N. V. Tatsenko introduction to theoretical phonetics of english

[з:] as in heard and third. This is a mid-central vowel which is used in most English accents as a hesitation sound (written 'er'), but which many learners find difficult to copy. The lip position is neutral.
^[э:] as in taught and port The tongue height for this vowel is between cardinal vowel No. 6 [o] and No. 7 [o], and closer to the latter. This vowel is almost fully back and has quite strong lip-rounding,
[a:] as in bar and father. This is an open vowel in the region of cardinal vowel No. 5 [a], but not as back as this. The lip position is neutral.
When sounds are used in connected speech they cannot help being influenced by one another. Duration is one of the characteristics of a vowel which is modified by and depends on the following factors:

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  its own length;
  
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  the accent of the syllable in which it occurs;
  
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  phonetic context;
  
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  the position of the sound in a syllable;
  
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  the position in a rhythmic structure;
  
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  the position in a tone group;
  
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  the position in a phrase;
  
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  the position in an utterance;
  
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  the tempo of the whole utterance;
  
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  the type of pronunciation;
  
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  the style of pronunciation.
  

The problem the analysts are concerned with is whether variations in quantity or length are meaningful (relevant), that is whether vowel length can be treated as a relevant feature of English vowel system. Different scholars attach varying significance to vowel quantity. The approach of D. Jones, an outstanding British phonetician, extends the principle, underlying phonological relevance of vowel quantity. That means that words in such pairs as [bid] - [bi:d], [sit] - [si:t], [fb:w9:d] (foreword) - ['fowad] (forward) are distinguished from one another by the opposition of different length, which D. Jones calls chronemes.
2. Stability of articulation specifies the actual position of the articulating organ in the process of the articulation of a vowel. There are two possible varieties: a) the tongue position is stable; b) it changes, that is the tongue moves from one position to another. In the first case the articulated vowel is relatively pure, in the second case a vowel consists of two clearly perceptible elements. So according to this principle the English vowels are subdivided into:

1.