Types of airstream mechanism
Any of the three initiators − diaphragm, glottis or tongue − may act by either increasing or decreasing the pressure generating the airstream. These changes in pressure often correspond to outward and inward airflow, and are therefore termed egressive and ingressive respectively.
Of these six resulting airstream mechanisms, four are found lexically around the world:
- pulmonic egressive, where the air is pushed out of the lungs by the ribs and diaphragm. All human languages employ such sounds (such as vowels), and nearly three out of four use them exclusively.
- glottalic egressive, where the air column is compressed as the glottis moves upward. Such consonants are called ejectives. Ejective and ejective-like consonants occur in 16% of the languages.
- glottalic ingressive, where the air column is rarefied as the glottis moves downward. Such consonants are called implosives. Implosive and implosive-like consonants occur in 13% of the world's languages. Despite the name, the airstream may not actually flow inward: While the glottis moves downward, pulmonic air passes outward through it, but the reduction in pressure makes an audible difference to the sound.
- lingual ingressive, AKA velaric ingressive, where the air in the mouth is rarefied by a downward movement of the tongue. These are the click consonants. Clicks are regular sounds in ordinary words in fewer than 2% of the world's languages, all in Africa.
These mechanisms may be combined into airstream contours, such as clicks which release into ejectives.
The Khoisan languages have pulmonic, ejective, and click consonants, the Chadic languages have pulmonic, implosive, and ejective consonants, and the Nguni languages utilize all four, pulmonic, click, implosive, and ejective, in normal vocabulary. Most other languages utilize only one or two airstream mechanisms.
In interjections, the other two mechanisms may be employed. For example, in countries as diverse as Sweden, Turkey, and Togo, a pulmonic ingressive ("gasped" or "inhaled") vowel is used for back-channeling or to express agreement, and in France a lingual egressive (a "spurt") is used to express dismissal. The only language where such sounds are known to be contrastive in normal vocabulary is the extinct ritual language Damin (also the only language outside Africa with clicks); however, Damin appears to have been intentionally designed to differ from normal speech.