SOUND (Lat. sonitus) is the impression produced on the ear (q.v.) by the vibrations of the elastic medium, such as air or water, in which it is plunged. That this is the ease is proved, first, by the fact that a bell or tuning fork in vacuo gives no sound when struck; second, by the fact that mere currents, as such (winds, running water, etc.), do not pro duce the sensation of sound until they are frittered down into vibratory motions by The most untutored ear distinguishes at once between a mere noise and a musical note. It of course distinguishes a loud sound from a faint one. Moreover, it distinguishes musical notes from one another by their shrillness or gravity, or, as it is technically called, pitch. Again, as in the case of vowel-sounds sung to the same musical note, or as in the case of different instruments (flute and violin, for instance) playing the same note, it distinguishes something furtherwhich is called the quality of the note. It is on the pitch of notes that the theory of music (q.v.) is based, for the quality is only of importance in giving variety, as in orchestral music—or in giving richness of tone in a solo. The most perfect music, so far as theory goes, may be executed on the poorest instrument, but it gives little pleasure from the want of richness or quality. In the same way a singer may possess faultless intonation, yet the performance, though musically perfect, may, from the harsh quality of the voice. be unpleasant. We intend in the present article to avoid everything connected with music, and have made these remarks to show that there is something in the theory of sound more profound than is contem plated in the theory of music.
The questions we have now to discuss are: 1. What constitutes the difference between a mere noise and a musical note? 2. On what does the pitch of a note depend? 3. On what does its quality depend? The answers to these queries are all contained in the following statement : Every musical note consists in the repetition at equal small intervals of time, of some defi nite noise; the pitch depends on the rate of repetition; and the quality upon the nature of the fundamental noise, Rough experimental illustrations of the parts of this statement are easily given, more refined ones will be afterward alluded to. If, for instance, the edge of a card be hold to
a revolving toothed-wheel, a definite noise is produced as each tooth bends the card and allows it to spring back. While theWheel revolves slowly, we can distinguish these sue cessive noises; but when it is revolving so fast that they are no longer separately distin guishable, the character of the sound changes completely. It now becomes continuous, and, so far as the ear can detect, uniform, and thus becomes a musical note (with such an apparatus, not a pleasing one). As the wheel is made to revolve faster and faster, the pitch of the note rises till it becomes a sort of shriek, and finally becomes inaudible. The sirens (q.v.) gives another excellent illustration. In this case the fundamental noise is produced by a puff of air escaping from an orifice; and we observe, just as before, that the greater the number of such puffsper second, after they have become so fre quent as to be separately undistinguishable, the higher is the pitch of the musical note produced.
Now, if by machinery we arrange matters so that the sirene and the toothed-wheel give the number of puffs and the number of impacts on the card the same per second, the musical note produced by each has the same pitch. But the notes differ greatly in quality, the one being exceedingly soft and pleasant, the other harsh and grating. The pitch, therefore, depends on the number of noises per second, and the quality upon the nature of the fundamental noise. We shall find a complete thedretical proof of this later.
The general nature of the mechanical process by which sound is propagated in the air will be illustrated, and compared with other cases of wave-motion, in our article on WAVES. Meanwhile, it is only necessary for us to observe that, as the velocity of sound is ten times greater than that of wind in the most violent hurricane, it is not air itself which is transferred from place to place, but a state of disturbance (condensation or rare faction) of the air. Each successive layer of air in the path of the sound suffers this dis turbance in turn, and by virtue of its elasticity (q.v.) passes it on to the next.