Vocal Sound Production

Diaphragm action pushes air from the lungs through the vocal folds, producing a periodic train of air pulses. This pulse train is shaped by the resonances of the vocal tract. The basic resonances, called vocal formants, can be changed by the action of the articulators to produce distinguishable voice sounds, like the vowel sounds.

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 illustration below shows approximate pitches for speaking voices related to an equal tempered piano keyboard based on A₄ = 440 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.

Vocal Folds in Phonation

The process of converting the air pressure from the lungs into audible vibrations is called phonation. When the air passes through the elastic vocal folds and causes them to vibrate, the type of phonation is called voicing. The vocal folds give the singer a wide range of control over the pitch of the sound produced. While "vocal folds" is more descriptive than "vocal cords", there is some similarity to a vibrating string in that the pitch produced depends upon the length, mass and tension of the vocal folds.

The excitation of the vocal folds is however very different from the excitation of a string in that is is caused by the passage of air through the opening between the folds. The muscles of the larynx change the elasticity and tension of the vocal folds to determine the pitch of the sound.

Vocal Fold Excitation

The vibratory cycle of the vocal folds is driven by aerodynamic phenomena. Driving air pressure from the lungs controls the opening of the folds, and the Bernoulli effect controls the closing phase. As the top of the folds is opening, the bottom is in the process of closing, and as soon as the top is closed, the pressure buildup begins to open the bottom. The vibration is then like a wave which travels from the bottom to the top of the vocal folds. Each vibration allows a brief puff of air to escape, producing an audible sound at the frequency of the opening; this process is called voicing. The voice intensity can be increased by increasing the air flow from the lungs and increasing the resistance from the vocal folds. The vocal folds are blown wider apart, and stay apart longer during the cycle. This increases the amplitude of the sound pressure wave produced. How about this for a "big word" description of the process: myoelastic aerodynamic theory of phonation