Phonemic differentiation is the phenomenon of a
language maximizing the acoustic distance between its
phonemes, presumably to minimize the possibility of misunderstanding.
For example, in many languages, including English, most front vowels are unrounded, while most back vowels are rounded. There are no languages in which all front vowels are rounded and all back vowels are unrounded. The most likely explanation for this is that front vowels have a higher second formant (F2) than back vowels, and unrounded vowels have a higher F2 than rounded vowels. Thus unrounded front vowels and rounded back vowels have maximally different F2s, enhancing their phonemic differentiation.
Phonemic differentiation can have an effect on diachronic sound change. In chain shifts, phonemic differentiation is maintained, while in phonemic mergers it is lost. Phonemic splits involve the creation of two phonemes out of one, which then tend to diverge because of phonemic differentiation.
In a chain shift, one phoneme moves in acoustic space, causing other phonemes to move as well to maintain optimal phonemic differentiation. An example from American English is the Northern cities vowel shift [1], where the raising of /æ/ has triggered a fronting of /?/, which in turn has triggered a lowering of /?/, and so forth.
![[1]](http://www.ling.upenn.edu/phono_atlas/ICSLP4/Figure_1.GIF){kind=link}
