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?

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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.

  • An interesting effect of this is that even though if you just judge in terms of phonological "features" like front/back and rounded/unrounded, a vowel like /ø/ is "closer" to /e/ or /o/ than it is to /ɤ/, in borrowings sometimes foreign /ø/ is adapted as /ɤ/ or vice versa: e.g. in Vietnamese loanwords from French, /ɤ/ is apparently used as the equivalent of French /ø/ and /œ/, even though Vietnamese has both /e/ and /o/ and makes use of /i/, /u/ or /wi/ for French /y/. Commented Mar 2, 2018 at 1:44
  • (Although I guess part of the reason for the use of Vietnamese /ɤ/ for French /ø/ and /œ/ might be phonological rather than phonetic: in French /ø/ and /œ/ are identified with the French schwa sound.) Another example could be the use of the NURSE vowel in English in place of German front rounded /øː/ (e.g. in Goethe). Commented Mar 2, 2018 at 1:45
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    I've been told by lots of American English speakers that "it sounds like there's an R in Goethe", which is because the American postvocalic /r/ is rounded like German /ø/, and the vowel in both stressed syllables is centralized.
    – jlawler
    Commented Mar 3, 2018 at 19:18

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