Voiced bilabial fricative

A voiced bilabial fricative is a type of consonantal sound, used in some spoken languages. The symbol in the International Phonetic Alphabet that represents this sound is either a Latin or Greek-style beta, ⟨β⟩.

This letter is also often used to represent a voiced bilabial approximant, though that is more precisely written with a lowering diacritic, that is ⟨β̞⟩. This sound may also be transcribed as an advanced labiodental approximant ⟨ʋ̟⟩, in which case the diacritic is again frequently omitted, since no contrast is likely.^[1][2] It has been proposed that either a turned ⟨β⟩ or reversed ⟨β⟩, among others, be used as a dedicated symbol for the bilabial approximant; however, despite occasional usage, none have gained general acceptance.^[3]

It is extremely rare for a language to make a phonemic contrast between a voiced bilabial fricative and a bilabial approximant. The Mapos Buang language of New Guinea contains this contrast. Its bilabial approximant is analyzed as filling a phonological gap in the labiovelar series of the consonant system rather than the bilabial series.^[4] Proto-Germanic^[5] and Proto-Italic^[6] are reconstructed as having had a contrast between the voiced bilabial fricative and the voiced labial–velar approximant /w/, albeit with [β] being an allophone for another consonant in both cases. In Bashkir language, it is an intervocal allophone of /b/, and it is contrastive with /w/: балабыҙ [bɑɫɑˈβɯð] 'our child', балауыҙ [bɑɫɑˈwɯð] 'wax'.

A bilabial fricative is diachronically unstable (likely to be considerably varied between dialects of a language that makes use of it) and is likely to shift to [v].^[7]

The sound is not the primary realization of any sound in English dialects except for Chicano English, but it can be produced by approximating the normal English [v] between the lips; it can also sometimes occur as an allophone of /v/ after bilabial consonants.