Nicolas Hermant

Effect of source-tract acoustical coupling on the oscillation onset of the vocal folds.

This paper analyzes the interaction between the vocal folds and vocal tract at phonation onset due to the acoustical coupling between both systems. Data collected from a mechanical replica of the vocal folds show that changes in vocal tract length induce fluctuations in the oscillation threshold values of both subglottal pressure and frequency. Frequency jumps and maxima of the threshold pressure occur when the oscillation frequency is slightly above a vocal tract resonance. Both the downstream and upstream vocal tracts may produce those same effects. A simple mathematical model is next proposed, based on a lumped description of tissue mechanics, quasi-steady flow and one-dimensional acoustics. The model shows that the frequency jumps are produced by saddle-node bifurcations between limit cycles forming a classical pattern of a cusp catastrophe. The transition from a low frequency oscillation to a high frequency one may be achieved through two different paths: in case of a large acoustical coupling (narrow vocal tract) or high subglottal pressure, the bifurcations are crossed, which causes a frequency jump with a hysteresis loop. By reducing the acoustical coupling (wide vocal tract) or the subglottal pressure, a path around the bifurcations may be followed with a smooth frequency variation.

Speech biomechanics: What have we learned and modeled since Joseph Perkell’s tongue model In 1974?

With his “physiologically oriented, dynamic model of the tongue, Joseph Perkell introduced in 1974 a new methodological approach to understanding the “relationships among phonetic models and the properties and capabilities of the speech-production mechanism.” This approach has guided a large part of our studies in the two last decades. In order to investigate how mechanical properties of the orofacial motor system constrain the degrees of freedom of speech articulation and contribute to shaping the speech signals exchanged between speakers and listeners, we, among other research groups, have developed increasingly more realistic 2D, and then 3D, finite element(FE) biomechanical models of the human vocal tract and face. After summarizing some of our modeling and simulation results that shed light on some basic characteristics of speech production, we present recent developments which aim to improve the realism of the models: evaluation of the links between the FE mesh structure (based either on tetrahedra, hexahedra, or mixed elements) and simulation accuracy; development of an active 3D element that simulates muscle mechanics and muscle force generation mechanisms; use of Diffusion Tensor Imaging to investigate muscle anatomy; design of an Atlas-based method (i.e., without manual image segmentation) for the automatic generation of subject-specific models.

Human Tongue Biomechanical Modeling

Abstract This chapter introduces the anatomy of the human tongue, with its complex interweaving of muscles, glands, and connective tissues, the shape of which is determined by the recruitment of approximately ten internal muscles. The most known constitutive models proposed in the literature, describing the complex mechanical behaviors of tongue tissues, are introduced and discussed. These models assume a hyperelastic material modeled using a strain energy approach. A finite element implementation of an incompressible two-parameter Yeoh strain energy was proposed to simulate deformation in tongue tissues in response to muscle activations. Such constitutive model was estimated from an indentation experiment done on the cadaver of a 74-year old woman. Simulated tongue deformations under muscle activations were then provided. It is shown that the very soft material chosen to account for tongue tissues elasticity allows simulating the very large strain values observed inside the tongue tissues with tagged MRI, while maintaining a level of stress in tongue muscles coherent with data provided in the literature. Such results show the importance of the choice for a constitutive law when the organ model is driven by muscles forces.