Initiation (phonetics)
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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.
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Pulmonic initiation
Initiation by means of the lungs (actually the diaphragm and ribs) is called pulmonic initiation. The vast majority of sounds used in human languages are pulmonic egressives. In most languages, including all the languages of Europe, all phonemes are pulmonic egressives.
The only attested use of a phonemic pulmonic ingressive is a lateral fricative in Damin, a ritual language formerly used by speakers of Lardil in Australia. This can be written with the extended version of the International Phonetic Alphabet as Template:IPA. !Xóõ has ingression as a phonetic detail in one series of its clicks, which are ingressive voiceless nasals with delayed aspiration, Template:IPA. Peter Ladefoged considers these to be among the most difficult sounds in the world. Other languages, for example in Taiwan, have been claimed to have pulmonic ingressives, but these claims have either proven to be spurious or to be occasional phonetic detail.
In interjections, but not in normal words, pulmonic ingressive vowels such as Template:IPA occur in languages as diverse as Swedish and Ewe. In Ewe, Template:IPA is used for backchanneling, to indicate that one is listening (like ah or I see in English).
Glottalic initiation
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 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. More open phonations than modal voice, such as breathy voice, are not condusive 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.
Velaric initiation
The third form of initiation in human language is velaric initiation: initiation at the velum, or soft palate. (In a few languages the closure may also be uvular, but the term 'velaric' covers both.) Velaric stops are also called clicks. This velaric airstream is almost universally ingressive.
To perform velaric ingressive initiation, one raises the back of one's tongue back so that air cannot pass it (as when pronouncing a velar or uvular stop), and simultaneously does the same with the front of the tongue or the lips (as when pronouncing a coronal or bilabial stop), then moves the middle of the tongue downward to create a partial vacuum in between. The closure at the front of the tongue is opened first, as the click "release"; then the closure at the back is released for the click "accompaniment" or "efflux", and this may be aspirated, affricated, or even ejective. Even when not ejective, it is also not uncommon for the glottis to be closed as well, and this third closure is released last to produce a glottalized click. Velaric ingressives are found in very few languages, notably the Khoisan languages of southern Africa and some nearby tongues. They are more often found in extra-linguistic contexts: for example, the "tsk tsk" sound used by many Westerners to express regret or pity is a dental click, and the clucking noise used by many equestrians to urge their horses on is a lateral click.
Velaric egressive initiation is performed by reversing the sequence of a velaric ingressive: the front and back of the tongue (or lips and back of the tongue) seal off the vocal cavity, and the middle of the tongue moves upward to increase oral pressure. The only attested use of a velar egressive is a bilabial nasal egressive click in Damin. Transcribing this also requires the use of the Extended IPA, Template:IPA.
Since the air pocket used to initiate velaric consonants is so small, and it is not considered possible to produce velaric fricatives, vowels, or other sounds which require continuous airflow.
Clicks may be voiced, but they are more easily nasalized. Because the vocal cavity behind the rearmost closure, into which the air passing through the glottis for voicing must be contained, is so small, uvular clicks cannot be voiced for long. Perhaps this is the reason voiced uvular clicks are often prenasalized.
In nasal clicks, the velum is lowered so as to direct air through the nasal cavity. This nasalization is pulmonic, and as it involves a second airstream, it may itself be egressive or ingressive, independently of the nature of the rest of the click. Nasal clicks may be voiced, but are very commonly unvoiced and even aspirated, which is rare for purely pulmonic nasals.