N. V. Tatsenko introduction to theoretical phonetics of english

Tenseness. Special instrumental analysis shows that long as opposed to short vowels also differ by being tense as opposed to lax:

1. 
   tense vowels are produced with a deliberate, accurate, maximally distinct gesture that involves considerable muscular effort. Tense vowels are either long vowels (e. g. [i:] in 'meet') or diphthongs (e.g. [ei] in 'say').
   
2. 
   non-tense (or lax) vowels are produced rapidly and are therefore short (e.g. [i] in 'pill').
   

3. 
   Another feature of English vowels which is sometimes included into the principles of classification is lip rounding. Traditionally three lip positions are distinguished, that is spread, neutral and rounded. For the purpose of classification, it is sufficient to distinguish between two lip positions: rounded or labialised and unrounded, or non-labialised (neutral). The fact is that any back vowel in English is produced with rounded lips, the degree of rounding is different and depends on the height of the raised part of the tongue; the higher it is raised the more rounded the lips are. So lip rounding is a phoneme constitutive indispensable feature, because no back vowel can exist without it.
   

It is well-known that a vowel in an unstressed syllable is perceived as very short, weak, and indistinct. The unstressed syllables are usually associated with vowels of central or
centralized quality [э], [i], sometimes [u] and the diphthongs [эо], [ai] (or a syllabic consonant), e. g. among [^mvq], before [bi'fo:], useful ['ju:sful], tomato [tэ'ma:tэu], exercise ['eksэsaiz], sudden ['sAdn].
Also vowels of full quality sometimes occur in unstressed positions, often in borrowed words of Latin and Greek origin, e. g. architect ['a:kutekt], paragraph ['psregra:fj, canteen [k®n'ti:n]. These nonreduced vowels in unstressed syllables are typical of all styles of pronunciation.
Then again partially reduced sounds are found in unstressed positions. They appear in more formal and careful style of pronunciation instead of the neutral sound used in informal casual speech. Cf.: phonetics [feu'netiks - fэ'netiks - fnetiks].
Our next point should be made in connection with the phonemic status of the schwa sound [э]. The phonological analysis marks the opposition of the schwa to other unstressed vowels, the most common among them being [i]. In the minimal pairs: officers ['nf^z] - offices ['ofisiz]; accept ^k'sept] - except [ik'sept], armour ['атаэ] - army ['a:mi] the neutral sound is phonologically opposed to the phoneme [i] with its own distinctive features capable of differentiating the meaning of lexical units. So the neutral sound [э] in officers, accept, armour is an independent phoneme opposed to the [i] phoneme of the minimal pairs given above.
On the other hand, the problem of the phonemic status of the schwa sound has a morphological aspect. In English as well as in Ukrainian there are numerous alternations of vowels in stressed and unstressed syllables between the derivatives of the same root or different grammatical forms of the same word. For example: