This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
Voice consists of sounds produced by the vibrations of two elastic bands called the vocal cords. These cords lie in the larynx, which is situated between the pharynx and the windpipe, and is a portion of the passage conveying air to the lungs specially modified to form a voice-organ.
The vocal cords project into the larynx so that but a narrow slit, called the glottis, is left between them. When the vocal cords are put in a certain position air driven through the glottis sets them vibrating and they give origin to sounds. The stronger the blast the louder the voice.
The pitch of the voice is primarily dependent on the size of the larynx. The larger it is, or what comes to the same thing, the longer the vocal cords are, the lower is the pitch of the voice. In children, therefore, the voice is shrill; and, as the female larynx is usually smaller than the male, a woman's voice is usually higher pitched than a man's. About sixteen or seventeen years of age a boy's larynx grows very fast, and his voice "breaks," becoming about an octave deeper in tone.
How is voice produced? Where do the vocal cords lie? Where is the larynx situated?
What is the glottis? When do the vocal cords give origin to sounds? On what does the loudness of the voice depend?
How does the size of larynx influence the pitch of the voice? Why is a woman's voice commonly higher pitched than a man's? Why does a youth's voice break?
While everyone's voice has a certain natural pitch which leads us to call it soprano, tenor, bass, and so forth, this pitch can be modified within limits, so that we each can sing a number of notes. This variety is due to the action of muscles in the larynx which alter the tension of the vocal cords; the more tightly these are stretched, other things being equal, the higher pitched is the tone which they emit.
The vocal cords alone would produce but feeble sounds. If a fiddle-string be attached to a hook on the ceiling and stretched by hanging a heavy weight on its lower end, we can get tones out of it when it is plucked or bowed; but the tones are feeble and deficient in character and fullness. In the violin the strings are attached to a hollow wooden box, and when the string is set in movement it causes the wood to vibrate, and this, in turn, the air contained in the cavity of the instrument; in this way the tone is intensified, and altered and much improved in quality. The air in the pharynx, mouth, and nose answers pretty much to that in the hollow of the violin; those cavities together form a resonance-chamber, and when the vocal cords vibrate they set this air in vibration also, and so the sound is made louder and is altered in character. By movements of throat, soft palate, tongue, cheeks, and lips, the size and form of the sounding chamber are varied, and with them the tone of voice; by movements of tongue, lips, and palate, the air-current, and therefore the sound, is interrupted from time to time; on other occasions the air is forced through a narrow passage in the mouth, giving rise to new sounds added on to those originated by the vocal cords. In such manners the primitive feeble monotonous tone due to the vocal cords is reinforced and altered in various ways in throat and mouth, and voice is developed into articulate speech.
How is it that we can sing a number of notes of different pitch?
Why is a hollow wooden box an essential part of a violin? How do the throat and mouth cavities influence the loudness and quality of the voice? How do tongue, lips, and cheeks co operate in converting voice into speech?