Aeroacoustics of [s

Abstract

The theory of the sibilant fricative [s] is formulated and solved as a mathematical problem of aeroacoustics. Air is forced through the constriction between the tongue and the hard palate by the intra-oral pressure, forming a jet that strikes the upper incisors and leaves the mouth through a gap between the upper and lower incisors. The principal source of sound is the diffraction of jet turbulence pressure fluctuations by the incisors. The spectrum of these pressure fluctuations incident on the teeth is modeled analytically using an empirical formula adapted from boundary layer theory. Predictions are made of the far field acoustic pressure spectrum by reference to measured and estimated values of vocal tract dimensions and intra-oral pressure. Predicted spectra compare well with observations. The principal spectral peaks are determined by vocal tract physiology anterior to the tongue–palate constriction. The theory furnishes the first correct predictions of the dependence of the overall sound pressure level on the intra-oral pressure. This presentation will interpret the mathematical model in a nonmathematical manner. [Work supported, in part, by Grant NIDCD-01247 to CReSS LLC.]