Vowel Inherent Spectral Change

Part of the series Modern Acoustics and Signal Processing pp 31-47


Theories of Vowel Inherent Spectral Change

  • Geoffrey Stewart MorrisonAffiliated withForensic Voice Comparison Laboratory, School of Electrical Engineering and Telecommunications, University of New South Wales Email author 

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In many dialects of North-American English, in addition to vowels which are traditionally described as true and phonetic diphthongs, several vowels traditionally described as monophthongs also have substantial formant movement. Vowel inherent spectral change (VISC) has also been found to be an important factor in the perception of vowel-phoneme identity. This chapter reviews literature pertinent to theories of the perceptually relevant aspects of VISC. Three basic hypotheses have been proposed, onset + offset, onset + slope, and onset + direction; each taking the position that initial formant values are relevant but then differing as to the relevant aspect of formant movement. Of these, the weight of evidence indicates that the onset + offset hypothesis is superior in terms of leading to higher correct-classification rates and higher correlation with listeners’ vowel identification responses. Models which fit curves to whole formant trajectories have, as yet, not been found to outperform simple models based on formant measurements taken at two points (onset and offset) in formant trajectories. A popular curve-fitting model (first-order discrete cosine-transform, DCT) is interpretable as a parameterization of the onset + offset hypothesis.