MODELING BIPHONATION - THE ROLE OF THE VOCAL-TRACT

Instabilities of the human Voice source appear in normal voices under certain conditions (newborn cries, vocal fry, creaky voice) and are sy mptomatic of voice pathologies. Vocal instabilities are intimately rel ated to bifurcations of the underlying nonlinear dynamical system. We analyse in this paper bifurcations in 2-mass models of the vocal folds and study, in particular, how the incorporation of the vocal tract af fects bifurcation diagrams. A comparison of a simplified model (Steine cke and Herzel, 1995) with an extended version including vocal tract r esonances reveals that essential features of the bifurcation diagrams (as e.g. frequency locking of both folds and toroidal oscillations) ar e found in both model versions. However, vocal instabilities appear in the extended model at lower subglottal pressures and even for weak as ymmetries. (C) 1997 Elsevier Science B.V.