Nonlinear behaviors of asymmetrical vocal fold vibration with fluid structure vibration simulation.

Vocal fold vibration is essential to human phonation. It is a fluid structure interaction (FSI) between glottal airway flow and vocal fold tissue. During the regular/healthy phonation, two vocal folds tend to have similar tissue properties and have symmetrical self-sustained vibrations. But in some voice disorder cases, two vocal folds may have significant differences in tissue properties, which lead to asymmetrical vocal fold vibrations. In this work, we focused on investigating the nonlinear behavior of vocal fold vibrations under different levels of stiffness discrepancies between two vocal folds. When the stiffness difference between two vocal folds is limited, symmetrical vocal fold vibrations can still be maintained. With the stiffness difference growing, different types of asymmetrical vibration happen by analyzing the vocal fold displacement waveforms, spectrograms, and phase-space reconstructions. Under symmetric baseline conditions, the model produced regular, quasi-periodic oscillations. With increasing asymmetry, the system gradually transitioned to frequency locking, T² quasi-periodic motion, T³ quasi-periodic motion and eventually chaotic vibration. These findings demonstrate that stiffness discrepancy alone can destabilize vocal fold synchrony and give rise to complex nonlinear dynamics.