A PERCEPTUAL AND ACOUSTIC STUDY OF THE EFFECTS OF SPEECH RATE ON DISTINCTIVE VOWEL LENGTH IN THAI

The difference between phonologically short and long vowels in Thai is mainly a temporal one, i.e., the relative durations of the vocalic segments. Yet such durational differences are also produced by changes in speaking rate. The present study explored the effects of speaking rate on 10 simple vowels of Thai. The first and second experiments were designed to see whether speech rates affects only vowel durations, or formant targets as well as durations. The results support only the effect on vowel durations. Acoustical measurements showed that formant-frequency targets were relatively unaffected by rate variations; however, vowel durations did vary significantly with change of rate. The vowel durations decreased with increase of speaking rate and vice versa.^ The third, fourth, and fifth experiments attempted to find out about the effects of speech rates on the perception of vowels of CV(V)C syllables. A potential perceptual problem would seem to be the confounding of temporal information for short and long vowels with temporal compensation between the rates of the carrier phrase and those of the key words. Experiments 3, 4, and 5 confirmed that listeners evaluated duration in the key words relative to the carrier-phrase rates. That is, the identification of a key word in its original carrier context was accurate; however, errors increased when there was a mismatch between the carrier rates and the rates of the key words. Not only the preceding-carrier rates but also the rates of the postcursive context influenced the vowel judgments for the key words. Thus, errors in vowel identification increased when only the postcursive rate was mismatched with the key-word rate, as compared with the identifications when the key word occurred in its original context. In addition, the influence of both preceding-context rate and following-context rate extends across words; it is not limited to a single syllable.^ For the phonological distinction between short and long vowels, the speaker of Thai takes considerable variation in speech rate into account, for both production and perception, in the processing of relative duration as the major phonetic feature.