Perception of vocal tremor.

Vocal tremors characterize many pathological voices, but acoustic-perceptual aspects of tremor are poorly understood. To investigate this relationship, 2 tremor models were implemented in a custom voice synthesizer. The first modulated fundamental frequency (F0) with a sine wave. The second provided irregular modulation. Control parameters in both models were the frequency and amplitude of the F0 modulating waveform. Thirty-two 1-s samples of /a/, produced by speakers with vocal pathology, were modeled in the synthesizer. Synthetic copies of each vowel were created by using tremor parameters derived from different features of F0 versus time plots of the natural stimuli or by using parameters chosen to match the original stimuli perceptually. Listeners compared synthetic and original stimuli in 3 experiments. Sine wave and irregular tremor models both provided excellent matches to subsets of the voices. The perceptual importance of the shape of the modulating waveform depended on the severity of the tremor, with the choice of tremor model increasing in importance as the tremor increased in severity. The average frequency deviation from the mean F0 proved a good predictor of the perceived amplitude of a tremor. Differences in tremor rats were easiest to hear when the tremor was sinusoidal and of small amplitude. Differences in tremor rate were difficult to judge for tremors of large amplitude or in the context of irregularities in the pattern of frequency modulation. These results suggest that difference limens are larger for modulation rates and amplitudes when the tremor pattern is complex. Further, tremor rate, regularity, and amplitude interact, so that the perceptual importance of any one dimension depends on values of the others.