Individual variation of the hypopharyngeal cavities and its acoustic effects

Morphological measurements of the hypopharynx are conducted to investigate the correlation between fine structures of the vocal tract and speaker characteristics. The hypopharynx includes the laryngeal tube and bilateral cavities of the piriform fossa. MRI data during sustained phonation of the five Japanese vowels by four subjects are obtained to analyze intra- and inter-speaker variation of the hypopharynx. Morphological analysis on the mid-sagittal and transverse planes revealed that the shape of the hypopharynx was relatively stable, regardless of vowel type, in contrast to relatively large inter-speaker variation, and these results are confirmed quantitatively by a simple similarity method. The small intra-speaker variation of the hypopharynx is confirmed by further morphological analysis using high-quality MRI data for one of the subjects, obtained by using the “phonation-synchronized method” and “custom laryngeal coil.” Furthermore, acoustical effects of the individual variation of the hypopharynx are estimated by using a transmission line model. Vocal tract area function of one of the subjects above the hypopharynx is combined with the hypopharyngeal cavities of other subjects, and their transfer functions are calculated. The results show that the inter-speaker variation of the hypopharynx affects spectra in the frequency range beyond approximately 2.5 kHz.