vocal cords

Anatomy

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/_/ .'      /
                \       `- `'-|||-'`  /       /
                 `\       \.__|||_.--'       /
                   \.         |||          ./
                    `\_                  _/
                       '-.     --     .-'
                          `'--`  `--'`

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.

Vocal Control

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.

Common problems

Needless to say, the vocal cords do not always function perfectly. Here are the most common problems speakers experience with their vocal cords.

• Laryngitis: inflammation of the vocal cords by irritants such as smoke, stomach acid, or overuse, causing hoarseness and pain
• Spasmodic dysphonia or vocal fold paralysis: nervous control of the muscles is erratic or impaired, causing choked or breathy voice.
• Motion impairment: Scar tissue on the muscles renders them less functional, or the vocal folds are mechanically held open or closed by a foreign object. This causes a breathy voice and is potentially life-threatening if it affects the ability to swallow correctly.
• Polyps, nodules, cysts, and contact granuloma: abnormal growths on one or both vocal folds caused by irritants, causing hoarseness and pain
• laryngeal cancer

Sources

• http://www.upmc.edu/upmcvoice/anatomy.htm
• http://www.med.jhu.edu/voice/
• http://www.worldzone.net/music/singingvoice/anatomy.html
• "The Physics of Musical Instruments" by Fletcher and Rossing, ISBN 0-387-98374-0
• Thanks to m_turner and Gritchka for advice and feedback

The vocal cords or vocal folds are necessary for speech, and we generally think of this as their prime function. This might even be their prime function, as they have many more fancy design features to maximize their the speech production than for any other function. But other functions they do have, some of them quite important.

The vocal cords can clamp shut very tightly, and do so automatically if anything coming down the windpipe touches them. This, together with the choking and coughing reflexes, have saved you from some very nasty lung infections, and has probably saved your life. Every time you 'swallowed something the wrong way', whether it was a sip of water or a splinter of bone, a death was narrowly averted.

Somewhat less dramatically, the vocal cords clamping down will also trap air in the lungs. This allows us to hold your breath, and it also provides a structural function. Filling your lungs with air and tightly shutting your vocal cords will 'brace' your thorax, providing a stiff framework for your muscles to work off of. This is called thoracic fixation, and we use it unconsciously, but constantly.

Perhaps the most obvious example of thoracic fixation is seen when you are lifting a heavy weight, and you grunt. This grunt pops out as your body braces itself by clamping down the vocal chords. You may also find yourself gasping or giving a deep sigh when dropping a heavy weight, and your body relaxes after a hard fixation.

Another good example of thoracic fixation is giving birth. Bracing the thorax gives the abdominals something to push from, and will make the baby come out faster. This is why you might hear the expectant mother being told to "breathe!" No, she isn't going to suffocate without the doctor's instructions. Concentrating on taking deep breaths will help the mother keep from pushing with the contractions until it's time for the baby to come out.

And now you know why the rest of the animal kingdom has vocal cords.

I thought it might be interesting (well, to some) to point out that you might also use thoracic fixation during a particularly difficult bowel movement, but I couldn't quite work it in to the paragraph on giving birth. Anyway, now you know.