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Naval Submarine Medical Research Laboratory > Research -- Open Ocean Trials

Research -- Open Ocean Trials

Open Ocean Trials of the Effect of Depth on Underwater
Sound Attenuation of a Neoprene Wetsuit Hood

Introduction

In an accompanying UHMS presentation we show how psychophysical measures of wetsuit hood sound attenuation change as a function of depth in a small non-parallel sided water tank located within a hyperbaric chamber. The chamber study revealed that decreases in wet suit hood sound attenuation with increasing depth appear only to occur at frequencies between 500 and 1500 Hz. At frequencies >1500 Hz there was no effect of depth on wet suit hood sound attenuation.

Objective

In view of the acoustic limitations of the small tank environment for high frequency sound (>1000 Hz) we sought to validate the chamber findings under conditions where the acoustic environment is not limited by the constraints of a small tank and is closer to that experienced during operational dives.

Methods

Subjects:

16 US Navy Divers (15 males, 1 female)
mean ± SD: height 181 ± 7.6 cm,   weight 80.5 ± 10.0 kg,  age 36.3 ± 8.8 years.

Dive Set Up:

Dive Site: Tongue of the Ocean, Bahamas (see Fig. 1). Chub Cay, Southern Berry Islands and off the coast of New Providence Island.

  Fig 1. Map of the Bahamas showing the locations of the Tongue of the Ocean,  New Providence Island (red square) and the Berry Islands where the dives were conducted.

Ocean Depth: from 40 to >200 fsw
Conditions: sunny, calm (<2 foot swells),  water temp = 72 °F.
Dive platform:  DREAM TOO, a 45 foot live aboard dive boat (Fig. 2) from which a “shark cage” (Fig. 3) was lowered from the stern to the stage depth.

Fig. 2: DREAM TOO the dive platform used for the diving experiments.During diving operations all power on the boat was switched off to prevent extraneous noise being transmitted from the boat into the water.
 
 
Fig. 3. A subject undergoing an underwater audiogram in the “shark cage”. The underwater projectors were attached to the cage via the bracket shown in the foreground. Diver positioning was maintained by sitting on the cross bar shown and locking the arms and fins between the bars on the cage. The subject response button shown in the right hand was used to relay detection of a tone to a topside computer.

Diver Dress: Standard SCUBA gear with 3-mm neoprene wetsuit. At depth diver switched from SCUBA to surface supplied open circuit hooker rig for compressed air gas supply (see Fig. 3).

Experimental Conditions:

  • Dive depths (fsw): 10 and 60

  • Test frequencies (Hz): 100, 250, 500, 750, 1000, 2000, 4000, 8000

  • Diver dress: Bare headed and wearing a 7-mm Neoprene wetsuit hood

Dependent Variables:

Underwater hearing thresholds measured using a computer administered adaptive tracking procedure (see accompanying UHMS presentation). Hood attenuation was derived from the difference between hooded and bare headed hearing thresholds.

Procedure: 

Map sound field at both depths. For each depth subject’s conducted bare headed and hooded thresholds on the same dive in a counter balanced order. Order for the dive depth was counter balanced. Subjects repeated the 10 and 60 fsw dive on a separate day. Data at each depth are the average of dive 1 and dive 2.

Results

Table 1
ANOVA Results

 Variable  F-Ratio  p-value
 Depth: (F 1,15)  8.05  <0.05
 Frequency: (F 7,105)   95.08  <0.0001
 Depth x Frequency: (F 7,105)  3.68  <0.01

Fig 4: Changes in hood sound attenuation with frequency and depth during the ocean dives. * = significant difference (p <0.05) for post hoc comparison (Holm t correction) between 10 and 60 fsw depths. Data are means ± SD (whiskers) and ± SEM (boxes).
 

Fig 5. Comparison of wetsuit hood sound attenuation between open ocean and chamber dives showing the significant 3-way interaction between diving environment, depth and frequency (F 7,189 = 8.24, p<0.0001). Note the difference in the slope of the blue and red lines and the significantly lower hood attenuation values for the ocean dives at 60 fsw for frequencies >1000 Hz.  * = significant difference (p<0.05) for post hoc comparison (Holm t correction) between ocean and chamber dives.

Conclusions

Differences in the pattern of hood sound attenuation with depth between the ocean and chamber trials at frequencies above 1000 Hz indicate that caution should be exercised when applying psychophysical acoustic data collected in a reverberant sound field to the open ocean environment. It is recommended that when setting guidance for diver exposure to underwater sound any correction factor used to account for protection by wet suit hoods take into consideration the lower hood attenuation values observed in these ocean trials at depth. 

Acknowledgements

The Captain and crew of DREAM TOO
The NSMRL dive team
Our US Navy Diver subjects for their participation in this arduous sea duty.
NSMRL Technical Support
Logistic support was provided by the AUTEC program manager
Funded by NAVSEA

 
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