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This study explores the phonetic nature of phonological stop voicing contrast in American English by investigating how phonetic implementation of the voicing contrast is modulated by the prosodic structure along the continuum of phonetic voicing. In particular, the present study examines (1) the effects of two kinds of prosodic strengthening that can arise with prosodic structuring, a boundary-related domain-initial strengthening (DIS) and a prominence-induced strengthening, and (2) the possible enhancement types of linguistic contrasts that can underlie prosodic strengthening. The phonetic voicing was estimated using the Integrated Voicing Index (IVI), taking into account both the voicing lag (positive VOT) and the voiced interval during the closure. Results obtained with initial stops in both trochaic and iambic words are encapsulated as follows. Under the influence of DIS, both voiced and voiceless stops were produced with an increase in voicelessness, showing an enhancement of structurally motivated syntagmatic (CV) contrast. The effect size was larger for voiced stops, yielding a boundary-induced phonetic reduction of voicing contrast. Under the influence of prominence (focus), both voiced and voiceless stops showed an increase in voicelessness only in trochaic words, but this time, it was voiceless stops that showed a far greater effect, resulting in a maximization of voicing contrast—i. e. , an enhancement of paradigmatic contrast. Moreover, the reduced voicing for voiced stops under prominence even in the medial position indicates that voiced stops are realized in reference to the phonetic feature vl. unaspirated rather than voiced. These findings imply that seemingly non-contrastive low-level variation is indeed systematically modulated by the prosodic structure in reference to phonetic representations that regulate the phonetic implementation of phonological contrast in a given language. An alternative account is also discussed in the framework of Articulatory Phonology.
Kim et al. (Sat,) studied this question.