While many languages possess unrounded front vowels (e. g. /e/ and /i/) and rounded back vowels (e. g. /o/ and /u/), rounded front vowels (e. g. /y/ and /ø/) as well as unrounded back vowels (e. g. /ɤ/ and /ɯ/) seem to be far less common. Is there a reason for the statistic correlation between vowel backness and rounding? Does for example rounding facilitate the pronunciation of back vowels?
The essential acoustic difference between front vowels and back vowels is their difference in F2: back vowels have a lower F2 than do front vowels. This is because the front cavity (which correlates with F2) of back vowels is longer than that of front vowels, and the longer the tube, the lower the resonance frequency. Lip rounding has a similar effect: rounding a vowel makes the front cavity longer, lowering F2. When backness correlates with roundness (front unrounded, or back rounded), vowel identification is easiest.