Small vocal-tract cavities revealed by magnetic resonance imaging and their acoustic influences.

Abstract

In natural speaking conditions, the vocal tract exhibits small cavities in the hypopharyx and oral cavity, while their acoustic effects have not been fully recognized. Acoustic studies based on static and dynamic magnetic resonance imaging of the vocal tract in three dimensional have shown that these cavities cause regional resonance and antiresonance in the vocal tract. The hypopharyngeal cavities, including the supraglottal laryngeal cavity and bilateral cavities of the piriform fossa, characterize vowel spectra in the higher frequencies and contribute to determining voice quality and speaker characteristics. The laryngeal cavity functions as a Helmholtz resonator to generate an extra formant in the vicinity of 3 kHz, and the piriform fossa forms a pair of side-branches of the vocal tract to cause spectral zeros in the vicinity of 4–5 kHz. The bilateral interdental spaces form a pair of side-branches in the oral cavity in nonlow vowels, and their geometry varies with articulation. Their antiresonance frequency is below 2 kHz in /i/, while it rises toward /a/ crossing the second formant in transition, and thus causing a small discontinuity of that formant between the two vowels.