Vocal fold function is the source of the voice sound. It can occur only if the vocal folds are brought together so they may act as a kind of valve that resists the air pressure brought to them by the singer until that air pressure and the vocal fold closure tension meets a balance at which time the vocal fold opens, allowing air to pass and immediately closes again only to have the same process continue at a rate the is represented by the pitch being sung. When singing the A4 immediately above middle C the vocal folds open and close 440 times a second. The A an octave higher will require the folds to open and close 880 times a second.
If the male vocal folds are shortened or elongated to a medium length, they are rather thick and they open first at the bottom of the vocal fold and that opening moves quickly upward until they open at the top of the vocal fold just before the bottom of the vocal fold closes again followed by the continued closing of the fold until the top of the fold is finally closed. This creates a wave action of opening and closing from the bottom of the vocal fold to the top of the fold.
And this action produces a tone that is rich in overtones and this voice quality is usually called chest voice. It is the sound of a bass or baritone or tenor when singing in the lower two thirds of their respective vocal range.
If the male vocal folds are elongated beyond their medium length until they finally reach their lengthened limit, the vocal folds gradually become more and more thin. Thin vocal folds will still display some degree of wave function as they open at the bottom and the opening action continues upward to the top of the fold but because the vocal fold is thinner such wave action is greatly reduced.
This minimized wave action produces a tone that, though less rich in number of harmonics, emphasizes those harmonics that are in a frequency range most sensitive, and thus more emotionally evocative to the ear. This voice quality is usually called head voice. It is the sound of a bass or a baritone or tenor when singing in the upper third or fourth of his respective ranges
There is another vocal fold configuration possible. The vocal folds can be kept slightly apart, that is, not completely closed. In this configuration the folds act as a very poor air valve. Air brought to the vocal folds by the singer is never completely restricted from passage but only slightly restricted and that restriction increases the speed of the air as it passes over the slightly closed vocal folds (Bernoulli Principle). The air makes the very edges of the vocal folds oscillate producing a tone but with no vocal fold wave action from bottom to top of the fold.
This configuration produces a tone that has very few overtones and this voice quality is usually called falsetto. A typical example of male falsetto is a baritone imitating a high woman’s voice. It does not sound like a woman’s voice but is only a caricature of the woman’s voice.
An addendum. Since the quality of falsetto is dependent on the vocal folds not being completely closed, obviously the quality of the falsetto voice will change as the vocal folds are brought closer together. This is sometimes called “supported” falsetto because the more closed configuration does make the vocal fold valve a bit more efficient and the singer senses a bit more resistance to the passage of his breath. This configuration also gives the falsetto tone a few more overtones.
If the folds are eventually closed completely then the falsetto voice has morphed into the head voice and no longer has a falsetto quality because of the presence of many more overtones.
This covers the action of the vocal folds and such action is called phonation. By the way, the sound produced by the vocal folds (phonation) very closely resembles the sound produced by the trumpet player when he “buzzes” his mouthpiece.
The other equally important part of the production of the singing voice is what happens to the “phonated” sound once it enters the vocal tract. This is called resonance and that will be discussed in other “Thoughts” on this site.