Dependence of phonation threshold pressure and frequency on vocal fold geometry and biomechanics.

Previous studies show that phonation onset occurs as two eigenmodes of the vocal folds are synchronized by the interaction between the vocal folds and the glottal flow. This study examines the influence of the geometrical and biomechanical properties of the vocal folds on this eigenmode-synchronization process, with a focus on phonation threshold pressure and frequency. The analysis showed that phonation threshold pressure was determined by the frequency spacing and coupling strength between the two natural modes that were synchronized by the fluid-structure interaction. The phonation frequency at onset was the root mean square value of the two natural frequencies plus a correction due to the added stiffness of the glottal flow. When higher-order modes of the vocal fold structure were included, more than one group of eigenmodes was synchronized as the system moved toward phonation onset. Changes in vocal fold biomechanics may change the relative dominance between different groups and cause phonation onset to occur at a different eigenmode, which was often accompanied by an abrupt change in onset frequency. Due to the synchronization of multiple pairs of eigenmodes and the mode-switching possibility, a complete and accurate description of vocal fold biomechanical properties is needed to determine the final synchronization pattern and obtain a reliable calculation of the dependence of phonation threshold pressure and frequency on vocal fold geometry and other biomechanical properties.