Visualization of Hypopharyngeal Cavities and Vocal-Tract Acoustic Modeling

The hypopharyngeal cavities constitute a bottom part of the vocal tract near the larynx consisting of the supraglottic laryngeal cavity and the bilateral cavities of the piriform fossa. Visualization of those cavities with custom magnetic resonance imaging techniques reveals that during speech the laryngeal cavity takes a form of a long-neck flask and the piriform fossa a goblet of varying shapes: the former diminishes greatly in whispering and the latter disappears in deep inhalation. Our previous studies based on MRI have indicated that those cavities exert significant acoustic effects at higher frequency spectra. In this study, acoustic experiments with MRI-based mechanical vocal-tract models were conducted for male and female vocal tracts with the results that acoustic effects of those cavities determine frequency spectra above 2.5 kHz, giving rise to peaks and zeros. Those findings led us to propose an acoustic model of vowel production with three components: voice source, vocal-tract proper, and hypopharyngeal cavities. This model provides effective means in controlling voice quality and for expressing individual vocal characteristics.