Sound propagation through the large front-cavity volume of American English rhotic sounds

Abstract

Rhotic sounds in American English (AE) are produced with a large front-cavity volume between the palatal constriction and the lip constriction which is the source of the characteristic low F3 of /r/’s. Depending on its shape, this volume may be modeled as (a) a single tube or (b) a single tube with a side branch. Both models produce a low F3, but through different mechanisms that involve differences in wave propagation direction. In this paper, we describe finite element studies of the acoustics of the front cavity, using dimensions derived from MRI studies of different subjects producing AE /r/. In particular, we discuss the case of a front cavity with minimal sublingual space. The sound-pressure distribution in the front cavity and the possible sublingual cavity was calculated at various harmonic excitations. The results show that at low frequencies (up to 2000 Hz, depending on specific dimensions) the front cavity is better modeled as a single tube, and at higher frequencies as a single tube with a side branch. The implications of accounting for wave propagation patterns (and thereby acoustically critical points along the vocal tract) on extraction of dimensional measures from MRI data will be discussed.