Abstract
Sibilant /s/, one of unvoiced sounds, is believed to be produced by a jet of air through a sibilant groove towards the sharp edge of the teeth. In the present study, we investigated effects of an expiratory flow rate on the acoustic characteristics of sibilant /s/. A large eddy simulation was implemented to investigate flow in an intra-oral cavity model which simplifies important anatomical features of an oral cavity when sibiliant /s/ is pronounced. The results illustrated that an increase in the expiratory flow rate caused to develop turbulent vortices in a separated flow from the edge of the obstacle wall which mimics a lower tooth. The increase in turbulent intensity induced a dramatic elevation in the Powell's sound in the flow separation region, which was regarded as a sound source of sibilant /s/. A frequency spectrum analysis of the flow velocity in the flow separation region showed broadband signals up to 20,000 Hz as consistent with characteristics of sibilant /s/. The maximum of the Powell's sound, which was located near the anterior edge of the obstacle wall at smaller flow rates, shifted downstream in the flow separation as the expiratory flow rate increased and whereby turbulence developed. These results demonstrated significant influences of the expiratory flow rate on the acoustic characteristics of sibilant /s/ in particular when the flow rate elevates to cause turbulence.
