The Acoustic Impedance Calibrator was developed as a test instrument to measure the acoustic impedance which is presented to the human vocal tract when a diving mask is worn. The impedance measurements were a part of a program (1) to investigate means of improving the intelligibility of speech transmitted from diving masks, especially during deep submergence. Microphones and mask cavity shapes were studied during this program. Subjectively, speech transmitted from a pressure microphone located in a diving mask or gas mask, exhibits pronounced emphasis of low frequenices, and sharp response peaks and valleys due to resonant modes in the mask cavity. Spectral analysis additionally demonstrates (2) an upward shift of speech formants, especially for small mask cavities. At depths greater than 200 feet, helium is introduced in the breathing gas to provide the increased partial pressure required to balance the water pressure. A high percentage of helium does not substantially change the voice fundamental frequency but does shift the formants of vowels and consonant sounds upward by a factor which would be proportional to the speed of sound in the exhaled gas if the walls of the vocal tract were completely hard. The unnatural speech produced in deep submergence, whether direct or processed through a heloum unscrambler, is made even more difficult to understand by the mask reaction on the voice and by the masking effect of inlet and outlet gas flow noise.
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