When the stem of a tuning-fork so small that it can be scarcely heard when in vibration, except by, the person holding it, is laid against a solid body, as a table, its sound is at once so increased that it can be heard in the most distant part of a large room. When the same fork is held over an empty jar of suitable size and shape, a similar but much, less marked increase of its tone is to be observed.
If a cord of but moderate thickness be fastened at each end to a thin piece of wood, say a split shingle, and a little block of wood, in imitation of the bridge of a violin, be placed under the cord so as to render it tense, we have the essentials of a stringed instrument, the pitch of which can be made to vary by moving the block about and thus varying the tightness of the cord. But the sound of such an improvised instrument, produced by drawing a bow across the cord, is ridiculously feeble.
In the actual violin the volume of sound, as well as its quality, depends on the size, shape, and weight of the instrument. The strings serve the purpose of causing the body of the instrument, the air within it, and, in consequence, the air without, between it and the ear of the auditor, to vibrate or move in a specific manner.
Similarly, the imposing size of the grand piano is associated inevitably with loudness, as compared with a smaller instrument. A violoncello must produce a larger tone than a violin, though not necessarily one more intense.
These principles of resonance apply in the case of the singer and the speaker. The bass and barytone produce tones of larger volume (as well as different quality) than those of the tenor, because their resonance apparatus is different in size and shape. It is true, their vocal bands, their wind-power, and the laryngeal muscles are different—they are not of the same size, etc.—and, in a more remote sense, this is the cause of the differences in the tones they produce; but the immediate cause is to be sought in the resonance mechanism, and, above all, in the resonance-chambers.
It is true that when one speaks or sings, the chest, windpipe, and larynx may be felt to vibrate, but the essential vibrations are supra-glottic—above the vocal bands.
These resonance-chambers are the mouth cavity, in the widest sense, and the nasal chambers. It is highly probable that the vibrations of the chest walls and of the bones of the head may to some degree modify the vibrations of the air within the resonance-chambers, chiefly in the direction of intensification; but the idea that the hollow spaces in certain of the bones of the head have any appreciable influence on the tones of the speaker or singer, can at best not be considered as demonstrated, and it serves no practical purpose to take into account this possibility.
Fig. 46 (Tyndall). Representing water being poured into the vessel A B, till the air-space is just sufficient to respond to the vibrations of the tuning-fork. The air thus becomes a resonator of the fork.
The great facts, the facts which are so plain that they may be demonstrated to a child, are these: that the quality of any tone—e.g., a vowel—is absolutely determined by the shape of these cavities, the mouth and nasal chambers. This subject will be treated further when the tones, etc., of speech are considered, but inasmuch as no one can sing, in the proper sense of the term, without the use of vowels, at least, and as we produce different vowels with ease, one may at once demonstrate to himself that this is done by altering the shape of his mouth cavity, and chiefly by the agency of the tongue and soft palate.