Acoustic properties of phonemes in continuous speech for different speaking rate

Investigations have been made on the perceptual and acoustic properties of individual phonemes in continuous speech for different speaking rate. Fifteen short sentences spoken by four male speakers have been used as the test material. Each speaker has been asked to pronounce the sentences with three different rates: normal, first and slow. For perceptual experiment, individual CV-syllables have been taken out from their contexts and presented to listeners in isolation to be identified. The results reveal that individual syllables in continuous speech do not have enough phonetic information to be correctly identified especially for the fast speech. The average identification of syllables for the fast speech is 35% and even vowels are identified less than 60%. Slow speech shows highest identification among the three rates; 86% for the syllables, 87% for the consonants and 91% for the vowels. Duration of consonants and vowels are both affected by the speaking rate and the latter has been found greater in change. An important finding is that the duration ratio between consonant and vowel of a CV-syllable in the fast speech is kept almost the same as that in the normal speech. Vowel lengthening in the slow speech becomes significantly large. Formant frequencies of individual vowels have largely shifted toward the neutral region in the conventional F1-F2 plane as the rate becomes fast and, at the same time, distribution of vowels in each category becomes large.