Effect of speaking rate on the perception of vowels.

Three experiments examined the conditions under which the speaking rate of a context sentence affects vowel identification. In these experiments, listeners identified the vowel in synthetic /b/-vowel-/t/ syllables that varied systematically in the duration (temporal), the formant frequencies (spectral), or both the duration and formant frequencies (temporal-spectral) of the steady-state portion of the syllable. These syllables were embedded in two synthetic sentence frames, one with the temporal characteristics of a natural fast sentence and one with those of a slow sentence. For two vowel distinctions that are specified in natural speech by both temporal and spectral characteristics, /I/-/i/ and /e/-/ae/, listeners adjusted their identification of the vowels according to the sentence rate in all three conditions. Although there was a trend for the rate effect to be reduced in the temporal-spectral condition, the influence of sentential rate was never eliminated. By contrast, for a vowel distinction that is naturally specified primarily by spectral characteristics, /e/-/I/, there was no effect of sentence rate in any of the conditions. We conclude that when vowels are differentiated in natural speech by both temporal and spectral information, listeners obligatorily use the duration of the vowel to identify it and do so in relation to the rate of the sentence in which the vowel occurs.