This is a question about phonetics. Turkish undotted ı is normally described as a high (or closed) back unrounded vowel. Russian ы is described either as a high (closed) back unrounded vowel or as a high (closed) central unrounded vowel. Vietnamese has a closed ư and a half-open ơ. Some authors (let’s call them “school 1”) describe them as back unrounded vowels, but others (“school 2”) call them central unrounded vowels. Vietnamese â is described by everyone (I think) as a mid-open central vowel. School 2 says that â is a shorter version of ơ, but school 1 says that â is central while ơ is back. I can hear the difference in length between â and ơ, but I am not able to perceive the alleged central/back distinction. The whole issue is further complicated by the fact that there are other conditions which lead to vowel shortening in Vietnamese, notably the fact that in the nặng tone all vowels are shortened, without (apparently) neutralising the difference between ơ and â. My question is first: Is there any audible difference between back unrounded and central unrounded vowels? And second: Is there any language (other than Vietnamese, as described by school 1) that has a phonological opposition between back and central unrounded vowels at the same level of openness? I am looking for serious phonetic resources. Please do not tell me to go to Wikipedia.