In all languages speech sounds are divided into two main types - vowels and consonants.
A vowel is a voiced sound produced in the mouth with no obstruction to the air stream. The air stream is weak. The tongue and the vocal cords are tense.
A consonant is a sound produced with an obstruction to the air stream.
Consonants are the bones of a word and give it its basic shape. English accents differ mainly in vowels; the consonants are more or less the same wherever English is spoken. So if your vowels are not perfect you may still be understood by the listener, but if the consonants are imperfect there may be some misunderstanding.
On the articulatory level the consonants change:
1. In the degree of noise (noise consonants – sonorants);
2. In the manner of articulation (complete, incomplete, momentary)
3. In the place of articulation
According to the position of the soft palate all consonants are subdivided into oral and nasal. When the soft palate is raised oral consonants are produced; when the soft palate is lowered nasal consonants are produced.
Vowels are sounds of pure musical tone while consonants may be either sounds in which noise prevails over tone (noise consonants) or sounds in which tone prevails over noise (sonorants).
The English vowel phonemes are divided first of all into two large groups: monophthongs anddiphthongs. This division is based on the stability of articulation.
A monophthong is a pure (unchanging) vowel sound.
A diphthong is a complex sound consisting of two vowel elements pronounced so as to form a single syllable. In the pronunciation of a diphthong the organs of speech start in the position of one vowel and glide gradually in the direction of another vowel, whose full formation is generally not accomplished.
The first element of an English diphthong is called the nucleus. It is strong, clear and distinct. The second element is rather weak. It is called the glide.
There are eight diphthongs in English.
Besides these diphthongs, there are two vowels in English ([i:] and [u:]) which may have a diphthongal pronunciation: in the articulation of these vowels the organs of speech change their position but very slightly. These vowels are called diphthongised vowels, or diphthongoids.
The English monophthongs may be classified according to the following principles:
I.According to the tongue position.
II.According to the lip position.
III.According to the length of the vowel.
IV.According to the degree of tenseness