Accounting for Diphthongs: Duration as Contrast in Vowel Dispersion Theory

This dissertation investigates the production and perception properties of diphthong vowels at different speech rates in order to advance the understanding of diphthong phonetics and to incorporate diphthongs into the phonological theory of vowel dispersion. Dispersion Theory (Flemming, 2004; Liljencrants & Lindblom, 1972; Lindblom, 1986) models vowel inventories in terms of contrast between all vocalic elements, yet currently only accounts for quality contrasts. Problematically, diphthongs have been excluded from previous acoustic and theoretical work due to their complex duality of being composed of two vowel targets while acting as one phonological unit. Two experiments are presented which test diphthong production and perception by altering speech rate and duration to determine fundamental properties of diphthongs cross-linguistically. In an elicitation experiment that uses a novel methodology for speech rate modulation, it is shown that speakers maintain diphthong endpoint targets in Vietnamese, Faroese, and Cantonese. Both diphthong endpoints and monophthong targets show similar movement as a natural effect of reduction of the vowel space at faster speech rates, unifying monophthongs and diphthongs in terms of their phonetic properties. Contra the predictions of Gay (1968), it is shown that diphthong slope is variable across speech rates and slope variability is language-dependent. The second section examines the effect of duration manipulation on diphthong perception with a vowel identification experiment. Results show that the effect of duration manipulation is dependent on phonological vowel length, but otherwise increasing duration improves perception through an increase in percent correct, lower confusability, and lower reaction times. Increasing duration also reduces confusability between diphthongs and monophthongs. This study finds that duration is an important dimension of contrast both within diphthongs and the vowel inventory as a whole. The analysis shows that in order to adapt Dispersion Theory to account for diphthongs, the theory must include an additional contrast dimension of time. Based on the results of the experiments, three constraints are proposed to initiate the inclusion of diphthongs into Dispersion Theory: *DUR, MINDIST ONSET, and MINDIST OFFSET. Including duration in theoretical models of vowel dispersion is the first step in accounting for vocalic elements that are contrastive along multiple dimensions