An important yet often overlooked aspect of speech is its melody, aka intonation, the systematic modulation of voice pitch for linguistic purposes. A better understanding is important because intonation plays a critical role in communication. It helps us organize speech into phrases, and is also used to highlight important words and convey nuances about our intended meaning. The importance of intonation becomes clear when it “malfunctions” during communication. This can happen, for example, when a listener fails to detect irony, or a learner transfers a melody from their native tongue to their second language, where the melody is used differently. As our research shows, such misapplication of intonation is also characteristic of AI speech, which can imitate the melodies used by humans but use them in the wrong context.
It is not possible to remedy such miscommunications until we have a good understanding of intonation’s structure and functions but, as noted, intonation has been overlooked. This has led to either neglecting intonation (e.g. studying conversation without considering intonation) or treating it as an acoustic signal without considering its linguistic structure. The aim of SPRINT has been to address the challenges that intonation variability poses, by documenting variability and advancing new methodological principles for handling it, with a view to developing a comprehensive theory of intonation structure and meaning.
SPRINT focused primarily on speech production, where intonation variability is considered most challenging. To address this challenge, we adopted established statistical modelling techniques, that had not been previously used for the study of intonation, and applied them to conversational English and Greek. We showed that with the right methodologies, not only is it possible to study the intonation of spontaneous speech but that doing so is critical for understanding complex intonational phenomena. We further tested our findings by conducting comprehension experiments.
A main conclusion from SPRINT is that intonation behaves similarly to other speech phenomena that pertain to consonants and vowels (segments). Intonation categories have distinct phonetic realizations which show some overlap, so that the same phonetic realization can belong to different categories. This, however, is not a problem for comprehension: although pitch is the main phonetic exponent of intonation, additional cues (e.g. loudness and duration) are also used, a phenomenon known as cue-redundancy. Further, some redundant cues are used by speakers to compensate for the lack of optimal pitch use, a phenomenon known as cue-trading. Overlap, cue-redundancy, and cue-trading are well-established features of segments. Their use in intonation provides evidence that intonation and segmental categories are comparable. We further found that the phonetic detail and pragmatic function of intonation categories may be more salient to some speakers, based on their levels of empathy, musicality, and autistic-like traits. The interplay between all these factors can lead to speakers of a language developing slightly different intonation systems. Our figure illustrates this point: in English, speakers barely differentiate between highlighting words that present contrastive information from those that are simply new (e.g. the pitch of "black" is virtually the same whether speakers say "A black coffee, please", where "black" is new, or "no, I want BLACK coffee (not a cappuccino)", where "black" is contrastive). Greek speakers, on the other hand, differentiate the two using a fall for new and a rise-fall for contrastive information.
Overall, our research has helped us understand how intonation is structured and has led to the development of new methodological approaches that can become a blueprint for others to follow, further advancing the study of intonation. Such advancement can lead to breakthroughs in the teaching of intonation in second language acquisition, and the development of more natural synthetic voices.
