Laryngeal System 3
self sustaining quasi-periodic oscillation of the vocal folds that results from the interaction of muscular and aerodynamic forces in the vocal tract.
essentially a series of puffs of air separated by closure (or partial closure) of the vocal folds between each “puff”.
Transient region of constriction (“Valving”)
Vocal folds function as the major source of periodic vibration for the production of sound for speech. The vocal folds act as a regulating valve because of their close mid-line approximation.
A single cycle of opening and closing takes approximately 1/100th second: therefore, the cycle repeats at rates in the region of 100 times per second (adult male speaker).
Forces that act to approximate the vocal folds at midline. Combination of LCA and IA to rotate the arytenoid cartilages.
Stretching forces applied to the vocal fold tissue.
Use of the CT, TA and (in some instances) the extrinsic musculature.
Sequence Summary for Voice Initiation
Vocal folds are initially in an abducted state.
Refill air supply (inhalation). Approximate arytenoids & adduct the vocal folds. Elongate & increase tension of vocal folds (variable condition during phonation). More Tense – Higher frequency of vibratory sound source. Less Tense – Lower frequency of vibratory sound source. Area of transient constriction at the glottis generates an area of resistance for the airflow stream moving up from the lungs. Rise in subglottal (tracheal) air pressure. Subglottal (tracheal) pressure increases until it is sufficient to overcome the elastic & muscular forces holding the vocal folds approximated at midline. The minimum subglottal (tracheal) pressure needed to set the vocal folds into vibration is called the phonatory threshold (~ 3cm H20 for low F0, ~ 6cm H20 for high F0).
The subglottal (tracheal) air pressure pushes the vocal folds laterally and superiorly, and the vocal folds are “blown apart”. Subglottal (tracheal) pressure is released & the vocal folds are re-approximated. Subglottal (tracheal) pressure begins to rise again, and the sequence continues. Once the folds are vibrating, a slightly lower tracheal pressure (less than the phonatory threshold) will sustain the oscillation. Continuing cycles of vibration are sustained by the recoil forces (muscle elasticity) of the vocal folds & aerodynamic forces (Bernoulli effect) below and within the glottis.
If the vocal folds are very tightly adducted
a large subglottal pressure would be required to blow them apart. Phonation may not be likely in this case.
If the vocal folds are not sufficiently adducted
phonation isn’t likely because air from the lungs will escape through the glottis, and no subglottal pressure will be built up.
Nerve impulses from the brain initiated each cycle of vibration. Faster nerve impulses would be required as F0 increased.
Not experimentally confirmed
(Muller, 1839; van den Berg, 1958)
Air from the lungs is the active element in voice production, causing vibration of the essentially passive vocal folds.
Validated by experiments.
As the speed (velocity) of a moving fluid (liquid or gas) increases, the pressure within that fluid decreases.
The total energy in a steadily flowing fluid system is a constant along the flow path. An increase in the fluid's speed must therefore be matched by a decrease in its pressure.
lower frequencies of vocal fold vibration
A Mucosal Wave (surface wave) forms: inferior-to-superior movement of the mucosa. This is evidenced by Vertical phase difference: the inferior margins of the vocal folds move in a different manner than the superior margins. Inferior separate first and close first.
Three Requirements of Myoelastic-Aerodynamic Theory
Active muscle contraction and positioning of the arytenoid cartilages.
i.e. Laryngeal engagement
Pressure (and flow) source: respiratory system.
Airflow through the glottis produces the Bernoulli effect.
Elastic properties of the vocal folds
Vibration of vocal folds is primarily a passive mechanism
Vocal Folds are “Blown Apart” due to Ptrach (tracheal or subglottal pressure)
Relation between airflow velocity and changes in pressure.
Vocal folds are “sucked” toward each other.
Margins of the vocal folds rebound due to tissue elasticity (elastic forces = restoring force in opposition to displacement).