Investigation of hearing ability of divers in hyperbaric helium-air mixture environment in submergence

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1. Frequency characteristics of six kinds of headphones were measured at ambient pressures of 1ATA to 31ATA. A YAMAHA HP-1 was best suited for the present purpose because its frequency characteristics were stable even at the high ambient pressures. 2. The minimum-audible sound pressure level (MAP) and the equal-loudness contours (ELC) were measured at 1ATA to 31 ATA with an audiometer RION AA-71. To prevent the background noise, the headphone unit of HP-1 was build in an ear-protector. MAPs in normal air and at 5 meter depth were properly measured. Since the lower frequency noise could not be reduced by the ear-protector, MAPs at deeper than 20 meter were measured only at higher frequency than 1.5KHz. ALCs were widely changed by the ambient preesures. The curve of ALC at deeper than 120 meter were looked like a letter "w", showing that ALCs have local minima at 2KHz and 6KHz, and a peak at 3KHz. 3. The bone-conduction hearing thresholds (BCT) were also measured. The variation of BCT by the ambient pressure was less than that of MAP. Using an electronic circuit modeling of the ear canal, a shift of peak frequency of MAP in hyperbaric pressures was partly reasoned by change in acoustic trans-mission character of the ear canal. 5. The sound absorption coefficients of three absorber materials were measured at hyperbaric N_2-air and He-air. The curves of absorption coefficients were shifted to higher frequency region. In other words the sound absorption coefficient were reduced in hyperbaric pressure. It was found that the frequencies at which the absorption coefficient remains unchanged were proportional with the exponent of the product of gas density and cubic power of velocity. The exponents are 0.7 for glass wool, 0.5 for felt, and 0.7 for absorption sponge. 6. We have implemented a helium speech database on a personal computer.At present, there are 3,000 data recorded with phoneme label.