FIG. 122.—Bon jour ("good day" in French) as represented by a wave picture. The picture was made by a mirror arranged to move under the influence of the voice and to cast a beam of light upon a strip of sensitized paper.
Sound has been likened to a picture painted not in the space and color of substance but in time and motion. What really passes out from the source is merely a rhythmical motion of the air particles, manifesting themselves as changes in pressure, spreading out in ever-widening spheres through the atmosphere. The order of these compressions is different for every sound. The musical sounds of an orchestra embody a different set of vibrations for each note of each particular instrument. If the fluctuations in pressure of a sound wave are irregular and non-periodic, the sound is called a noise; if they are cyclic, and follow a regular and sufficiently rapid periodic lag, the sound is musical.
We may easily satisfy ourselves that in every instance in which the sensation of sound is produced the body from whence the sound comes must have been thrown into a state of rapid tremor, implying the existence of a motion to and fro of the particles of which it consists.
FIG. 123.—Experiment showing sounding bodies are in vibration.
If the face of a tuning fork prong be touched with a small ball of cork suspended from a fine silk fiber, after the fork has been struck and caused to emit its note, the cork will be violently repelled from the latter. Why? Because the prong of the fork is in vibration.
If a small wire or bristle is fastened to the prong of the fork and a piece of smoked glass drawn across it while the fork is giving forth a sound, the trace of the point will appear as a wavy line, showing that while the glass was drawn along the prong went to and fro many times.
The vibrations or disturbances set up in the air by a sound emitting body are known as sound waves. These waves consist of a series of condensations and rarefactions succeeding each other at regular intervals, each air particle swinging to and fro in a very short path.
FIG. 124.—Method of registering vibrations of a tuning fork.