A study of vowels, diphthongs, and tones in Thai

This dissertation investigates the perceptual cues for length distinctions on vowels and diphthongs in Thai. Thai has nine vowels, each both short and long, three diphthongs, on which a length distinction is disputed, and five tones. Previous research established duration as the principal cue differentiating the phonologically long and short vowels but that differences in vowel quality and the duration of post-vocalic nasals (which show an inverse correlation with the duration of the preceding vowel) may also play a perceptual role. This dissertation presents the results of perceptual studies reaffirming vowel length as the primary cue for this distinction but also showing that the duration of a following nasal also assists. Acoustic studies suggest that all post-vocalic consonants show the inversely correlated duration with the preceding vowel. Acoustic analysis failed to find any systematic correlation with the duration of the pre-vocalic consonants. Length on diphthongs, though not phonemic, is shown to play a perceptual role in differentiating phonetically longer and shorter allophones when other cues are removed.;It was found that tone shape varies with the duration of the tone-bearing-unit (TBU) they are superimposed on and may also assist the long vs. short distinction. When the TBU is shorter, the end of the tonal contour is frequently truncated, the remainder of the tone being realized on the beginning of the next syllable. With high tones, truncation of the beginning of the contour was also observed. Shortening may also produce changes in the slope of the tonal contour---typically an increase in rate of change of fundamental frequency (F0)---and in the F0 range---typically a lessening of the range but occasionally an increase. When a contour tone (the rising-falling tone and the falling rising tone) is especially long, a 'plateau', i.e., a period of relatively flat F0, appears between the steep initial and terminal contours. The duration of this plateau varies directly and lawfully with the duration of the TBU. Tonal shape is also influenced by adjacent tones with carryover assimilation being stronger than anticipatory assimilation. Many of the above generalizations are presented quantitatively using novel signal-processing techniques.