The airstream (the source of sound production) is initiated by the lungs and respiratory airways. A constriction in the airways (glottal or supraglottal) modifies the flow.
During inspiration, the lungs expand, causing the air to flow from the mouth to the lungs with the glottis relatively open (the adult tracheal area ranges from 3.0 to 4.9 cm2) (Shadle, 1996: 42). During expiration, the lungs contract, pushing the air from the lungs toward the mouth. For most languages phonation (the production of sounds) occurs during expiration. In this case the flow of air is relatively small because of constrictions in the vocal tract and a nearly closed glottis. During normal breathing expiration the glottal area (the opening between the vocal folds) is in he order of 1 cm2 while during phonation the average glottal area is 0.05 to 0.1 cm2 (ibid.). Follow this link if you want to observe the larynx during quite breathing, swallowing and articulation.
The air pressure in the trachea, which is virtually equal to the sub-glottal air pressure (denotes the pressure just below the vocal folds), is almost the same as in the lungs but the pressure above the glottis is nearly zero (i.e. like in the surrounding air). The "glottal valve" can control the airflow in such a way that it is more constant and can be used more economically (longer). For short utterances at normal loudness, normal (mid-range of total lung capacity) expiration is sufficient and ca. 25% of the total lung capacity is used (1.2 to 1.5 l of air is involved) (Warren, in: Lass, 1996:53). For louder or longer speech one needs to respire more deeply (lung volumes of 4 to 5 l are used). Shadle (ibid.) states that during speech it is our goal is to hold the subglottal pressure ranging from 3 to 30 cm H2O depending on the desired level of loudness. Subglottal pressure is one of the most important factors in speech production and primarily affects pitch and loudness.
Although the egressive pulmonic airstream mechanism is the most efficient, in some languages other mechanisms are used. The pulmonic ingressive airstream is not used on the linguistic layer of communication but is sometimes used paralinguistically (Laver, 1994:169-170). There is also the possibility of glottalic airstream mechanisms where the whole larynx is pulled up or down to produce ejectives or implosives, respectively.
Ejective stops, fricatives and affricates are found in African and American languages, but also in Asian languages such as Gujarati or Caucasian languages. About 18% of all languages contain ejectives (Ladefoged & Maddieson 1996:78). Implosives (only oral stops) are found in about 10% of all languages (ibid.:82), especially in West African languages. They are often articulated with vibrating vocal folds (voiced implosives).
A final airstream mechanism (velaric) results from the movements of the tongue. is produced by the release of a closure at the front part of the tongue while the back of the tongue forms a second closure. The blade, tip or side of the tongue move down, releasing the front closure so that air rushes into the mouth, equalizing air pressure (ibid.: 247). The sounds produced by velaric airstream are called clicks. Clicks are part of the consonant systems of almost all Khoisan and numerous southern Bantu languages (Dogil et al., 1996).
Listen to clicks
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