In a phonetic account we seek to be able to identify the sounds which make up a spoken language and to describe the variation in those sounds which may occur both within the pronunciation of different speakers and within the pronunciation of an individual speaker. The primary mechanism for identifying, classifying and labelling sounds is by reference to their manner of articulation - that is to say, by reference to the parts of the mouth and vocal tract that are involved in making the sound and the way those parts are used in the making of a sound. Thus we might be concerned with the position of the tongue when the sound is made, whether the lips are spread or rounded and whether the vocal folds (otherwise known as the vocal chords) are vibrating or not.
The first major distinction in types of sounds is between vowels and consonants. Most of you will probably be familiar with the difference and will be able to list examples of both - a, e, i, o and u are vowels and b, d, f, v, and m are examples of consonants. But what the basis of these two groupings? What properties do vowels all have which separate them from consonants, and what phonetic feature is shared by all consonants but not by vowels?. The issues is relatively straightforward for most vowels and consonants and not quite so simple for a small set of what are usually considered consonants. I'll come back to this last grouping later on.
The distinction turns on the way the air passes through the vocal tract when the respective sounds are being made. With vowels the air passes through relatively unimpeded. We can say that there is no obstruction of the air as it passes from the vocal folds to the lips. In contrast, with consonants there is some narrowing, constriction or closing of some part of the vocal tract so as to produce an audible friction.
In English, there are several problematic sounds which don't fit easily into either categories. In some ways the resemble vowels and in others way they resemble consonants. These include r at the beginning or red, l at the beginning of label, and the w at the beginning of way. You probably think of these sounds as consonants. This is most probably because of the positions they fill in words. They are like consonants in that they come at the beginning and ends of syllables rather than in the middle. Yet they are like vowels terms of their manner of articulation - there is no actual audible friction produced in the pronunciation of sounds such as w, l and r. Accordingly these sounds are seen as representing a kind of in-between class - they are therefore often termed semi-vowels.
