Acoustic characteristics of the piriform fossa in models and humans

The piriform fossa forms the bottom part of the pharynx and acts as a pair of side branches of the vocal tract. Because of its obscure form and function, the piriform fossa has usually been neglected in the current speech production models. This study examines the geometric and acoustic characteristics of the piriform fossa by means of MRI-based mechanical modeling, in-vivo experiments and numerical computations. Volumetric MRI data showed that the piriform fossa is 2.1 to 2.9 cm3 in volume and 1.6 to 2.0 cm in depth for four Japanese subjects (three males and one female). The results obtained from mechanical models showed that the piriform fossa contributes strong troughs, i.e., spectral minima, to speech spectra in a region of 4 to 5 kHz. The antiresonances were identified with increasing frequency when water was injected into the piriform fossa of human subjects in in-vivo experiments. Antiresonances obtained from the experiments and simulations were confirmed to be consistent with those in natural speech within 5%. Acoustic measurements and simulations showed that the influence of the piriform fossa extends to the lower vowel formants in addition to the local troughs. This global effect can be explained by the location of the fossa near the glottal end of the vocal tract.