Asymmetries of the vocal folds and vocal fold vibration are key features underlying unilateral vocal fold motion impairment (VFMI). The knowledge of what particular asymmetries contribute to breathy voice and which asymmetries must be eliminated to re-establish normal voice will be important to improving evaluation and treatment of VFMI. It was hypothesized that several structural and vibratory asymmetries should lead to predictable changes in the glottal area, flow, and acoustic waveforms, and subsequently a perceived breathy voice quality. The purpose of this project was threefold: 1) to determine how specific vocal fold structural and vibratory asymmetries alter vocal function and perceived voice quality, 2) to determine the improvement in vocal function and voice quality in an abnormal voice with elimination of individual asymmetries, and 3) to develop a battery of vocal function measures that vary with dysphonia in a predictable manner. The approach was to use a computational kinematic model of vocal fold vibration that allows for differential left/right control of parameters such as vocal fold adduction, medial surface bulging, vibratory nodal point, phase, amplitude of vibration, and fundamental frequency. The resultant signals were subjected to aerodynamic and acoustic measurement as well as perceptual rating of voice quality. Results revealed that the degree of vocal process separation was the most influential parameter tested, though asymmetry of bulging, nodal point ratio, and starting phase worsened normal voice quality. Conversely, increased symmetry of bulging, nodal point ratio, amplitude of vibration and starting phase improved disordered voice quality. The amount of improvement to disordered voices varied based on the number of other asymmetries present. None of the six vocal function measures tested were primarily responsive to one particular model parameter, though four measures generally decreased as vocal process separation increased: maximum flow declination rate (MFDR), spectral slope (B0-B2), cepstral peak prominence (CPP), and harmonics-to-noise ratio (HNR). Two of the measures, MFDR and CPP, co-varied with each of the five parameters and robustly correlated with perceived severity.

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