Acoustic properties of non-native clear speech: Korean speakers of English

The present study examined the acoustic properties of clear speech produced by non-native speakers of English (L1 Korean), in comparison to native clear speech. L1 Korean speakers of English (N=30) and native speakers of English (N=20) read an English word-list in casual and clear speaking styles. Analysis included clear speech correlates thought to be universal across languages (vowel space expansion and stressed vowel lengthening) and clear speech modifications believed to function in a languages-specific way (increased mean voice fundamental frequency and fo range and acoustic enhancement of English voicing contrast via voice onset time and onset fo). The results showed that across the two groups of participants clear speech was acoustically distinct from casual speech in every parameter. In addition, the presence and direction of the acoustic difference between the two speaking styles was largely the same for both native and non-native speakers. Nevertheless, non-native clear speech differed from native one in the magnitude of acoustic modifications. Specifically, L2 speakers implemented less vowel space expansion, less increase of mean fo, and less positive and negative VOT lengthening in clear speech than native speakers. Many, but not all differences between native and non-native clear speech could be attributed to the effect of participants' L1, for example, lower functional load of VOT in the differentiation of Korean laryngeal categories and absence of pitch-dependent system of prosodic prominence in Korean. Overall, we conclude that Korean speakers were successful in producing native-like English clear speech, although the observed deviations in the extent of acoustic modifications suggest some reliance on L1-specific prosodic system and system of phonetic implementation and enhancement of phonological contrasts.