The Vocal Folds
Positioned at the base of the larynx in the vocal tract, these twin infoldings of mucous membrane act as the vibrator or "reed" during phonation. Open during breathing, the folds are closed by the pivoting of the arytenoid cartilages for speech or singing. Positive air pressure from the lungs forces them open momentarily, but the high velocity air produces a lowered pressure by the Bernoulli effect which brings them back together. The folds themselves have a resonant frequency which determines voice pitch.
In an adult male, the vocal folds are usually 17-23 mm long, and12.5 -17 mm in an adult female (Kaplan). They may be stretched 3 or 4 mm by action of the muscles in the larynx.
The male speaking voice averages about 125 Hz, while the female voice averages about 210 Hz. Children's voices average over 300 Hz
The front end of the vocal folds is attached to the thyroid cartilage, the "Adam's apple". The back end is attached to the arytenoid cartilages, which move to separate the folds for breathing.