N. V. Tatsenko introduction to theoretical phonetics of english

monophthongs,

diphthongs.

BBC pronunciation has a large number of diphthongs - sounds which consist of a movement or glide from one vowel to another. In terms of length, diphthongs are similar to the long vowels described above. Perhaps the most important thing to remember about all the diphthongs is that the first part is much longer and stronger than the second part; for example, most of the diphthong [ai] (as in the words 'eye', ’I) consists of the [a] vowel, and only in about the last quarter of the diphthong does the glide to [i] become noticeable. As the glide to [i] happens, the loudness of the sound decreases. As a result, the [i] part is shorter and quieter. Foreign learners should, therefore, always remember that the last part of English diphthongs must not be made too strongly.
A diphthong is defined as a unisyllabic complex in the articulation of which the organs of speech start from one position then glide into another position. A diphthong may be falling - when the nucleus is stronger than the glide, and rising - when the glide is stronger than the nucleus. When two elements are equal such diphthong is called level. English diphthongs are falling.
There are two categories of diphthongs in English which differ according to the direction of the vowel movement - centering diphthongs and closing diphthongs. The vowels [i:] and [u:] are diphthongized in modern English and the tendency of diphthongization is becoming gradually stronger - some phoneticians call them diphthongoids.
T
he total number of diphthongs is eight (though [оэ] is increasingly rare). The easiest way to remember them is in terms of three groups divided as in this diagram:
The centring diphthongs glide towards the [э] (schwa) vowel, as the symbols indicate.