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Phoneticians define phonation as "use of the laryngeal system to
generate an audible source of acoustic energy [i.e. sound] which can then be modified by the articulatory actions of the rest of
the vocal apparatus."
A voiced sound is produced when the vocal folds vibrate,
by an overpressure of air in the lungs, thus chopping the airstream into a turbulent
airflow. This produces an audible hiss, depending on the volume of the airflow and the degree of constriction of the folds. This
gives rise to the glottal fricative [h].
If the vocal folds do not vibrate, then the sound (usually a consonant) is
called voiceless.
Other sounds may be produced by completely blocking off the airstream and then releasing the folds. The sound of this type
produced at the glottis is called a glottal stop.
The voice source is used to change intonation and the tone of words by varying the
subglottal pressure as well as the tension of the vocal folds. This leads to changes in the frequency of vibration, which are in
turn perceived by the listener as modifications in pitch and/or in loudness. During speech the
flow of air is relatively small because of constrictions of the folds.
Subglottal pressure is regulated by a number of factors, namely: the respiratory muscles, gravity and elasticity.
See also
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