Computational Modeling of the Vocal Tract : Applications to Speech Production

Sammanfattning: Human speech production is a complex process, involving neuromuscular control signals, the effects of articulators' biomechanical properties and acoustic wave propagation in a vocal tract tube of intricate shape. Modeling these phenomena may play an important role in advancing our understanding of the involved mechanisms, and may also have future medical applications, e.g., guiding doctors in diagnosing, treatment planning, and surgery prediction of related disorders, ranging from oral cancer, cleft palate, obstructive sleep apnea, dysphagia, etc.A more complete understanding requires models that are as truthful representations as possible of the phenomena. Due to the complexity of such modeling, simplifications have nevertheless been used extensively in speech production research: phonetic descriptors (such as the position and degree of the most constricted part of the vocal tract) are used as control signals, the articulators are represented as two-dimensional geometrical models, the vocal tract is considered as a smooth tube and plane wave propagation is assumed, etc.This thesis aims at firstly investigating the consequences of such simplifications, and secondly at contributing to establishing unified modeling of the speech production process, by connecting three-dimensional biomechanical modeling of the upper airway with three-dimensional acoustic simulations. The investigation on simplifying assumptions demonstrated the influence of vocal tract geometry features — such as shape representation, bending and lip shape — on its acoustic characteristics, and that the type of modeling — geometrical or biomechanical — affects the spatial trajectories of the articulators, as well as the transition of formant frequencies in the spectrogram.The unification of biomechanical and acoustic modeling in three-dimensions allows to realistically control the acoustic output of dynamic sounds, such as vowel-vowel utterances, by contraction of relevant muscles. This moves and shapes the speech articulators that in turn dene the vocal tract tube in which the wave propagation occurs. The main contribution of the thesis in this line of work is a novel and complex method that automatically reconstructs the shape of the vocal tract from the biomechanical model. This step is essential to link biomechanical and acoustic simulations, since the vocal tract, which anatomically is a cavity enclosed by different structures, is only implicitly defined in a biomechanical model constituted of several distinct articulators.