Analysis of the pressure-wall interaction at the release of a stop closure

Abstract

In producing a stop consonant, a soft tissue articulator, such as the lower lip, the tongue tip, or the tongue body, is raised to make an airtight closure. Stevens [I] pp 32 9-3 30 hypothesized that the interaction of the air pressure with the yielding soft-tissue wall would lead to a plateau-shaped release trajectory, and the duration of the plateau is progressively longer for bilabial, alveolar, and velar (Fig. 1-1). This thesis analyzes the pressure-wall interaction when a stop closure is released. Three flow models are implemented to derive the release trajectory: quasi-steady incompressible, unsteady incompressible, and unsteady compressible flow. Results from the models confirm Stevens' hypothesis. In the unsteady flow models, this thesis contributes a new method - deformable control volume analysis - to the pressure-wall interaction for small openings. This method may also be applied to quantify the unsteady effect during the closing and opening of the vocal folds and during the initial transient phase of a stop consonant, when the cross-sectional area is small. Indirect means of measuring an unknown parameter in the pressure-wall interaction analysis is discussed with the aid of a closure model which derives the condition of retaining a complete closure against air pressure buildup. In comparison with real speech data, an acoustic measure is defined for determining the duration of the frication noise of voiceless alveolar and velar stop consonants in syllable initial positions. This newly defined measure is based on the time variation of the average FFT magnitude in the whole frequency range and the magnitude in a 50-Hz-wide frequency band containing the front cavity resonance for the signal in every 5 milliseconds (a moving 5-ms window). This measure is found applicable to 25 releases out of 32 releases from TIMIT database. The means of the collected durations are found closest to the estimated duration calculated with the unsteady compressible flow model.