In phonetics, initiation is the action by which an air-flow is created through the vocal tract. Along with articulation, it is one of the two mandatory aspects of sound production: without initiation, there is no sound. The means of initiating a phone is called its airstream mechanism.
Initiation may be divided into pressure and suction. In the former, the organ performing the initiation—called the initiator—builds up pressure within the vocal tract, creating an outward airflow. In the latter, the initiator reduces pressure within the vocal tract, creating an inward airflow. Phones pronounced with pressure initiation are called egressive, and those pronounced with suction initiation are called ingressive.
Initiation by means of the lungs is called pulmonic initiation. The vast majority of sounds used in human languages, including all vowels, are pulmonic egressives. In many languages, all phones are pulmonic egressives. The only attested use of a pulmonic ingressive is a lateral fricative in Damin, a ritual language formerly used by speakers of Lardil in Australia; however, no International Phonetic Alphabet symbol exists for it.
Pulmonic egressive stop consonants are also called plosives.
It is possible to initiate an airflow in the upper respiratory tract by means of the vocal cords or glottis. This is known as glottalic initiation.
To perform glottalic pressure initiation, one lowers one's glottis (as if to sing a low note), closes it as if for a glottal stop, and then raises it, building up pressure in the upper trachea and oral cavity. Glottalic egressives are also called ejectives. Since the glottis must be fully closed to form glottalic egressives, it is impossible to pronounce voiced ejectives.
To perform glottalic suction initiation, the sequence of actions performed in glottalic pressure initiation is reversed: one raises one's 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 also called implosives.
It is possible to pronounce voiced implosives, although the initiation is slightly different. 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. Methods of phonation other than voice, such as breathy voice, are not considered applicable to glottalic sounds because 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, vocoids and approximants cannot be pronounced with glottalic initiation.
The third and final known form of initiation in human language is velaric initiation: initiation at the velum, or soft palate. Velaric stops are also called clicks. The IPA does not provide separate symbols for egressive and ingressive clicks, as no known languages differentiate between them phonemically.
To perform velaric suction initiation, one draws the base of one's tongue back so that air cannot pass it (as when pronouncing a velar stop), then slides it downwards to create a vacuum behind the articulator. Velaric ingressives are found in very few languages, notably the Khoisan languages of Africa and some nearby languages. They are more often found in extra-linguistic contexts: for example, the "tsk tsk" sound used by many Westerners to express regret is a palatal click, and the clucking noise used by many equestrians to urge their horses on is an alveolar lateral click.
Velaric pressure initiation is performed by reversing the sequence of action used to produce velaric suction initiation: the base of the tongue closes low in the throat and moves up to build up pressure. The only attested use of a velar egressive is a bilabial one in Damin, which lacks an IPA symbol.
It is not possible to produce velaric glottal, pharyngeal, epiglottal, or uvular stops, as those places of articulation are below the initiator. Since the airspace for velaric consonants is so small, the potential pressure difference is very limited, so it is not considered generally possible to produce velaric fricatives, approximants, or vocoids—velaric laterals are actually stops with a lateral release.
Since initiation occurs above the glottis in velaric consonants, they cannot be voiced. However, it is possible to produce nasalized clicks by lowering the velum so as to direct air up through the nasal cavity and tensing the glottis for voice. The nasalization is technically pulmonic. By redirecting the airstream in this way, it is actually possible to pronounce clicks while breathing in and out normally through the nose.