How do vocal folds behave according to the mucosal wave theory?

The mucosal wave theory explains the complex movement of the vocal folds during phonation. According to this theory, when air is pushed through the closed glottis, the vocal folds do not simply open and close entirely; instead, they exhibit a wave-like motion. This wave results in a vertical opening and closing of the vocal folds, meaning that different parts of the vocal folds can move at different times.

As the airflow increases, the lower edges of the vocal folds start to open first, followed by the upper edges, creating the mucosal wave that travels along the length of the folds. This effect is crucial for producing sound, as it contributes to the rich harmonic spectrum of the voice and is essential for proper vocal function. Thus, the vertical opening during the vibration cycles is a defining characteristic of how the vocal folds behave in accordance with the mucosal wave theory.