Tech Briefs

Data gathered during testing will help develop strategies to protect Navy divers from loud underwater sounds.

The U.S. Navy is interested in strategies that divers could employ to protect them from loud underwater sounds. Sonar transmissions and other forms of underwater sound, such as that produced by noisy underwater tools, are an occupational hazard for U.S. Navy divers.

Apparatus and experiment setup. Testing was performed in an acoustic open-air water-filled holding tank located within the inner lock of the NSMRL Genesis Hyperbaric Chamber. The holding tank was designed to have nonparallel, asymmetrical sides; its base dimensions are illustrated in the lower figure. Sound stimuli were produced by a USRD J-11S transducer. An underwater response button was placed in the tank for the subjects. A hookah rig was used to supply subjects with air. Subjects placed their head at the correct distance from the transducer by touching their mask to a plumb line positioned 18-inches from the transducer. Diver head positioning was monitored throughout testing by the diver tender who remained outside the tank.

At the time this research was conducted, the U.S. Navy guidance for diver exposure to underwater sound accounted for the diver wearing a wetsuit hood but did not account for any changes in the hood’s sound attenuation properties with dive depth. Characterizing how the hood’s attenuation properties change with depth and frequency is critical to creating appropriate guidance for diver’s exposure to underwater sound.

Underwater hearing thresholds were collected from 13 U.S. Navy trained divers while bare-headed and while wearing a 7 mm neoprene wetsuit hood. Testing frequencies and depths ranged from 100 Hz to 12,000 Hz and from near surface to 132 feet of sea water (fsw), respectively. All dives were conducted in a small immersion tank in NSMRL’s hyperbaric chamber.

Wetsuit hood attenuation was calculated from the difference between hooded and unhooded hearing thresholds. The attenuation of underwater sound provided by a 7 mm neoprene wetsuit hood at the threshold of hearing was measured as being from 0 dB to approximately 34 dB. The amount of attenuation is dependent on at least two factors: frequency of the sound and ambient pressure (simulating depth). In general, the lower frequency sounds are attenuated less and the higher frequency sounds are attenuated more when a neoprene wetsuit hood is worn. The greatest attenuation (~20-34 dB) was above 4000 Hz; and almost no attenuation was measured at 100 and 250 Hz. From 500 to 1000 Hz, the amount of sound attenuation decreased as the ambient pressure increased.

This work was done by David M. Fothergill, Ph.D.; Edward A. Cudahy, Ph.D.; Derek W. Schwaller; Olha Townsend; and Michael K. Qin, Ph.D. NAVSYS-0004

This Brief includes a Technical Support Package (TSP).

Sound Attenuation of Neoprene Wetsuit Hoods as a Function of Dive Depth and Acoustic Frequency: Hyperbaric Chamber Trials (reference NAVSYS-0004) is currently available for download from the TSP library.

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