Reciprocity Calibration of Microphones in High Ambient Pressure

Abstract

Research concerning man's ability to speak and hear within ambient conditions of many times normal atmospheric pressure poses special difficulties in calibrating psychoacoustic equipment. In this experiment, a system for obtaining reciprocity calibrations within hyperbaric environments was employed. By means of the reciprocity system, microphone sensitivities were calculated from measures taken with a piezoelectric microphone. Frequencies were 0.1 6 kHz, and conditions of ambient pressure within a helium-filled hyperbaric chamber were equivalent to pressures encountered from surface to 600-ft submersion in sea water (1–19 ata absolute). The results indicate that, at frequencies below 1 kHz, increases in ambient pressure from surface to depths of 600 ft were accompanied by consistent decreases in microphone sensitivity. At 2 kHz, sensitivity did not continue to decrease for pressures equivalent to submergence from 300 to 600 ft. Resultant corrections to the calibrated microphone for increases in ambient pressure permit accurate measures to be made of stimuli for psychoacoustic experiments, and precise acoustic analyses can be made of speech produced under hyperbaric conditions.