

As part of a coordinated research program designed to define the parameters of underwater speech com- munication, further investigation of the effects of ear depth appeared warranted. As it seemed that increases in water depth--with concomitant increases in ambient pressure (water) upon the peripheral auditory system-- might possibly be a limiting factor in underwater hearing, free-field audibility thresholds for pure tones were obtained at ear depths of 35, 70, and 105 ft by means of the fixed-frequency B•k•sy technique.

I. PROCEDURE

A. Test Facility and Apparatus

As the detailed procedure has been described previ- ously (Brandt and Hollien, 1967; Hollien and Brandt, 1969), only a brief description is given here. The Bugg Springs field facility of the Naval Research Laboratory, Underwater Sound Reference Division, Orlando, Florida, was the site of the present research. Located directly

* Presented at the 74th Meeting of the Acoustical Society of America, November, 1967 •J. Acoust. Soc. Amer. 42, 1149(A) (•)•.

over a deep fresh-water spring (temperature, 22øC) is a large floating barge, with two laboratory rooms situated one on either side of a well, through which the Diver Communication Research System, DICORS, (Hollien and Thompson, 1967) was lowered to the proper depth. D ICORS is essentially an open-frame- work diving cage, constructed of polyvinyl chloride tubing, and is used to support subjects and equipment used in diver-communication research. It can be sus-

pended in the water at any desired depth and is kept in place by guy wires.

The stimulus-generating equipment and response units also have been described (Brandt and Hollien, 1967; Hollien and Brandt, 1969). Sinusoidal test stimuli gen- erated by a beat-frequency oscillator (General Radio, type 1304-B) were passed through an electronic switch (Grason-Stadler, model 829D) and associated equip- ment to a type J9 transducer mounted on the frame of DICORS, 1 m in front of the listener's ears. The J9 transducer was used as a sound projector (loudspeaker) for the audio range from 40 to 20 000 Hz. Calibration was accomplished by an F36 hydrophone, fixed to DICORS at the position of the diver's left ear. Acoustic signals emitted from the projector were transduced by the hydrophone and transmitted by cable to surface monitoring equipment.

Sinusoidal stimuli of 125, 250, 1000, 2000, and 8000 Hz were gated oN and o•r with a period of 500 msec, a 50% duty cycle, and a 2.5-msec rise-and-fall time. The attenuation rate of the recording attenuator was 8 dB/sec. Air-conduction thresholds were obtained by using a Rudmose (model ARJ-4) automatic audiometer,

The Journal of the Acoustical Society of America 89•

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BRANDT AND HOLLIEN

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125 250 500 1000 2000 4000 8000

F R EQU ENCY (H z)

Fzo. 1. Mean threshold SPL (decibels re 0.0002 •bar) as a function of test frequency in air and water at three depths. N = 6 diver/listeners.

modified to allow presentation of frequencies of 250, 500, 1000, 2000, 3000, and 4000 Hz.

B. Method

The research was carried out on six adult listeners

(4 males and 2 females) who were competent divers with experience in taking hearing tests in air and water. When the diver was in position, had equalized the air pressure in the middle ear against the water pressure in the external auditory meatus, and was ready to begin the threshold test, he signaled the ex- perimenter at the surface. The SPL of the test stimulus was first presented at a high enough SPL to be clearly audible. The diver/listener varied the SPL of the stimulus around the audibility-threshold level in the manner of the B•k•sy technique by activating a water- and pressure-proofed hand switch connected through a control box to the recording attenuator. As in earlier experiments (Brandt and Hollien, 1967; Hollien and Brandt, 1969), the threshold measures were taken while the listener was holding his breath. This pro- cedure reduced the noise level of the medium to a minimum, since considerable noise is generated around the diver's ears when air bubbles are exhaled.

II. RESULTS AND DISCUSSION

The investigation was concerned with underwater hearing thresholds for pure tones at three ear depths, 35, 70, and 105 ft, and in air. Mean threshold SPLs for six divers as a function of frequency (Table I) are

TABr.•. I. Mean threshold SPL (decibels re 0.0002 ubar) in air and water (three ear depths) as a function of frequency for six listeners. Standard deviations (decibels) are in parentheses.

Frequency Conditions 125 250 1000 2000 8000

Water (35 ft) 66.8 68.5 64.3 68.8 (5.6) (5.7) (7.9) (6.6)

Water (70 ft) 66.7 66.2 65.3 71.3 (5.8) (6.6) (7.2) (9.5)

Water (105 ft) 68.2 67.0 72.8 74.8 (9.2) (8.4) (12.6) (13.7)

Mean Threshold 67.2 67.2 67.5 71.6

(taken over depth) (7.3) (7.1) (10.3) (10.6) Air 37.5 17.8 11.2

(2.5) (3.4) (7.3) Difference 29.7 49.7 60.4

(Water-Air)

81.2

04.0) 78.2

(14.6) 80.8

(14.4) 80.1

(14.4)

graphed in Fig. 1. The lower curve represents air- conduction thresholds and the three upper curves represent underwater thresholds. No statistically sig- nificant differences in threshold SPL due to ear depth were apparent. The increased thresholds at 1000 and 2000 Hz, at 105 ft, were probably the result of prac- tice effects, as those conditions were the initial thresh- old conditions for all divers.

The underwater thresholds are from 30 to 60 dB

higher than the air conductions, the difference increas- ing with test frequency. The underwater thresholds vary in frequency from 67 to 80 dB SPL, a range of about 13 dB, with a mean threshold of about 70 dB SPL. The threshold SPLs in air and water are in

excellent agreement with our previously reported data (Brandt and Hollien, 1967; Hollien and Brandt, 1969).

The experiment can be summarized by reporting that increases in ear depth from 12 ft (previous experiment) to 105 ft (present experiment) and the concomitant positive increases in water pressure (5.3, 15.6, 31.2, and 46.7 psi at 12, 35, 70, and 105 ft, respectively) or corresponding increases in atmospheric pressure of 1.4, 2.1, 3.1, and 4.2 arm have no effect upon free-field underwater hearing thresholds in the frequency range between 125 and 8000 Hz.

ACKNOWLEDGMENTS

This work was supported by the Physiological- Psychology Branch, Office of Naval Research, and the National Institutes of Health. The authors wish to

thank the Underwater Sound Reference Division, Orlando, Florida, for their support, and, especially, Jimmy Walker. Excellent support was also provided by the U.S. Navy Mine Defense Laboratory, Panama City, Florida.

REFERENCES

BRAND:r, J. F., and Hor.r.zm% H. (1967). "Underwater Hearing Thresholds in Man," J. Acoust. Soc. Amer. 42, 1966-1971. Ho•.r.•.N, H., and BR•DT, J. F. (1969). "The Effect of Air Bubbles in the External Auditory Meatus on Underwater Hearing Thresholds," J. Acoust. Soc. Amer. 46, 384-387.

HorJ.zm% H., and Taom?sota, C. L. (1967). "A Diver Communica- tion Research System (DICORS)," Prog. Rep. CSL/ONR No. 2, ONR Grant Nonr 580 (20), 15 Jan. 1967. AD-648-935. Pre- sented at 73rd Meeting of Acoustical Society of America, New York, April, 1967 [J. Acoust. Soc. Amer. 41, 1603(A) (1967)•.

894 Volume 46 Number 4 (Part 2) 1969

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