Both “physical” and “auditorily transformed” spectra of English sibilant fricatives were investigated for evidence that might: (a) discriminate between /s/ and /∫/; (b) do so independently of vowel context; and (c) normalize the substantial speaker-sex differences found. Measurements included the frequency location of the first peak of fricative noise, the location of the low-frequency edge descending from this peak, the amplitude range of this descent, and its gradient or orientation. Analyses of variance on our data demonstrated a marked superiority for the orientation of the spectral edge, which differentiated the phonemes reliability, while remaining independent of vowel context or speaker sex. This was true. however, only of the edge as transformed in auditory space. Given the analogous ability of the eye-to-track edges, the notation appears to have some plausibility for speech perception theory and an obvious implication for automatic speech recognition. Perceptual confirmation of the importance of spectral edges in fricatives is now being sought.
This research began out of two overlapping motives. First, we have been observing what have impressionistically been termed ‘whistling’ fricatives in Shona (and also, though not reported here, in another language, Jibbali); we felt that there was room for a fuller analysis of some aspects of their production characteristics. Second, sibilance in general offers plenty of scope for what Delattre called the ‘quest of the Holy Grail’ in phonetics: the search for features of relevance to perception. More background to both the production and the perception issues will be given as we proceed.
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