Measuring lingual coarticulation from midsagittal tongue contours: Description and example calculations using English /t/ and /ɑ/

The purpose of this study was to test the feasibility of a protocol for measuring coarticulation using tongue surface outline data derived from ultrasound imaging. Ultrasound and acoustic data were collected from three speakers of Southern British English while they repeated a list of three sentences 15 times. Tongue surface outlines for the consonant /t/ in /ɑtɑ/ (in “…Ma tasked…”) were compared with those for the /t/ in /iti/ (in “… Leigh teased…”) and tongue surface outlines for the vowel /ɑ/ in /ɑkɑ/ (in “… Ma cast…”) were compared with those in /ɑtɑ/. Nearest neighbour distance calculations were used for the comparison of tongue surface outlines. Mean distance in midsagittal tongue surface outline between tokens of the same phoneme across two different environments was taken as a measure of the phoneme's susceptibility to environmental influence. The calculations show that the tongue contour during /t/ adapts to the influence of the neighbouring vowels approximately three times more than the tongue contour during /ɑ/ adapts to the influence of the neighbouring consonants. The applicability of the measure proposed in this paper to future speech research using ultrasound and other articulatory techniques is discussed.