Rhythm typology:: acoustic and perceptive studies

This thesis is mainly concerned with rhythm typology, that is to say with the categorisation of languages into rhythm classes. Studies in this field go back to Pike (1945) and Abercrombie (1967), who introduced the dichotomy of stress-timed vs. syllable-timed languages (the former supposedly exhibiting isochrony at the foot level, the latter at syllable level). After several experiments disproved these claims, it was proposed (cf. Bertinetto, 1981, and Dauer 1983) that the perceptual impression of stress-timing and syllable-timing could depend on structural properties of the languages, namely the presence/absence of complex consonantal clusters and vowel reduction. More recently, some authors proposed acoustic correlates of these properties (cf. Ramus, Nespor & Mehler, 1999, Grabe & Low, 2002, etc.), based on durational measures of C and V sequences. The research presented in this thesis is collocated within this framework. A number of experiments are presented, mostly exploiting a corpus containing comparable audio samples of 61 speakers of 21 languages, which were gathered and manually segmented for this purpose. The structure of the thesis is slightly peculiar as it is not barely split into two separate parts, the first expounding the state of the art, the second presenting the experiments, but attempts to mingle these two aspects. Every chapter deals with one or more issues of speech rhythm and contains a survey of the studies on the topic as well as an analysis of the experiments conducted. Previous research in the field of rhythm typology, from its beginning to present day models, is unfolded in Chapter 2 by following the thread of the traditional dichotomy that opposes stress-timing and syllable-timing. The chapter also presents an experiment inspired by recent studies (cf. Wagner & Dellwo, 2004, and Asu & Nolan, 2006): the inter-onset distance (a unit which has been abandoned in modern mainstream approaches, but which has been the focus of past research) is taken as a cue of syllable complexity. These measures provide encouraging results on multilingual productions of a multilingual speaker, but their values are less convincing when applied to data of more speakers and more languages. Chapter 3 zooms in to rhythm correlates. The main studies on this topic are outlined along the illustration of the formulae and a discussion of practical and methodological issues, such as speech rate normalisation and segmentation criteria. The values of several acoustic correlates of speech rhythm (%V, ΔC, ΔV, varcoC, varcoV, rPVI, nPVI, cCCI, vCCI) for the entire corpus of 21 languages are presented and discussed. Different correlates provide in some cases different results for specific languages, but the general representation is roughly the same: supposedly stress-timed languages tend to have higher variability of both C and V measures than supposedly syllable-timed and mora-timed languages. A final small-scale experiment provides terrain for speculation about the inclusion of pitch and intensity within rhythm accounts. The implementation and functioning of Correlatore is illustrated in Chapter 4. This program has been developed within the thesis for the automatic calculation of rhythm correlates and as a framework for the study of speech rhythm. Chapter 5 introduces the theme of speech rhythm variation and variability. After a discussion of the rare studies adopting this approach, it presents the results of the rhythm correlates on selected samples and outlines a framework that opens interesting perspectives. It is proved that the variability of rhythm correlates is not simply a reflection of instability, but follows a coherent scheme, intra-speaker variability being smaller than inter-speaker variability, which is in turn smaller than inter-language and inter-dialect variability. Finally, the focus moves to perception as only few studies have investigated the ability of listeners to discriminate languages on the basis of rhythm. Different de-lexicalisation procedures are discussed and a multi-task perceptive test on 43 listeners is described. Its results challenge the claim that the stress-timed vs. syllable-timed dichotomy is rooted in perception. The conclusion discusses the efficacy of rhythm correlates and the possibility of including pitch and intensity in a model of speech rhythm. It finally advocates for a scalar and bi-polar representation of speech rhythm.