Phonation is the opening and closing cycle of the vocal folds, which repeats at up to 400 times per second. What kind of mechanical structure allows for such rapid movement and fine control?

The Aerodynamic Myoelastic theory suggests that, rather than any mechanical muscular action, the airflow itself, and the elasticity of the folds, combine to produce this action (known as a ‘mucosal wave’). Here’s how the cycle works:

• When the folds close, the pressure of the air below them increases. When this pressure exceeds the pressure holding the folds together, they burst apart and air flows again. This airflow again drops the pressure (the Bernoulli effect), and the folds get sucked back together again.
• It is these pressure changes created by regular puffs of air coming through the folds that produce sound, not the folds clapping together or vibrating.
• The vocal folds have a certain thickness, and the lower edge of the folds opens before the upper edge, so the opening moves upwards. Then the lower edge of the folds closes before the upper edge, so the closure moves upwards.

This vibration cycle is animated for you on the right.  Play the animation, then play the video for slow motion effect stroboscopy of voiced [i]. Although the quality is not great, you can just get a sense of the lower edge of the folds opening and closing before the upper edge.