Distinctive vowel length: duration vs. spectrum in Thai

Many languages are described as having a phonological distinction of length in vowels or consonants or even both. If the term is to be taken literally, we would expect to find that the underlying mechanism is control of the relative durations of the articulations for the phonemes in contrast. In languages with short and long vowels, however, it is often observed that short and long counterparts have somewhat different vowel qualities. Previous work on the vowels of Central Thai has shown that relative duration is a sufficient acoustic cue for the length distinction. Since, however, short vowels in portions of the vowel space tend to be more open, we have sought to learn how much of the perceptual burden is borne by relative duration, the major physical correlate of length, and how much by the spectrum (formant pattern), the major physical correlate of vowel quality. We have incrementally lengthened original short vowels and shortened original long vowels in minimal pairs of words embedded in sentences, thus preserving the spectral differences. The stimuli were played to native speakers for identification. The results show that the dominant cue is relative duration; however, for all the vowel pairs, the category boundary is influenced by spectral differences with, perhaps, some effect of the timing of the context as well.