How phonetically distinct in terms of quality (tongue gesture) are /ɜː/ and /ə/ in Received Pronunciation?

Question

The English Wikipedia article on Received Pronunciation uses two particular vowel charts adapted from two sources, an article by Peter Roach titled British English: Received Pronunciation published in the Journal of the International Phonetic Association, and Alan Cruttenden's book titled Gimson's Pronunciation of English. While both sources are reasonably detailed in describing the qualities of /ɜː/ and /ə/ in isolation, as far as I can tell, neither elaborates on quality comparison between these two vowels. The Wikipedia charts are consequently also cursory and just map the two vowels at the same location.

My interpretation is that the quality of these vowels are virtually identical (they have the same tongue gesture and height), with /ɜː/ realized as either a raised [ɜ̝ː], a plain [əː], or a shortened [ə] in unstressed closed syllables. Meanwhile /ə/ could be anything between a plain [ə], a raised [ə̝], a lowered [ə̞], a lower [ɜ], even a really low [ɐ]. /ɜː/ might involve much more tenseness of the tongue than /ə/, but tenseness alone is not exactly audible or perceptible. Length is not that salient either, because in unstressed syllables, especially closed ones, /ɜː/ will naturally be shortened to probably about the same length as /ə/. As you can see, there's simply way too much overlap between these two vowels, that I feel any attempt of figuring out their quality differences is borderline futile.

And in all honesty, in vanishingly scant "minimal pairs," I'm not certain if there's any difference at all. Based on audio evidence from, say, John C. Wells's Longman Pronunciation Dictionary, I can't tell the difference between forward (/ə/) and foreword (/ɜː/). The more I listen to these audio samples, the more I'm convinced that any "phonemic contrast" between /ɜː/ and /ə/ exclusively in unstressed syllables is purely imaginary. I'd even venture that there's simply no point in teaching these two "phonemes" to my ESL students, because I myself, as reasonably competent in the field of phonetics as I'd like to believe, don't know if they truly are phonemes or not, or rather, allophones in free variation in unstressed syllables.

So what would be the most noticeable and understandable phonetic distinction between /ɜː/ and /ə/ in Received Pronunciation?

Answer 1

According to LPD (Longman Pronunciation Dictionary, J. C. Wells), the quality of those vowels are not the same but it can be; the more instructive constatation is that /ə/ covers /ɜː/ in both BrE and GenAm. This can be seen in the diagrams provided in LPD.

        

BrE /ɜː/

        

GenAm /ɜː/ — This vowel is pronounced with the mouth less open and slightly more towards the front, but in fact there is not a great difference.

        

BrE and GenAm /ə/ — It can be seen that there is a much greater possibility of variation in quality for this vowel than there is for the /ɜː/ vowel in both varieties of the language, and that the extent of this variation covers entirely that for BrE /ɜː/ and almost all for GenAm.

You might make a clear distinction in this case: "pilfer/pill fur", where you pronounce the second one as a corrective for "pill" and so put stress on it instead of on "fur". You should feel the necessity of having a long enough "ur" vowel. Syllabification is also a clue: "/'pilf. ə/, /'pil. fɜː/ (strong vowel draws in consonnant).The case you chose appears difficult (forward/foreword); however, think that the strong final vowel in "forword" is tantamount to a third degree of stress, a quasi-ternary stress and it should not exist for "ward".