Acoustic and perceptual effects of clear speech on duration-dependent vowel contrasts
Ph.D. Dissertation, 1997
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.
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