Types of Rounding
There are two types of vowel rounding: protruded and compressed. In protruded rounding, the corners of the mouth are drawn together and the lips protrude like a tube, with their inner surface visible. In compressed rounding, the corners of the mouth are drawn together, but the lips are also drawn together horizontally ("compressed") and do not protrude, with only their outer surface visible. That is, in protruded vowels the inner surfaces of the lips form the opening (thus the alternate term endolabial), while in compressed vowels it is the margins of the lips which form the opening (thus exolabial). Catford (1982:172) observes that back and central rounded vowels, such as German /o/ and /u/, are typically protruded, while front rounded vowels such as German /ø/ and /y/ are typically compressed. Back or central compressed vowels and front protruded vowels are uncommon, and a contrast between the two types has been found to be phonemic in only one instance. There are no dedicated IPA diacritics to represent the distinction.
The distinction between protruded and compressed holds for the semivowels and as well as labialization. In Akan, for example, the is compressed, as are labio-palatalized consonants as in Twi "Twi" and adwuma "work", whereas and simply labialized consonants are protruded. In Japanese, the /w/ is compressed rather than protruded, paralleling the Japanese /u/. The distinction applies marginally to other consonants. In Southern Teke, the sole language reported to have a phonemic /ɱ/, the labiodental sound is "accompanied by strong protrusion of both lips", whereas the found as an allophone of /m/ before /f, v/ in languages such as English is not protruded, as the lip contacts the teeth along its upper or outer edge. And in at least one account of speech acquisition, a child's pronunciation of clown involves a lateral with the upper teeth contacting the upper-outer edge of the lip, but in crown a non-lateral is pronounced with the teeth contacting the inner surface of the protruded lower lip.
Some vowels transcribed with rounded IPA letters may not be rounded at all. An example is /ɒ/, which in English has very little if any rounding of the lips. The "throaty" sound of English /ɒ/ is instead accomplished with sulcalization, a furrowing of the back of the tongue also found in non-rhotic /ɜː/ (Lass 1984:124).
It is possible to mimic the acoustic effect of rounded vowels by narrowing the cheeks, so-called "cheek rounding", which is inherent in back protruded (but not front compressed) vowels. This technique is used by ventriloquists to mask the visible rounding of back vowels like . It is not clear if it is employed by languages with rounded vowels that do not utilize visible rounding.
Read more about this topic: Roundedness
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