Modeling vocal fold asymmetries with coupled Van der Pol oscillators

Abstract

Models of the glottal sound source are being developed to extend a recent synthesizer of disordered voices [Fraj et al., J. Acoust. Soc. Am. 132, 2603–2615 (2012)]. The synthesizer was based on a nonlinear wave-shaping algorithm, which generates a glottal excitation to a concatenated-tube representation of the trachea and vocal tract. The purpose of the present work is to incorporate a physics-based model of the vibrating vocal folds in order to increase the anatomical fidelity of the synthesizer. Further, the model will permit to characterize left-right fold asymmetries and explore the effect of those asymmetries on the resultant vocal timbre. In this report, the vocal folds are represented as a system of two coupled Van der Pol oscillators with noise terms and a detuning factor between their natural frequencies. Regions of phase locked and unlocked oscillations are determined and illustrated with bifurcation diagrams. Also, the effect of frequency detuning on the resultant frequency jitter is analyzed. The results are discussed in terms of their implications for modeling abnormal vocal fold behavior. [Work supported by CNPq (Brazil) and FNRS (Belgium).]