The voiced labio-velar approximant is a type of consonantal sound, used in certain spokenlanguages, including English. It is the sound denoted by the letter ⟨w⟩ in the English alphabet; likewise, the symbol in the International Phonetic Alphabet that represents this sound is ⟨w⟩, and the equivalent X-SAMPA symbol is w. In most languages it is a labialized velar approximant[ɰʷ], and the semivocalic counterpart of the close back rounded vowel[u] - i.e. the non-syllabic close back rounded vowel. As labio-velar consonants do not easily fit into consonant charts with only labial and velar columns, [w] may be put in either the velar column, (bi)labial column, or both, though putting it in both columns is rare; the placement may have more to do with phonological criteria than phonetic ones.
Features of the voiced labialized velar approximant:
Its manner of articulation is approximant, which means it is produced by narrowing the vocal tract at the place of articulation, but not enough to produce a turbulent airstream. The type of approximant is glide or semivowel. The term glide emphasizes the characteristic of movement (or 'glide') of /w/ from the /u/ vowel position to a following vowel position. The term semivowel emphasizes that, although the sound is vocalic in nature, it is not 'syllabic' (it does not form the nucleus of a syllable).
Landau, Ernestina; Lončarić, Mijo; Horga, Damir; Škarić, Ivo (1999), "Croatian", Handbook of the International Phonetic Association: A guide to the use of the International Phonetic Alphabet, Cambridge: Cambridge University Press, pp. 66–69, ISBN0-521-65236-7
Martínez-Celdrán, Eugenio; Fernández-Planas, Ana Ma.; Carrera-Sabaté, Josefina (2003), "Castilian Spanish", Journal of the International Phonetic Association33 (2): 255–259, doi:10.1017/S0025100303001373
Pukui, Mary Kawena; Elbert, Samuel H. (1986), Hawaiian Dictionary, Honolulu: University of Hawaiʻi Press, ISBN0-8248-0703-0
Rogers, Derek; d'Arcangeli, Luciana (2004), "Italian", Journal of the International Phonetic Association34 (1): 117–121, doi:10.1017/S0025100304001628
Šuštaršič, Rastislav; Komar, Smiljana; Petek, Bojan (1999), "Slovene", Handbook of the International Phonetic Association: A guide to the use of the International Phonetic Alphabet, Cambridge: Cambridge University Press, pp. 135–139, doi:10.1017/S0025100300004874, ISBN0-521-65236-7