Vowels are normally made with the air stream that meets no closure or narrowing in the mouth, pharyngal and nasal cavities. That is why in the production of vowel sounds there is no noise component — characteristic of consonantal sounds.
On the articulatory level the description of vowels notes changes:
1. In the stability of articulation.
2. In the tongue position.
3. In the lip position.
4. In the character of the vowel end.
5. In the vowel length and the degree of tenseness.
1. Stability of articulation.
All English vowels are divided into three groups: pure vowels or monophthongs, diphthongs and diphthongoids.
Monophthongs are vowels the articulation of which is almost unchanging (the organs of speech do not change their position throughout the duration of the vowel). The English monophthongs are: [I, e, x, a:,O, L, u, A,W, q].
In the pronunciation of diphthongs the organs of speech glide from one vowel position to another within one syllable. The starting point, the nucleus, is strong and distinct. The glide which shows the direction of the quality change is very weak. The English diphthongs are: [eI, aI, OI, au,qu, Iq, Fq, uq].
In the pronunciation of diphthongoids the articulation is slightly changing but the difference between the starting point and the end is not so distinct as it is in the case of diphthongs. There are two diphthongoids in English: [i:, u:].
2. Tongue positions.
The tongue may move forward and backward, up and down, thus changing the quality of vowel sounds.
a) When the tongue moves forward and backward various parts of it may be raised in the direction of the palate.
When the tongue is in the front part of the mouth and the front part of it is raised to the hard palate a front vowel is pronounced. This is the position for the English vowels [i:, e, x].
When the tongue is in the front part of the mouth but slightly retracted and the part of the tongue nearer to centre than to front is raised, a front-retracted vowel is pronounced. Such is the position for the English vowel [I].
When the central part of the tongue is raised towards the juncture of the soft and hard palate the vowel is called mixed. This is the position for the English vowels [A, E:, q].
When the tongue is in the back part of the mouth and the back part of it is raised towards the soft palate a back vowel is pronounced. This is the position for the English vowels [O, O:, u:].
When the tongue is in the back part of the mouth but a slightly advanced and the central part of it is raised towards the front part of the soft palate a back-advanced vowel is pronounced. This is the position for the English vowels [V, a:].
b) Moving up and down in the mouth various parts of the tongue may be raised to different height towards the roof of the mouth.
When the front or the back of the tongue is raised high towards the palate the vowel is called close. This is the way the English vowels [i:, I, u, u:] are pronounced.
When the front or the back of the tongue is as low as possible in the mouth open vowels are pronounced. These are [x, a:, O, A].
When the highest part of the tongue occupies the position intermediate between the close and the open one mid vowels are pronounced. These are [e, E:, L, q ].
To make the classification more precise it is necessary to distinguish broad and narrow variants of close, mid and open vowels. For instance, both English vowels [i:] and [I] belong to the group of close vowels, but when the vowel [I] is articulated the front of the tongue is not so high in the mouth as it is in the case of the vowel [i:].
3. Lip position.
The shape of the mouth cavity is also largerly dependant on the position of the lips. When the lips are neutral or spread the vowels are termed unrounded. Such is the position of the lips for the English vowels [i:, I, e, x, a:, A, E:,q].
When the lips are drawn together so that the opening between them is more or less round the vowel is called rounded. These are [O, O:, u, u:].
4. Character of vowel end.
The quality of all English monophthongs in the stressed position is strongly affected by the following consonant of the same syllable. If a stressed vowel is followed by a strong voiceless consonant it is cut off by it. In this case the end of the vowel is strong and the vowel is called checked, e.g. better, cart, city.
If a vowel is followed by a weak voiced consonant or by no consonant at all the end of it is weak. In this case the vowel is called free, e.g. before, money, begger, seed.
5. Vowel length and degree of tenseness.
All English vowels are historically divided into long and short. Long vowels are always tense, short vowels are always lax.
Long, tense vowels are: [i:, a:, O:, u:, E:].
Short, lax vowels are: [I, e, O, u, A, q].
The vowel [x] is not included in the category of short vowels because of specific length associated with it.
But for the purpose of practical speech training it is not enough to distinguish two degrees of length. In the similarly accented position all English vowels are fully long when they are final, e.g. see, bar, sore, fur. They are almost as long as that when a weak voiced consonant follows them in the closed syllable, e.g. seed, arm, form, bird, big, bed, song. They are considerably shorter before strong voiceless consonants in closed syllables, e.g. seat, lark, look, first, bit, set.
Diphthongs vary in length in the same way as long vowels, cf. play – played – plate, toy – toys – voice, fear – fears – fierce. Variations of length affect mainly the nucleus, not the glide. Such variations might be represented in the following way: play [ple:I] – plays [ple˙Iz] – plate [pleIt].
All English vowels are longer when they are strongly stressed, cf. in'form – 'uninform.
All English vowels are longer in the nuclear syllable, cf. It is six o'clock now. – They are only six.
It should be noted that in similar phonetic contexts traditionally long vowels are always longer than traditionally short vowels, cf. see – sin, calm – come, cord – cod.
The classification of English vowels looks like this (see table 1):