I will not attempt to go into the matter fully, but the general principles involved are quite easily grasped.
Taken broadly, then, it will be understood that vocal sound is produced by a column of air passing from the lungs through a small aperture formed by the vocal cords within the larynx (see diagrams). When we breathe in the ordinary way the air passes in and out as we inspire and exhale, without any sound being produced. This is because the passage through the larynx is then quite clear. No obstruction is offered to the air current, and in consequence the process is quite noiseless.
When, however, we wish to utter a sound, Nature provides for this by enabling us to interpose an obstruction to the air current by means of the vocal cords, and the air then has to pass through a small slit or aperture, sometimes called the “vocal chink,” formed by their being drawn closely together curtainwise, as it were.
THE VOCAL CORDS DURING DEEP BREATHING.
When the vocal cords—or ligaments, as they are perhaps better described—are drawn together in this manner the passage of the air is so restricted that it can only pass in short rapid pulsations, instead of, as before, in a continuous stream, and the result of these pulses or vibrations is the production of sound or tone.
The aperture, or chink, is called the glottis, and the character of the tone resulting, in particular the pitch of it, is regulated by the precise disposition and proximity to one another of the two bands or cords or ligaments—sometimes they are known as the vocal lips—by which the chink or opening is formed.
THE VOCAL CORDS DURING THE SINGING OF A HIGH NOTE.
The process itself of regulating the opening of the vocal cords in this way is entirely automatic and subconscious. We merely will, to produce a tone of a certain pitch and the vocal cords automatically, and without any conscious effort on our part, are brought together to precisely the right degree necessary to produce that particular tone.