MULTIDIMENSIONAL ANALYSIS OFABNORMAL VOCAL QUALITY

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The need for quantitative methods to evaluate voice disorders has motivated a good deal of research into the acoustic correlates of pathological vocal quality. The goal of most acoustic-based research is to find acoustic measurements that: (1) can be used in the detection of laryngeal pathology and (2) correlate well with perceived degree and/or perceived type of dysphonia. For example, it has been shown that voice samples from dysphonic speakers exhibit abnormally large cycle-to-cycle variations in fundamental frequency (jitter) and voice amplitude (shimmer). It has also been demonstrated that dysphonic voices show unusually large amounts of additive noise; i.e. signal-to-noise ratios are poor compared to measurements made from normal speakers. However, relatively little is known about how individual acoustic parameters combine to influence a listener's overall impression of vocal quality. The present research was designed to study both the individual and interactive contributions of four acoustic parameters in controlling perceived hoarseness: (1) jitter, (2) shimmer, (3) signal-to-noise ratio and (4) mean fundamental frequency (FO). A laboratory computer will be used to synthesize sustained vowels varying in jitter, shimmer, signal-to-noise (S/N) ratio and mean fundamental frequency (FO). These stimuli will be used in a series of psychoacoustic experiments designed primarily to answer two questions: (1) what are the individual or univariate relationships between perceived hoarseness and jitter, shimmer and S/N ratio, and (2) how do jitter, shimmer, S/N ratio and mean FO combine to affect perceived hoarseness. The results of the multivariate listening experiments will be analyzed using a statistical procedure called multidimensional scaling (MDS). MDS analysis will be used to determine: (1) the subjective dimensions underlying perceived hoarseness, (2) the relative perceptual weight given to each dimension and (3) the relationship between these dimensions and the four acoustic properties being manipulated. An attempt will then be made to extend these findings to hoarseness ratings and acoustic measurements obtained from naturally occurring normal and disordered voices. The results of these experiments should prove very useful in developing an acoustically based index that is sensitive to both the quantity and quality of hoarseness. A measurement technique of this type would be useful both to voice clinicians and to voice scientists studying the relationship between perceived voice quality and the acoustic attributes of speech.