Figure VI
III
What was said about the uncertainty of our present knowledge concerning the laryngeal action applies with almost equal force to the other operations of tone production. Beyond the basic facts that the tones of the voice are produced by the vibration of the vocal cords and are reinforced and modified by the influence of the resonance cavities, little can be stated with absolute certainty. There is, of course, no question as to the definiteness of our knowledge of the anatomical structure of the parts involved. But, with regard to the muscular operations of the resonance cavities and the application of acoustic and mechanical principles in these operations, the same uncertainty is encountered as in the laryngeal actions. We shall continue therefore to outline the most widely accepted theories, without entering into a discussion as to their soundness.
Considered acoustically, the voice is a wind instrument of the reed class. It differs, however, from all other reed instruments in several particulars. It is capable of producing a wide range of pitches, covering more than three octaves in many cases, by the operation of a single pair of reeds. Further, it has command of an immense range of tone qualities, through the combined action of its reed mechanism and its resonating cavities.
For the production of tone the vocal cords are brought together and held on tension with sufficient strength momentarily to close the glottis and check the outflow of the expired breath. As a slight degree of condensation takes place in the air behind the cords, they are forced apart and a tiny puff of air is allowed to escape. Immediately the cords spring back, once more close the glottis, and again check the outflow of the breath. This is repeated a varying number of times per second, the rate of rapidity of the succession of puffs being regulated by the degree of tension of the vocal cords, and by their effective vibrating length. The pitch of the tone thus produced is determined by the rate of the air puffs; these range from 75 per second for the lowest usual bass note,
to 1,417 per second for the highest usual soprano note,
.