Acoustic Analysis of Speech Produced in Air Compressed from 14.7 to 147 psia

Abstract

Confusions in studies of speech in the deep undersea environment are caused by variations in the response of microphones under abnormal ambient pressures as well as an interactive effect upon vocal production caused by increases in ambient pressure and changes in gas mixture make up. In the present study, speech was analyzed when the gas mixture, normal air, was held constant, and reciprocity calibrations were made at those pressures under study of the microphone used in the recording of speech samples. Taped recordings were collected of the speech of six Navy divers reading lists of words and a paragraph inside a pressure chamber at normal atmospheric pressure and at pressures found 50, 100, 150, 200, 250, and 300 ft underwater (equivalent to 14.7 to 147 psia). The mean time required to read the paragraph increased 17% from 14.7 to 147 psia. Spectrographic analysis below 4 kHz showed an upward shift of formant centers of about 175 cycles, regardless of vowel, formant frequency, or speaker. The effect of a lower limiting frequency of resonance for the human vocal mechanism is discussed in light of the above results.