On the Pitch of Musical Sounds.
In order to fully describe a Musical Sound, it is necessary, besides specifying its duration, to particularize three things about it, viz., its Pitch, its Intensity, and its Quality. By pitch is meant its height or depth in the musical scale, by intensity its degree of loudness, and by its quality, that character which distinguishes it from another sound of the same pitch and intensity.
In the present chapter we have only to do with Pitch. Inasmuch as we have seen that the Sensation of Sound is due to the vibratory motion of some body, the first question that arises is,—What change takes place in the vibratory movement, when the pitch of the sound changes ?
If we stretch a violin string loosely, and pluck it, we get a low sound; stretch it more tightly, and again pluck it, a higher sound is obtained, and at the same time, we can see that it is vibrating more rapidly. Again, take in the same way a short and a long tongue of metal, and the former will be found to give a higher sound and to vibrate more quickly than the latter.
But the instrument that best shows the fact, that the more rapid the rate of vibration, the higher is the pitch of the sound produced, is perhaps the Wheel Syren (fig. 19). This consists of a disc of zinc or other metal, about 18 inches or more in diameter, mounted on a horizontal axis, and capable of being rapidly revolved by means of a multiplying wheel. The disc is perforated by a number of holes arranged in concentric circles, all the holes in each circle being the same distance apart. In order to work the instrument, a current of air is blown through an India-rubber tube and jet, against one of the circles of holes while the disc is slowly revolving. Whenever a perforation comes in front of the jet, a puff of air passes through,