Elastic lips and vocal folds self-oscillate once energy input begins.

Evidence

The lips of a brass player (or vocal folds of someone who is singing or speaking) are elastic; that means that if they are displaced from their rest position, they will oscillate around it due to the restoring force that acts on each lip (or vocal fold) to move it back to its rest or equilibrium position. Because they will be moving when they arrive at their equilibrium positions, they will, like a displaced pendulum, move past their equilibrium position, where they will be slowed down, because the restoring force is now acting to return them to their rest position, stop and return toward their rest position. Provided there is some energy input to the system, in this case causing the lips (or vocal folds) to move toward each other, the movement can be started and, once started, continue as a sustained oscillation. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1237

Categorization

• DEFINE: Explains the mechanism of self-sustained oscillation in sound-producing membranes.
• XREF: connects to: Lips only begin oscillating as a reed once airflow passes a threshold — David Howard, Jamie Angus, coustics and Psychoacoustics, 1263; Lip and vocal fold vibration is driven by Bernoulli pressure reduction from airflow — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1240; Lip oscillation sustains continuously while airflow and adduction persist — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1242