Modulation of Voice Related to Tremor and Vibrato

Abstract

Modulation of voice is a result of physiologic oscillation within one or more components of the vocal system including the breathing apparatus (i.e., pressure supply), the larynx (i.e. sound source), and the vocal tract (i.e., sound filter). These oscillations may be caused by pathological tremor associated with neurological disorders like essential tremor or by volitional production of vibrato in singers. Because the acoustical characteristics of voice modulation specific to each component of the vocal system and the effect of these characteristics on perception are not well-understood, it is difficult to assess individuals with vocal tremor and to determine the most effective interventions for reducing the perceptual severity of the disorder. The purpose of the present studies was to determine how the acoustical characteristics associated with laryngeal-based vocal tremor affect the perception of the magnitude of voice modulation, and to determine if adjustments could be made to the voice source and vocal tract filter to alter the acoustic output and reduce the perception of modulation. This research was carried out using both a computational model of speech production and trained singers producing vibrato to simulate laryngeal-based vocal tremor with different voice source characteristics (i.e., vocal fold length and degree of vocal fold adduction) and different vocal tract filter characteristics (i.e., vowel shapes). It was expected that, by making adjustments to the voice source and vocal tract filter that reduce the amplitude of the higher harmonics, the perception of magnitude of voice modulation would be reduced. The results of this study revealed that listeners' perception of the magnitude of modulation of voice was affected by the degree of vocal fold adduction and the vocal tract shape with the computational model, but only by the vocal quality (corresponding to the degree of vocal fold adduction) with the female singer. Based on regression analyses, listeners' judgments were predicted by modulation information in both low and high frequency bands. The findings from these studies indicate that production of a breathy vocal quality might be a useful compensatory strategy for reducing the perceptual severity of modulation of voice for individuals with tremor affecting the larynx.