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."
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]. 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.