Rhythm in English clear speech

Abstract

This study investigates the effect of hyperarticulated, intelligibility-enhancing clear speech on English speech rhythm. Ramus et al. [Ramus et al., ‘‘Correlates of linguistic rhythm in the speech signal,’’ Cognition 72, 265–292 (1999)] showed that temporal properties of a speech signal such as the percentage of vocalic intervals (%V) and variability of consonantal and vocalic intervals (C, V) can be related to phonological properties such as presence/absence of unstressed vowel reduction and syllable structure complexity, and are consequently quite successful at grouping languages into the traditional rhythmic classes (stress-, syllable-, and mora-timed). Here, we explore whether/how clear speech affects stress-timed characteristics of English sentences in terms of these measures. Results revealed that the proportion of vocalic intervals (%V) within sentences remained stable across speaking styles, i.e., consonants and vowels were lengthened equally in clear speech. However, variability of both vocalic and consonantal intervals (V, C) was higher in clear than in conversational speech. The increase in V and C was achieved primarily through insertion/strengthening of short vocalic and consonantal segments that were dropped or coarticulated with surrounding sounds in conversational speech, making word and syllable boundaries more salient. These results suggest that increased intelligibility of clear speech can in part be attributed to prosodic structure enhancement by means of enhanced syllable and word boundaries demarcation.