English Phonology

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The vocal tract  

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DEFINITIONS:

• Consonant: An alphabetic or phonetic element other than a vowel; an elementary sound of speech which in the formation of a syllable is combined with a vowel. Applied both to the sounds and to the letters (the latter being the historically prior use).
    While a vowel sound is formed in the larynx, and only receives its special quality by the conformation of the oral cavity through which it is sounded, a consonant sound is wholly or mainly produced in the mouth, or the mouth and nose. Vowels thus consist of pure voice or musical sound; consonants are either simple noises or noises combined in various degrees with voice. But a noise may itself be of a continuous and rhythmical character, as a friction, trill, hiss, or buzz, and those consonants in which this is markedly the case approach closely to vowels, and may perform the function of a vowel in a syllable. Hence ‘the boundary between vowel and consonant, like that between the different kingdoms of nature, cannot be drawn with absolute definiteness, and there are sounds which may belong to either’ (SWEET Handbk. Phonetics §164). And there is in the consonants a regular gradation from those which come nearest to vowels and may function as vowels, to those which are most remote, and never so function. From this point of view, elementary sounds have been classed as (1) vowels, (2) semi-vowels (Eng. j and w), (3) liquids (l, , r), (4) nasals (m, n, , ), (5) fricatives or spirants, voice (v, , z, , ), and breath (f, , s, , x), (6) mutes or stops, voice (b, d, g), and breath (p, t, k). Class 2 are more strictly the vowels i, u, functioning as consonants, and classed as consonants; classes 3, 4, 5, are capable, in a decreasing measure, of functioning as vowels; only class 6 have the consonantal function exclusively, p, t, k, being the most typical consonants. The use of the liquids and nasals as vowels or sonants is a prominent feature in Indogermanic Phonology. (See VOWEL.) Consonants may also be classed, according to the part of the mouth where they are formed, into labials (p, b, f, v, m, w), dentals, palatals, gutturals, and other minor groups. (See these terms.) In the Roman alphabet (with its Greek accessions), the historical vowels are a, e, i, o, u, y; down to the 16-17th c., i and u were used both as vowels and consonants, a double function served by y and w in various modern languages.

• Vowel: A sound produced by the vibrations of the vocal cords; a letter or character representing such a sound (as a, e, i, etc.).
    ‘A vowel may be defined as voice (voiced breath) modified by some definite configuration of the super-glottal passages, but without audible friction (which would make it into a consonant)’ (Sweet Primer of Phonetics, ed. 2, §32).

• Diphthong: A union of two vowels pronounced in one syllable; the combination of a sonantal with a consonantal vowel.
    The latter is usually one of the two vowels i and u, the extremes of the vowel scale, which pass into the consonants y, w. When these sounds, called by Melville Bell glides, follow the sonantal vowel, the combination is called a ‘falling diphthong’, as in out, how, boil, boy; when they precede, the combination is a ‘rising diphthong’, as in It. uovo, piano. It is common in the latter case to consider the first element as the consonant w or y.

From the OED