Basic criteria for classifying English vowels:
– pronunciation stability
– tongue bulk position in the mouth cavity (in its front, central, or back part)
– tongue bulk rise (high, mid, or low position)
– speech apparatus tension (muscular tongue tensioning)
By the 1st criterion, English vowels divide into monophthongs, diphthongs and diphthongoids. Monophthongs are vowels pronounced by the fixed speech organs with stable vowel quality.
Diphthongs feature sliding articulation with radical sound quality change. The speech organs take a certain position to articulate the 1st stable vowel sounding distinct and drawling. Then they reshape for another position without reaching it. This slide is created toward one of 3 timbres: [i], [u] and [ә] – they sound short and indistinct. The 1st element is a diphthong nucleus, the second slide.
They’re midway between monophthongs and diphthongs. Like diphthongs, diphthongoids feature sliding articulation though with much weaker slide. They don’t radically change vowel sound quality, so traditionally belonging to the monophthong group.
English has 12 monophthongs: [i:], [i], [e], [æ], [u], [u:], [o:], [o], [a:], [a], [e:], [ә], of them 2: [i:] and [u:] are diphthongoids and 8 diphthongs: [ei], [ai], [oi], [au], [әu], [iә], [eә], [uә].
If the tongue bulk is advanced to the front teeth (with the tongue’s front part touching the lower teeth) and the middle part is bent up, vowels formed thereby are front (monophthongs [i:], [e], [æ] and the [eә], [ei] diphthongs’ 1st elements).
The [u:], [o:], [o] monophthongs and the [oi] diphthong’s 1st element form with the tongue bulk pulled back and its back part bent up.
Back Advanced Vowels
In articulating [u], [a:] and starting [uә] the tongue bulk is slightly advanced compared to its position in the back mouth cavity.
Central (Mixed) Vowels
Central vowels ([e:], [ә], [a] and [әu]’s 1st element) are pronounced with the tongue bulk in the mouth cavity center. Here both the mid and back parts are equally risen.
[i:], [i], [u], [u:] and [iә], [uә]’s 1st elements are pronounced with the tongue bulk high in the mouth cavity.
[e], [e:], [ә], [o:] and [ei], [әu], [eә]’s 1st elements are pronounced with the tongue bulk amid the mouth cavity, its mid and back parts equally risen.
[a], [æ], [a:], [o] and [oi], [ai], [au]’s 1st elements are pronounced with the tongue bulk low in the mouth cavity.
Monophthongs can be pronounced with some muscular tension like in [a:], [o:] or without it like in [i], [a]. The [i:], [u:], [o:], [a:], [e:] vowels are tense with the rest as lax. Diphthongs are considered semi-tense as tension typically weakens toward their articulation. Diphthongoids feature some tensioning up to vowel ending as a diphthongoid’s sliding sector is in the vowel sound beginning, not at its end like in diphthongs.
Vowels are historically divided into long and short. However their distinction is based on quality characteristics as the factual sound length of historically long vowels may change. Positionally conditioned length variants depend on syllable type, syllable-ending consonant type, word accent structure, and word intonation.