How the vocal folds vibrate

The vocal folds form a kind of valve. Their primary function is to prevent anything entering the lungs, such as food or water, by forming a stoppage in the windpipe. For example, if when you swallow something ‘goes down the wrong way’ (a description which is actually rather accurate), the reflex reaction is to close the vocal folds tightly together, and then cough. Coughing involves an increase of air pressure below the closure at the glottis, and then releasing the closure forcefully in an attempt to expel anything that has fallen down too far. You can make a cough and then release it more gently: this release of the cough is a glottal stop, transcribed [ʔ].

For breathing, the vocal folds are open and held wide apart so that air can pass in and out of the lungs unimpeded. If you breathe with your mouth open, you will hear only a gentle noise as the air moves in and out of your body. However, you can make a little more tension across the vocal folds, and you will get a [h] sound.

Sounds that are made with the vocal folds open, allowing the free passage of air across the glottis, are voiceless. In English, voiceless sounds include [p t k f θ s ʃ]. Voiceless sounds often have a more open glottis than the state of the vocal folds for breathing.

Voiced sounds are made with a more or less regular vibration of vocal folds. They include:  and all the vowels. As we will see, the way the contrast between voiced and voiceless sounds is accomplished phonetically involves more than the presence or absence of vocal fold vibration.

We will now take a look at the mechanism by which voicing is produced. The vibration of the folds is not caused directly by commands from the brain telling the folds to open and close: it is caused by having the right amount of tension across the folds. When the folds are shut, the air below them cannot escape, yet the pressure from the intercostal muscles has the effect of forcing the air out. So the pressure builds up below the glottis. Once this pressure is great enough, it forces the folds to open from below, until eventually they come open. Once they are open, and air can pass through the glottis, the air pressure above the glottis and below the glottis equalizes. Now the tension across the vocal folds forces them back together again, making a closure again. The process now repeats itself: the folds are closed, air cannot escape through the glottis, so the pressure builds up, the folds are forced open, the pressure equalizes, the folds close again.

This cycle of opening and closing is an aerodynamic effect called the Bernoulli effect.

When the vocal folds vibrate making complete closure along their full length (that is, with no gaps in contact between the vocal folds), with regular vibration, and with no particular tension in the folds to make them especially thick (and short) or thin (and long), this is called modal voicing. Few speakers really achieve modal voicing, but since most people have a ‘normal’ setting (that is, one that has no particular distinguishing features for them), we often speak of modal voicing to mean a person’s default voice quality.