Abstract
A mathematical model was developed to investigate possible causes of jitter and shimmer. The model builds on the classic, lumped element model of Ishizaka and Flanagan and allows for asymmetric motions of the vocal folds and aerodynamic imbalances. The intraglottal pressures were derived from empirical pressure data obtained from a static physical model of the larynx (M5). The mathematical model is based on ten, second-order, nonlinear, coupled, ordinary differential equations that were solved simultaneously using the software Mathematica. The solutions were analyzed graphically and numerically to identify perturbations in the fundamental frequency and amplitude of the glottal airflow. Jitter and shimmer were quantified using the jitter factor and the amplitude variability index. The results indicate that only time-dependent variations in biomechanical and aerodynamic parameters result in jitter and shimmer. The magnitudes of jitter and shimmer tend to be less than those observed in the natural sounding voice, even when the asymmetries are large. Although time-independent asymmetries may cause the vocal folds to oscillate out of phase or with different amplitudes, they tend to entrain and vibrate at a common frequency. [Work supported by NIH R01DC03577.]
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