A comparative study of the impact of voiceless alveolar and palato-alveolar sibilants in English on lip aperture and protrusion during VCV production

Lip rounding and protrusion during sibilant production are known. Whether these features are discriminative among sibilants and how the discrimination changes in different vowels context are not well explored. In this work, we consider two voiceless sibilants, namely, /s/ (alveolar sibilant) and /ʃ/ (palato-alveolar sibilant) in English during VCV production and show that lip aperture (LA) and lip protrusion (LP) are significantly higher in case of /ʃ/ than /s/ irrespective of the vowels (/a/, /i/, /u/) context. Using the USC Speech MRI database comprising 74 subjects speaking VCV sequence, we also show that, when used for /s/ vs /ʃ/ automatic classification, LA provides the highest classification accuracy of 90.57%(±5.91%) in case of /i/ followed by 85.09%(±5.19%) and 82.38%(±8.76%) in case of /u/ and /a/, respectively. The change in LA and LP from /s/ to /ʃ/ are seen as an effect of higher displacement of lower lip than that of upper lip for /a/ and /i/ unlike that for /u/.