Characteristics of phonation onset were investigated in a two-layer body-cover continuum model ofthe vocal folds as a function of the biomechanical and geometric properties of the vocal folds. Theanalysis showed that an increase in either the body or cover stiffness generally increased thephonation threshold pressure and phonation onset frequency, although the effectiveness of varyingbody or cover stiffness as a pitch control mechanism varied depending on the body-cover stiffnessratio. Increasing body-cover stiffness ratio reduced the vibration amplitude of the body layer, andthe vocal fold motion was gradually restricted to the medial surface, resulting in more effective flowmodulation and higher sound production efficiency. The fluid-structure interaction inducedsynchronization of more than one group of eigenmodes so that two or more eigenmodes may besimultaneously destabilized toward phonation onset. At certain conditions, a slight change in vocalfold stiffness or geometry may cause phonation onset to occur as eigenmode synchronization due toa different pair of eigenmodes, leading to sudden changes in phonation onset frequency, vocal foldvibration pattern, and sound production efficiency. Although observed in a linear stability analysis,a similar mechanism may also play a role in register changes at finite-amplitude oscillations.
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