Perception and production of English lexical stress by Thai speakers

This study investigated the effects of first language prosodic transfer on the perception and production of English lexical stress and the relation between stress perception and production by second language learners. To test the effect of Thai tonal distribution rules and stress patterns on native Thai speakers' perception and production of stress in English, paired experiments using English non-words with controlled syllabic structures were used. Native speakers of Thai (NT) and American English speakers (AE) were first asked to concatenate two aurally-presented monosyllabic syllables and to produce them as disyllabic non-words with initial or final stress. They were then asked to identify stress location in the same set of disyllabic non-words produced by a trained phonetician. Acoustic analyses of both groups' production indicated that they implemented word-initial stress with a higher mean F0 range, a higher average F0 and a higher average intensity than word-final stress. However, word-final stress was realized with longer vowel duration than word-initial stress. The results obtained also revealed that, while both groups' production was equally (~70%) heard as intended overall, their production of word-initial stress was heard as intended at a higher rate than their word-final stress production (73% vs.67%). In addition, it was found that a syllable with a short vowel followed by an obstruent (CVO) was the most difficult syllabic structure on which accurate stress production could be heard. Results of a stepwise regression analysis showed that native judges relied heavily on vowel duration in their perception of NT word-initial as well as word-final stress production. On the other hand, intensity and average F0 were their main perceptual cues to AE word-initial and word-final stress respectively. Even though F0 patterns of NT stress production could be identified as similar to a Thai tone, little evidence of Thai tonal distribution rule transfer on their production of stress was found.;The stress perception results showed that NT identified stress position as accurately as AE and both showed initial stress preference. Both groups were more accurate at identifying word-initial stress than word-final stress even though final stress was produced with more acoustic correlates than initial stress. Reaction time analysis also revealed that Thai speakers spent more time on final stress identification. In addition, stepwise regression analyses revealed no significant predictor for both AE and NT perception accuracy scores for initial stress. For final stress, vowel duration was the predictor for AE while intensity, duration and mean F0 were strong predictors for NT. A careful examination of the acoustic data suggested that average F0 may have been responsible for their superior initial stress perception. However, NT did not show the same sensitivity toward vowel reduction as a cue for stress as AE did. Furthermore, a moderate degree of correlation between the perception and production scores and acoustic reliance was observed among NT, but not among AE, suggesting a relatively closer relation between stress production and perception among NT. The overall findings are inconsistent with the Stress Typology Model and suggest that tonal speakers with L1 fixed stress patterns could accurately produce and perceive variable stress in L2 and that acoustic features used contrastively in L1 are exploited in L2 stress perception and production. More detailed acoustic studies on L2 stress acquisition among tone language speakers will deepen our understanding of L1 prosodic system influence on L2 stress learning, thus leading to a more comprehensive and accurate model of cross-language stress acquisition.