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The Navy needs to control noise exposure in hyperbaric chambers and diving helmets but the applicability of existing hearing-conservation standards to those environments is uncertain. Consequently, a program of research has been undertaken to develop suitable control measures. The specific goals of the research described in this report were to develop means of insuring accurate calibrations of microphones and earphones in hyperbaric environments, to determine the feasibility of measuring valid auditory thresholds in the NSMRL hyperbaric facility, and to obtain preliminary data bearing on the effects of noise in compressed air on the ear. Condenser microphones were calibrated in compressed air to 10 atmospheres and the results were highly similar to previous studies. Providing care is used in handling them, and calibration are preformed in suitable gas it should not be necessary to calibrate condenser micro-phones in diving environments more than once a year for routine noise measurement (sound survey) purposes. Calibration studies should be undertaken to determine the performance of ceramic microphones that are less expensive and more rugged than condenser microphones for use in compressed air environments. Several earphones were also calibrated in compressed air but the results were not consistent for most earphones and instrumentation for in situ calibration of earphones on each dive should be developed. However, careful selection of earphones and pre- and post experiment calibrations may be relied upon for experiments in compressed air. It was demonstrated that with minimal precaution, valid auditory threshold measurements can be obtained in the NSMRL hyperbaric chamber. The ear appears to be less sensitive to noise in compressed air at 3 atm than it is in air at 1 atm if sound is measured in terms of sound pressure.