Modal locking between vocal fold oscillations and vocal tract acoustics

During voiced speech, vocal folds interact with the vocal tract acoustics. The resulting glottal source-resonator coupling has been observed using mathematical and physical models as well as in in vivo phonation. We propose a computational time-domain model of the full speech apparatus that contains a feedback mechanism from the vocal tract acoustics to the vocal fold oscillations. It is based on numerical solution of ordinary and partial differential equations defined on vocal tract geometries that have been obtained by magnetic resonance imaging. The model is used to simulate rising and falling pitch glides of [A, i] in the fundamental frequency (f_o) interval [145 Hz, 315 Hz]. The interval contains the first vocal tract resonance f_R1 and the first formant F₁ of [i] as well as the fractions of the first resonance f_R1/5, f_R1/4, and f_R1/3 of [A]. The glide simulations reveal a locking pattern in the f_o trajectory approximately at f_R1 of [i]. The resonance fractions of [α] produce perturbations in the pressure signal at the lips but no locking.