Perceptual evaluations of spectral and temporal modifications of deaf speech

The perceptual effect of modifying speech produced by deaf talkers was investigated to discover the changes necessary for disordered speech to be judged normal. Recordings of passages read by three deaf talkers were used as material. For the first two experiments, a three-syllable word was extracted from the deaf talkers' passages and from a similar passage recorded by a hearing talker. Each of the deaf speech samples was paired with the normal speech sample to generate various continua that differed in the spectral and temporal modifications applied to them. Within each continuum, the individual stimuli varied in the shape of the spectrum envelope and were produced by linear interpolation of LPC analysis parameters between the deaf and normal speech end points. Results suggest that correcting the temporal component of deaf speech alone is not enough to make it sound normal. Spectral corrections that approximate about 70% of normal appear to be necessary for the deaf speech samples to be judged normal. A third experiment made use of a 10-syllable segment of speech in which the relative contributions of spectral and temporal adjustments were investigated. The general conclusion of these three experiments is that spectral adjustments are more important to perceptual judgments of normality than temporal adjustments.