Acoustic and perceptual effects of clear speech on duration-dependent vowel contrasts

Jean DeMerit

Ph.D. Dissertation, 1997

Abstract

This study investigates acoustic and perceptual effects of clear speech, focused on three duration-dependent vowel contrasts in American English, /$/varepsilon,$ ae/, /$/Lambda,/ /alpha$/ and / scI, i/. Eight female native speakers produced the vowels in two consonantal contexts under two speech style conditions, conversational versus clear. A querying technique was used in which the speakers were engaged in a conversation-like task. Measurements of vowel duration and formant frequency were made. For each vowel, duration was significantly longer in clear speech. However, only one vowel pair, /$/varepsilon,$ ae/, indicated an increase in the durational contrast between the two vowels. In terms of clear speech effects on formant frequencies, F1 was significantly higher for the vowels of both the lower pairs while there was a significant increase in F2 for / scI, i/. With respect to the vowel contrasts, F1 increases were greater for /$/alpha$/ than for /$/Lambda$/. A perceptual test, using stimuli edited from the speakers' natural vowel productions, was employed to assess the relative importance of durational versus spectral changes with respect to improved identification of these vowels in clear speech. Two types of stimuli were created: silent-center stimuli, in which a large portion of the vowel nucleus was removed but transitional components remained in their original temporal positions, and fixed-center stimuli, which contained a central portion of the vowel nucleus of a single given duration. The results suggest that--although speaking clearly yields improvements in listeners' vowel perception--these effects are dependent on the vowel category and individual speaker. In particular, only the long vowels showed overall identification improvements across speakers. There was no strong indication of a difference in the relative perceptual contributions of the durational and spectral consequences of clear speech. Connections between the perceptual outcomes and the observed changes in acoustic properties are discussed.