Alignment of Acoustic and Coupled Mechanic-Acoustic Eigenmodes in Human Phonation

Abstract: The human sound generation and the underlying fluid–structure–acoustic interaction that describes the mechanism of sound production are not yet fully understood. Previously, an experimental study indicated that the coupling of the acoustic effects with the structural dynamics causes a deviation in the vibration frequency. This happens whenever the primary frequency of the vocal fold motion is close to the lowest acoustic resonance frequency of the vocal tract. Presuming that this effect is a pure coupling of the acoustical effects, a numerical simulation model is established based on the computation of the mechanical-acoustic eigenvalue. With varying pipe lengths, the lowest acoustic resonance frequency was adjusted in the experiments and so in the simulation setup. In doing so, the evolution of the vocal folds coupled eigenvalues and eigenshapes is investigated, which confirms the experimental findings. Finally, it was shown that for normal phonation conditions, the mechanical mode is the most efficient vibration pattern whenever the acoustic resonance of the pipe (lowest formant) is far away from the vocal folds' vibration frequency. Whenever the lowest formant is slightly lower than the mechanical vocal fold eigenfrequency, the coupled vocal folds motion pattern at the formant frequency is the dominating one.