On Medumba bilabial trills and vowels

The Medumba consonant inventory includes plain and prenasalized bilabial trills /ʙ, mʙ/, which contrast with bilabial stops. The trills occur most often before the central vowels /ʉ̜ ə/, while they are not attested before /u/. The central vowel /ʉ̜/ has a vowel posture that is particularly conducive to trilling of the lips: it does not form a circular opening, the corners of the mouth are drawn slightly apart, the lips are tensed, particularly at the corners of the mouth, and there is a narrow aperture between the lips. This suggests that it is close lip aperture of the following vowel— rather than rounding—that provides the most conducive environment for bilabial trill production.