Glottalic Initiation
It is possible to initiate airflow in the upper vocal tract by means of the vocal cords or glottis. This is known as glottalic initiation.
For egressive glottalic initiation, one lowers the glottis (as if to sing a low note), closes it as for a glottal stop, and then raises it, building up pressure in the oral cavity and upper trachea. Glottalic egressives are called ejectives. Since the glottis must be fully closed to form glottalic egressives, it is impossible to pronounce voiced ejectives.
For ingressive glottalic initiation, the sequence of actions performed in glottalic pressure initiation is reversed: one raises the glottis (as if to sing a high note), closes it, and then lowers it to create suction in the upper trachea and oral cavity. Glottalic ingressives are called implosives, although they may involve zero airflow rather than actual inflow. Because it is not necessary to fully close the glottis, implosives may be voiced; indeed, voiceless implosives are exceedingly rare.
It is usual for implosives to be voiced. Instead of keeping the glottis tightly closed, it is tensed but left slightly open to allow a thin stream of air through. Unlike pulmonic voiced sounds, in which a stream of air passes through a usually-fixed glottis, in voiced implosives a mobile glottis passes over a nearly motionless air column to cause vibration of the vocal cords. Phonations that are more open than modal voice, such as breathy voice, are not conducive to glottalic sounds because in these the glottis is held relatively open, allowing air to readily flow through and preventing a significant pressure difference from building up behind the articulator.
Because the oral cavity is so much smaller than the lungs, vowels and approximants cannot be pronounced with glottalic initiation. So-called glottalized vowels and other sonorants use the more common pulmonic egressive airstream mechanism.
There is no clear divide between pulmonic and glottalic sounds. Some languages may have consonants which are intermediate. For example, glottalized consonants in London English, such as the t in rat, may be weakly ejective. Similarly, fully voiced stops in languages such as Thai, Zulu, and Maidu are weakly implosive. This ambiguity does not occur with the next airstream mechanism, lingual, which is clearly distinct from pulmonic sounds.
Read more about this topic: Airstream Mechanism
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