The vocal cords or "vocal folds" are a part of the larynx. In the diagram below, which represents the larynx as viewed from above, they are shaded with M's.
/ \ / \
| / ___,--.___ \ \
| / ,--’ ‘-._ | |
\ \__ / . |___...___| . \ / /
| ``-/ “ ” \--’’ /
/ \.._ <-’`-> __../ |
| ._| |Mi-. | |.-iMMM/| . |
| | \ |MMMM|| ||MMMM| /`| \
\ \_ \\MMMM\\ //MMMMM// _/ |
\ \ \\MMM\\ //MMMM// / /
\ | \.\MMM|\ //MMM/ /| /
| \ \\MM|| ||MMM/_’ / /
\ \ \ \M\V/M/_/ .’ /
\ ‘- `’-|||-’` / /
`\ \.__|||_.--’ /
\. ||| ./
’-. -- .-’
Unfortunately, the ascii art is unable to convey the full complexity
of the muscle
s that make up the vocal cords. There are actually several different muscles that work together to produce the fine control
we have over our voices. They are connected to the vocal ligament
, which forms a V-shape in the center of the larynx, and two structures of cartilage
- the cricoid cartilage
, at the top
of the diagram, and the thyroid cartilage
, forming a U-shape around the sides and bottom
of the diagram.
The thyroarytenoid muscle or vocalis muscle makes up most of the vocal folds. This muscle is larger in adults than in children, and larger in males than in females. The vocal folds shorten and thicken as this muscle contracts.
The cricothyroid muscle lies below the plane of the diagram, slanting from the bottom front of the adam's apple to back center of the larynx. When this muscle contracts, the adam's apple pitches forward, putting tension on the vocal folds (thus, lengthening and thinning them).
The arytenoid muscles, at the top of the V-shaped vocal ligament, contract to close the vocal folds together.
Above and to the outside of the true vocal folds are a set of similarly shaped muscles called "false vocal cords".
How they work to produce sound
Acoustically, the vocal folds act as a blown-open reed. That is to say that at the start of sound production, the arytenoids have closed the vocal folds together. The pressure of air generated from below by the lungs forces the vocal folds open, against the action of the arytenoids. The thickness of the vocal folds, determined by the action of the vocalis muscle and the cricothyroid muscle together, then determines how far the vocal folds will open (the thicker they are, the farther they open).
The springlike arytenoids fight this motion, and after the inertia of the opening vocal folds carries them past a certain point, the arytenoids start to successfully counteract the opening force of the air. The vocal folds accelerate towards closing, and their inertia leads them to fully close despite the opening force of the airstream. This process, a single cycle of vibration for the vocal fold, usually takes between 2 and 5 milliseconds. It repeats as long as the speaker continues to contract the arytenoids and exhale.
In the short time the vocal folds are open, the airstream escapes upward past the larynx. This pulse of air continues upward through the vocal tract. The pulse is changed by its transit through the vocal tract, as portions of it are scattered and reflected back by the palate, nose, tongue, teeth, and lips. The shape and tension of these reflecting surfaces introduce resonances to the pulses escaping from the vocal folds- and gives vocal sounds their recognizable character.
The false vocal cords, by default in an open position, can only cause small pressure variations in the airstream, compared to the on-or-off blasts of the true vocal folds. For this reason, their effect on the overall sound produced is minimal.
By increasing the tension on the arytenoid muscles, more powerful airstreams can be used successfully to produce sound. In this way, the arytenoid muscles help increase the volume of the the sound produced. By contracting the vocalis, the vocal folds are thickened, lowering the pitch of the resulting vibration. Vocalis muscles that are naturally thicker (in adults, especially males) produce naturally lower pitches. By contracting the cricothyroid muscles, the vocal folds are thinned, raising the pitch.
Needless to say, the vocal cords do not always function perfectly. Here are the most common problems speakers experience with their vocal cords.
- "The Physics of Musical Instruments" by Fletcher and Rossing, ISBN 0-387-98374-0
- Thanks to m_turner and Gritchka for advice and feedback