The way air is moved about in the vocal tract to produce sound is referred to as the airstream, or airstream mechanism. Airstreams are described in terms of the generator of movement and the direction of movement.
Generators of movement:
- Pulmonic: lungs
- Glottalic: glottis/larynx
- Velaric: velar closure with the back of the tongue
Directions of movement:
- Ingressive: inwards (away from lips)
- Egressive: outwards (towards lips)
There are four main airstreams used in human speech.
- Pulmonic egressive. This is by far the most common airstream used. About 82% of languages (including English and most other European languages) use this airstream exclusively. This airstream generates sound by moving air outward from the lungs.
- Glottalic egressive. This airstream produces consonants known as ejectives, transcribed in the IPA with an apostrophe after a voiceless obstruent symbol, such as [p']. These sounds are produced with a closed larynx moving up, pushing air ahead of it.
- Glottalic ingressive. This airstream produces consonants known as implosives, transcribed in the IPA using characters with a right hook off an ascending portion of the glyph for a voiced pulmonic plosive, such as [ɓ].
- Velaric ingressive. This airstream produces consonants known as clicks, of which there are five recognized in the IPA and transcribed with five different symbols: [ʘǀǃǂǁ].
The pulmonic ingressive airstream is physiologically achievable, and is used paralinguistically in some cultures [ref needed]. However, it is never used contrastively, and so the IPA does not contain symbols or diacritics to represent such productions.