What two conditions must be met for vocal folds to vibrate?

For vocal folds to vibrate, they must be adducted, meaning they are brought together to close the glottis, which is the space between them. This closure is critical as it allows for effective build-up of air pressure beneath the vocal folds, known as subglottic pressure. When this pressure becomes higher than the pressure in the supraglottic area (the space above the vocal folds), the vocal folds are pushed apart.

As the air escapes through the glottis and the pressure differences persist, the vocal folds are then forced back together due to the Bernoulli effect—whereas the air flowing between the folds creates a decrease in pressure that draws them closer together. This cycle of opening and closing due to the interplay between the subglottic and supraglottic pressures results in vibration, leading to sound production.

The requirement for vocal folds to be adducted ensures that a firm closure is achieved for the efficient generation of sound. The need for the subglottic pressure to be higher than the supraglottic pressure is essential for initiating the periodic opening of the vocal folds necessary for vibration. Together, these two conditions create the fundamental mechanics required for phonation.