Laryngeal Physiology• The vibrating vocal folds are the major source of periodic sound for speech. The vocal folds may also produce aperiodic sounds.• In this section, we discuss two main aspects of phonatory physiology: – 1. How are the vocal folds put into vibration? – 2. How do we change the manner in which the vocal folds vibrate?
Glottal Tone Initiation• Before sound can be produced from the vocal folds, several conditions must be established:• 1. The vocal folds must be approximated or at least brought into the phonatory position. Phonation may also be initiated after complete closing of the vocal folds.
• 2. It is also necessary to properly tense and elongate the vocal folds prior to actually producing sounds. Length and tension are important determinants of the fundamental vibrating rate of the vocal folds.
• 3. Finally, there must be airflow from the lungs. In order to be able to produce the required flow of air from the lungs, there must be a sufficient quantity of air in the lungs.• Once the aforementioned conditions have been met, phonation can start.
The Phonatory Cycle: The Aerodynamic-Myoelastic Theory• The vocal folds are brought to the midline of the glottis via muscular forces.• Vocal fold adduction must occur in coordination with the expiratory air stream.• Subglottal air pressure builds below the closed folds.• The pressure eventually overcomes the resistance of the folds and separates them.
The Bernoulli Force• The vocal folds return to the midline of the glottis due to (a) elasticity, and (b) the Bernoulli force: – When a gas/fluid moving through a tube reaches a constriction, its velocity increases. – As the gas speeds through the point of constriction, a pressure drop occurs perpendicular to the direction of flow.• The vocal folds are sucked back together in this zone of low pressure.