Changes in shape, space and time: the impact of position on the spatiotemporal and configurational articulatory properties of liquid consonants.

The speech sounds 'L' and 'R' are often grouped together as a class (called 'liquid consonants'), because they are similar in a number of ways. For example, although they function as consonants in speech, they have a vowel-like phonetic quality. The are also among the most complex speech sounds to produce (and may be late acquired by children or hard for adult learners to master). They vary widely in different accents of the same language. Finally, their production can involve the tongue forming multiple constrictions in the vocal tract and they sometimes involve specific movements of the lips as well.

Although speakers are not always aware of it, the 'L' sounds at the beginning and end of a word like 'level' do not sound exactly the same. Likewise the R' sounds at the beginning and end of a word like 'roar' (for those so-called 'rhotic' speakers who pronounce an 'R' at the end of 'roar' at all!) do not sound exactly the same. Behind the difference in sound quality is complex variation in (i) the way the articulatory organs synchronise their movements (ii) the strength of the production of the speech sound and (iii) the shape of the tongue when the speech sound is produced. When an 'L' or 'R' at the beginning of a word is pronounced, the speech organ movements involved tend to be more tightly synchronised than for an 'L' or 'R' at the end of a word. Also, 'L' and 'r' at the beginning of words are produced with more effort than they are at the ends of words. Finally, the tongue shapes involved in the production of 'L' and 'R' at the beginning and ends of words can be radically different from one another. These remarkable differences are very hard to measure, but research over many decades has addressed and raised a number of theoretical questions.

Variation in these three parameters can cause very noticeable changes in the way 'L' and 'R' sound, explaining why, at the end of words, they seem less like consonants and more like vowels, e.g. making 'foal' and 'foe' sound very similar. The consonant might even disappear altogether, as occurred 200 years ago to 'R' at the end of the words in the RP accent of English. Thus, subtle variation in speech production can result in big changes in the long term. However, not all accents of English show the same patterns, or change at the same rate. While American and Irish English mostly have strong 'R' sounds at the end of words, word-final 'R' is starting to sound very weak and even be lost in some Scottish accents.

This project will use a vocal-tract imaging technique, ultrasound tongue imaging (UTI), to directly study the way the tongue moves inside the mouth when it is producing 'L' and 'R', informing theories of speech articulation. The movement of the lips will also be recorded, as they play an important part in the production of English 'L' and 'R' too. We will record differences in the timing of movements of different parts of the tongue and the lips, how extreme the movements are and how different the shape the tongue is when it is producing 'L' and 'R' in different positions within the word. We will also look at what happens to 'L' and 'R' across longer domains too, as it has been shown that the greatest changes in the way these sounds are produced are found when 'L' and 'R' occur at the beginning and end of speech utterances longer than single words. We will study how changes in the movements of the vocal organs correlate with changes in the acoustic speech signal and we will identify which kinds of variation in vocal organ movement are most likely to make 'L' and 'R' sound weak, vowel-like or missing.

Our research will focus on three key varieties of a single language in which 'R' is pronounced at the beginnings and ends of words, i.e. Scottish, Irish and American English. We will thus be able to address regional and historical variation within an otherwise well-understood language using novel methods to address theoretical questions relevant to all languages.

Who will benefit from this research?

While this project will provide methods and findings which are fundamentally useful to academic researchers in phonetics and linguistics, the research will also be of practical use to non-academic users, speech and language therapists and their clients. The findings of this research is also likely to be useful to those working on text-to-speech synthesis. Economic benefit will come by keeping UK institutions ahead of the curve in terms of developments in speech imaging technology, developed in collaboration with Articulate Instruments Ltd. Finally, the public will benefit from this study through educational outreach.

How will they benefit?

Speech and Language Therapists (SLTs) and their clients will benefit from this research as the liquid consonants are often delayed or disordered in child speech. The articulatory complexity of liquid consonants perhaps explains the difficulty children have in acquiring these speech sounds. The fact that the sequencing of gestures varies depending on the position of the liquid segment in a syllable is a further complexity that children have to master. Nevertheless the kind of variation we see diachronically in liquids is often mirrored in disordered speech, e.g. coda liquids are produced as vocalized variants or are deleted. Our study can show SLTs the impact that different gesture sequencing can have on the auditory quality of liquids in order to help them identify what is happening in the vocal tract of their client and to help them develop remediation strategies. Our study can help to inform SLTs about the gestural complexity of liquids and the types of variation in tongue shape and tongue-gesture sequencing that we find across speakers and varieties of English. Our study will be able to provide clinical researchers with detailed information about the way that liquids are articulated in onset and coda position, across a variety of English accents, in a normative speech corpus.

Those who work with text-to-speech systems can benefit from out study. Speech synthesizers generally function most effectively when producing single sentences, but they are less successful at producing longer stretches of speech that sound natural. One challenge in synthezising longer speech stretches is to duplicate the prosody of natural speech. Phonetic variation found at utterance boundaries provides important prosodic perceptual cues for listeners. Our study will provide information on the types of acoustic variation that occur at prosodic boundaries, e.g. preboundary syllable lengthening, but also extreme changes in phonetic quality associated with prosodic boundaries.

The Speech and Hearing Sciences (QMU) impact return for REF2015 was rated at 3*(70%) or 4* (30%). The impact return included a case study on economic development via Articulate Instruments Ltd. Working in close partnership with Articulate Instruments has helped to keep both parties (AI and QMU's researchers) at the forefront of articulatory speech research and development. The current project pushes forward the technology by developing the first ever synchronised continuous ultrasound tongue imaging and lip video recording system. This system will prove invaluable to researchers around the world who wish to study articulation in spontaneous speech.

The public will benefit from this research through teaching outreach that will take place at "meet the scientist" events. We will be able to show, thought ultrasound demonstration, how the tongue moves inside the vocal tract. This has proved to be an engaging way to get people interested in speech research and ultrasound.