Ibrahim, Omnia

Speaker Adaptations as a Function of Message, Channel and Listener Variability

University of Zürich, Zürich, Switzerland, 2022.

Speech is a highly dynamic process. Some variability is inherited directly from the language itself, while other variability stems from adapting to the surrounding environment or interlocutor. This Ph.D. thesis consists of seven studies investigating speech adaptation concerning the message, channel, and listener variability. It starts with investigating speakers’ adaptation to the linguistic message. Previous work has shown that duration is shortened in more predictable contexts, and conversely lengthened in less predictable contexts. This pervasive predictability effect is well studied in multiple languages and linguistic levels. However, syllable level predictability has been generally overlooked so far. This thesis aims to őll that gap. It focuses on the effect of information-theoretic factors at both the syllable and segmental levels. Furthermore, it found that the predictability effect is not uniform across all durational cues but is somewhat sensitive to the phonological relevance of a language-specific phonetic cue.
Speakers adapt not only to their message but also to the channel of transfer. For example, it is known that speakers modulate the characteristics of their speech and produce clear speech in response to background noise – syllables in noise have a longer duration, with higher average intensity, larger intensity range, and higher F0. Hence, speakers choose redundant multi-dimensional acoustic modifications to make their voices more salient and detectable in a noisy environment. This Ph.D. thesis provides new insights into speakers’ adaptation to noise and predictability on the acoustic realizations of syllables in German; showing that the speakers’ response to background noise is independent of syllable predictability.
Regarding speaker-to-listener adaptations, this thesis finds that speech variability is not necessarily a function of the interaction’s duration. Instead, speakers constantly position themselves concerning the ongoing social interaction. Indeed, speakers’ cooperation during the discussion would lead to a higher convergence behavior. Moreover, interpersonal power dynamics between interlocutors were found to serve as a predictor for accommodation behavior. This adaptation holds for both human-human interaction and human-robot interaction. In an ecological validity study, speakers changed their voice depending on whether they were addressing a human or a robot. Those findings align with previous studies on robot-directed speech and confirm that this difference also holds when the conversations are more natural and spontaneous.
The results of this thesis provide compelling evidence that speech adaptation is socially motivated and, to some extent, consciously controlled by the speaker. These findings have implications for including environment-based and listener-based formulations in speech production models along with message-based formulations. Furthermore, this thesis aims to advance our understanding of verbal and non-verbal behavior mechanisms for social communication. Finally, it contributes to the broader literature on information-theoretical factors and accommodation effects on speakers’ acoustic realization.