Bernoulli Effect

Bernoulli Effect and Voicing

To understand how the Bernoulli effect contributes to vocal fold vibration, it is necessary to consider how air from the lungs flows through the glottis into the supraglottal vocal tract. The glottal aperture during voicing is approximately 10 to 12 mm long, reaching a maximum width during normal voicing of about 2.5 mm. The depth of the gap bordered by the edges of the folds is 4 to 5 mm. The internal diameter of the trachea is around 20 to 25 mm, and the lower pharynx, about 22 to 29 mm. The trachea-glottis-pharynx configuration is thus a tube that narrows and then widens out again (see Figure 1).

The narrow section, the glottis, functions as a Venturi tube (named after an Italian physicist, Giovanni Battista Venturi) in that the flow of air has to speed up to get through without causing the flow in the trachea to slow down. This increase in velocity brings with it the Bernoulli effect: As the air accelerates through the glottis, the pressure it exerts on the vertical edges of the vocal folds decreases, causing a suction effect that brings the folds together. The glottis remains closed only momentarily because the air coming up the trachea pushes the folds apart again and, once again, accelerates through the gap—and the cycle continues.

[Page 269]The opening and closing actions of the vocal folds are aided by the elasticity of the muscle fibers composing the vocal folds. When they are pushed apart by the subglottal pressure, starting at the lower edges, they are waiting for an opportunity to spring back together again. The opportunity comes when the air accelerates past their surfaces, which also happens first at the lower edges. The speed with which they are pushed apart is slower than the speed with which they snap together again; the snapping together produces the acoustic energy of voicing and excites the air in the supraglottal chambers to create resonances that are then shaped by the articulators into vowels and voiced consonants.

The complete process by which voicing is produced is known as “aerodynamic-myoelastic action.” The term aerodynamic refers to the movement of air, and myoelastic refers to the muscle elasticity (“myo” is from the Greek word for muscle). The Bernoulli effect is a key part of this action and is brought into play by the Venturi tube properties of the glottis. The action continues for as long as the vocal folds are held in position by the laryngeal adductor muscles and there is sufficient air being pumped from the lungs.

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