The vocal tract

Speech sounds are produced by the organs of the vocal tract, which modify the airstream from the lungs. In the larynx or ‘voice box’, situated at the top of the windpipe, are the vocal cords, the gap between which is called the glottis. When these cords are kept close enough together and made to vibrate as air passes through the glottis, the sound produced is voiced; where there is no vibration the sound is voiceless. All vowels in English are voiced; consonants may be voiceless or voiced.

Speech sounds may also be oral or nasal, depending on whether or not the velum (or soft palate) is lowered to allow air to pass through the nose as well as the mouth. English has only oral vowels, but nasal and oral consonants; French, Portuguese and Polish have nasal vowels as well as nasal consonants. Consonants are produced by full or partial obstruction of the airstream, while vowels are produced by positioning the tongue in different configurations which do not impede the flow of air. The distinction, however, is not a clear-cut one and some sounds classified as consonants have vowel-like qualities.

You might find it helpful to think in terms of a continuum or sonority hierarchy with highly vocalic or ‘vowel-like’ sounds like [a] (as in British English rat) at the top and strongly consonantal sounds like [p] in pip at the bottom. Stressed syllables in English must have a vowel as their head or nucleus, while Slovak, for example, allows sounds further down the sonority hierarchy, such as [r] in the place name Brno, to occupy this position.