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.

 

I can't give you an authoritative answer, but in the case of high vowels I'd suggest you consider:
1. Russian ы (which you've mentioned)
2. The unstressed vowel of English (common varieties of Amer Eng, at least) words like "roses".
3. Japanese 'u'
All seem unrounded and non-front to me, and I hear them as three different vowels. And based on my understanding of a what a back high unrounded vowel sounds like, I wouldn't put #1 or #2 in that category. But all this is the opinion of a non-expert.

So I think the answer to your question is yes. Of course, for the three vowels I mention, it's likely that there are secondary characteristics (beyond backness) that contribute to the perceptual differences among them.

 

I would not describe Russian "ы" as a back vowel. It is definitely central, and even a good bit closer towards the front than to the back. It is very close to English "i" in "bit", or German in "i" in "bitte".
See the chart below. the sign ɨ.

 

The three you've mentioned and Japanese are the only reference I have for such vowels - that is, where I've heard the vowel repeatedly while being aware of its phonetics - so this is a limited sample. I can't think of a language that is described as having a high back versus central contrast. To me, Turkish ı sounds more central, and I believe that's how it's generally described. The backness of the Japanese and Vietnamese vowels might actually be spread lips, as opposed to slack or neutral for more central sounds. Phonetically that would (I think) be a higher formant than F1 and F2. Perhaps the backness (F1 and F2) is not actually so relevant as the lip effects - but this is a sheer guess on my part. Now that I try, I find it difficult to retract my tongue root from [ɨ] to [ɯ] without varying the lip spreading too.

 

That was fdb's second question, one I didn't attempt to answer. I was responding to (and quoted):

I attempted to answer this question in the affirmative (at least for high vowels) by pointing to the Russian ы vowel and the English "roses" vowel and suggesting that these presumed central high unrounded vowels are perceptually distinct from the back high unrounded vowel (in my non-expert opinion). (ы and "roses" in addition seem different from each other, suggesting additional perceptual sensitivity in that region of vowel space.)

 

fdb, arguably Thai has the same opposition that Vietnamese does, although not in relation to vowel length.

In a related area you mention, my humble opinion is - after attentively listening a million times! - that school 2 is wrong about Vietnamese ơ and â being long and short versions of the same sound. In fact, ơ approaches French œ more than anything else that I can think of. I've never run across a source describing it that way, though.

As for Russian ы, it is IMHO definitely central. The reason that it doesn't seem to be in certain environments is because it radically velarizes the consonant that comes before it. Ask a Russian to pronounce it in isolation and the centrality is more obvious. Of course, since the sound does not occur initially or in isolation in Russian, you hardly ever hear it like that.

 

French /œ/ and /ø/ are produced with prominent lip rounding. Do you hear/see any lip rounding with ơ ?

 

True, ơ isn't identical with œ - there is no lip rounding. But the tongue position seems to be the same, and pronouncing it like [ə:] is definitely wrong, as far as I can tell. Pronouncing the soup phớ like [fə:] is very likely to be misunderstood, and lớp and lấp definitely do not sound the same.

But as I once heard said (and I have learned to be true, the hard way), the phonetic space between the different vowels in Vietnamese is incredibly small - actually smaller than in any other language I've ever studied - so don't be surprised if the distinctions sometimes seem to be difficult to ascertain.