Effects of ambient pressure and gas mixture on a numerical model of subglottal acoustics and vowel spectra.

Abstract

Divers’ speech in hyperbaric heliox environments becomes significantly distorted, making communication between divers and with surface-based personnel difficult. Previous research identified two basic mechanisms underlying this distortion. First, the speed of sound increases significantly in heliox, causing a linear increase in the second and higher formant frequencies. Second, in hyperbaric conditions the specific impedance of air in the vocal tract approaches that of the vocal tract walls, leading to a nonlinear increase in the first formant frequency. Effects of ambient pressure and gas mixture on subglottal acoustics and subglottal-vocal tract coupling in vowel spectra have not been sufficiently investigated. In this paper, such an investigation is carried out using a numerical model of subglottal and vowel acoustics. It is shown that subglottal resonance frequencies increase linearly as the speed of sound increases, and the corresponding subglottal pole-zero pairs in vowel spectra become disproportionately prominent as ambient pressure increases. Increased acoustic coupling between the subglottal system and the vocal tract in hyperbaric heliox therefore constitutes a third significant source of speech distortion. The results have implications for understanding subglottal coupling in vowels and for heliox speech unscrambling techniques. [This work was supported in part by NSF Grant No. 0905250.]