Naturally produced English clear speech has been shown to be more intelligible than English conversational speech. However, little is known about the extent of the clear speech effects in the production of nonnative English, and perception of foreign-accented English by younger and older listeners. The present study examined whether Cantonese speakers would employ the same strategies as those used by native English speakers in producing clear speech in their second language. Also, the clear speech effects on sentence intelligibility of Cantonese-accented English for younger and older English-speaking listeners were explored. In the production study, native speakers of Cantonese and English produced English sentences conversationally and clearly. Some productions were subjected to acoustical measurements and selected for a subsequent perceptual task. Both groups of speakers showed a decrease in speaking rate and in articulation rate, and an increase in total pause time and in sentential fundamental frequency in their clear speech productions. However, the Cantonese speakers spoke slower in both speaking styles and lengthened the pauses significantly more than did the English speakers in clear speech. In the perception study, selected conversational and clear sentences were duplicated and mixed with 12-talker babble at a constant signal-to-babble ratio, and presented along with noise-free stimuli to English-speaking younger and older adults. The listeners assessed intelligibility by transcribing the sentences and verifying their truth value. Results showed that overall, clear speech was better perceived than conversational speech. The Cantonese speakers’ utterances were less intelligible than those of the English speakers in all conditions. The performance of the older listeners was poorer than that of the younger listeners, especially in noisy conditions. Native-produced English clear speech was more intelligible than conversational speech, but no significant difference in intelligibility was found between conversational and clear Cantonese-accented English for both groups of listeners. Overall, the findings suggest that the acoustic-phonetic characteristics of the conversational-to-clear speech transformations are in general similar for nonnative and native English speakers in producing clear speech. Nonetheless, a clear speaking style adopted by nonnative English speakers seems not to be an effective speech intelligibility enhancement strategy for native English listeners.
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