Effects of lip protrusion on measurements of lip aperture

Lip movement during speech has been measured by many techniques. One common class of methods uses one marker for each lip, usually located midsagittally at the vermilion border. However, equating the distance between markers with the distance between the lips is problematic. Westbury and Hashi [J. Phonet. 25, 405–419 (1997)] found that compression of the lips during stop closure precluded using a single value for the minimum distance when the lips were together. Thus lip trajectories alone cannot determine the point of closure nor estimate lip aperture during vowels. The present experiment used video images collected simultaneously from front and side views of phonemes varying in lip shape (three vowels, two fricatives). The distance between the vermilion borders (i.e., typical marker locations) was about 3 mm larger than the actual aperture for /u/ relative to /a/ and /i/ and for the palatal fricative relative to /s/. This difference correlated best with the degree of upper lip protrusion. Outward curling of the lips, bringing the inner margins closer during rounding, seems the likeliest explanation. Predicting the actual aperture from marker locations must consider both the vertical separation and the degree of protrusion. [Work supported by NIH Grant No. DC‐02717.]