Lip protrusion/rounding dissociati consonants: /w/ vs

In this study of facial movements during speech, we found a consistent tendency for two native French speakers and an American English speaker (also reported in Toda et al. [1]): there exist two separate lip gestures for consonants involving ‘labialization’. The first one is a lip protrusion with rounding (narrow lip area) observed for /w/, and the second is a protrusion with a large lip opening observed for postalveolar sibilants /S, Z/. Furthermore, this separation of lip patterns is supported by a factor analysis on the movement data, indicating that these gestures are functionally autonomous. We can relate this fact to the assumed modes of resonance for the consonants: rounding is a requirement for the Helmholtz’ resonance of /w/ being responsible of low F2, whereas protrusion can be intended to lower the frequency of frication noise of postalveolars, enhancing the contrast with their alveolar counterparts. The acoustic/aerodynamic function of the large lip opening for postalveolars remains to be explained.