Production and perception of vowel length in spoken sentences

A series of experiments was conducted to determine (1) the accuracy with which vowel segment durations in spoken sentences can be represented in auditory sensory storage and (2) the extent to which phoneme boundaries in the identification of phonemic vowel length in Dutch are affected by syntactic and/or auditory-phonetic context. In a preliminary production test it was found that durations of both long and short Dutch vowel phonemes in monosyllabic words embedded in sentences are systematically affected by word positions in the sentences. In an initial perceptual experiment, phoneme boundaries and slopes of identification curves were measured for 12 listeners in five different test utterances in a binary forced choice identification test. Perceptual accuracy of vowel duration perception as determined from the slopes of the identification curves corresponds on the average to a just-noticeable difference (JND) of about 5 ms with a test segment duration of about 90 ms. Phoneme boundary values are systematically affected by context in ways predictable from syntactic structure and the auditory-phonetic environment. In a second perceptual experiment it is shown that a major effect on phoneme boundary can be brought about by perceived properties of the test utterance following the monosyllable containing the test segment. In a third perceptual experiment it is shown that the difference between phoneme boundaries in utterance final syllable and in embedded syllable is related to the presence or absence of a perceived speech pause following that syllable. The results of these experiments are interpreted in terms of a simple decision model with a noisy auditory representation of embedded vowel duration, lasting a few hundredths of milliseconds, and a noiseless internal criterion for vowel length identification which is systematically affected by the auditory-phonetic environment.