Towards a definition of schwa : an acoustic investigation of vowel reduction in English
Bates, Sally Alexandra Rosemary
This thesis reports a single speaker acoustic study of vowel variability in connected speech. Over eight thousand vowel tokens taken from a corpus of read sentences are examined. The aim of the thesis is to achieve a better understanding of the nature of vowel reduction in English. Three questions are addressed. The first of these concerns the phonetic characterisation of schwa, the central or 'reduced' vowel. Schwa's contextual variability is assessed with reference to the question of whether or not it has an independent phonetic target. The second question concerns the role of stress in conditioning vowel reduction. Patterns of variability for sententially stressed and unstressed vowel tokens are examined in order to determine how far stress and context effects interact to influence vowel quality. The final question concerns the potential ditf erences between vowels with respect to inherent variability, that is, whether some vowels are inherently more susceptible to coarticulatory effects than other vowels. Maximal context-dependency for schwa strongly supports the hypothesis that it is completely unspecified for tongue position. The data indicate that it is also highly unspecified for jaw position. Evidence that schwa is targetless and can occupy almost any position in the vowel space depending on context, argues against the traditional concept of vowel reduction as an independent process of articulatory and/or acoustic centralisation. Greater context sensitivity for sententially unstressed vowels compared with their sententially stressed counterparts also supports an account of vowel reduction in terms of contextual assimilation. The results also indicate a continuum of underspecification. This ranges from-the more peripheral vowels /i, a, ɑ, ɒ, Ɔ/ which show the least contextual variability and which may be thought of as the most narrowly specified vowels to the more central vowels /I, ε , з, ᴧ, Ʊ/ and, in the present data, /u/, which show greater overall context dependency. It is proposed that greater acoustic stability for the more peripheral vowels reflects quantal acoustic properties and tighter articulatory and/or perceptual constraints on variability.Overall, the results support the view that vowel reduction represents a means of economising on articulatory effort. Schwa, the endpoint of the reduction process represents minimal articulatory effort insofar as it represents the straight-line interpolation between consonants and hence minimal resistance to coarticulatory effects. Shorter durations, greater context dependency and, in the case of the peripheral vowels, less extreme formant values for sententially unstressed compared with sententially stressed vowels reflects a reduction in articulatory effort and consequently less displacement from neutral. In view of the greater contextdependency observed for the more central vowels generally compared with the more peripheral vowels, the tense/lax alternation in phonological vowel reduction can also be interpreted as a saving on articulatory effort. A principal advantage of an account of English vowel reduction in terms of phonetic underspecification is that phonetic and phonological vowel reduction may be accounted for by the same mechanism.