The human vocal folds are a highly articulate muscular structure which work in cooperation with the diaphragm, lips, tongue, and muscles in the throat to produce speech. The myoelastic-aerodynamic theory of phonation is an attempt to describe the process by which the vocal folds produce sound, relying on Bernoulli's principle and a method of vibration very similar to that of an harmonica or another free reed musical instrument. Janwillem van den Berg is credited with devising this theory.
According to this model, negative air pressure forms below the vocal folds in the respiratory tract, sucking the folds tightly against each other and forming an enclosed space of air below the glottis. Air pressure increases in the lungs and below this point of closure, and once the air pressure reaches a critical high point, it forces the folds back open, blowing them up and outward as the pressure releases in a plosive puff of air. The vocal folds have lateral movement that claps the two sides of the larynx against each other lightly until their natural elasticity overcomes this movement and pulls the folds back into a position from which the air pressure change can once again suck them closed. This entire cycle occurs several or even hundreds of times each second, each cycle releasing a puff of air and laterally clapping the larynx. The sound produced by this rapid process is the human voice. Increased forcefulness of expelled air increases loudness of the voice, and muscular adjustments to the speed of movement of the vocal folds result in different pitches, as when singing.
This theory does a poor job of explaining how the vocal folds sustain their oscillation; quite a bit of energy is lost in the transfer of movement between the air stream and the soft tissues of the larynx and throat. Bernoulli forces alone are inadequate to describe this aspect of phonation. This is not, however, sufficient to disprove the theory. An alternative theory of phonation is the neurochronaxic theory, which has been disproven.
Iron Noder 2015, 21/30