Air waves cannot be seen by the naked eye, but their nature may be easily represented or illustrated. Fig. 126 gives a pictorial representation of the crowding together of the air particles during the passage of a wave. The loudness of the sound depends upon the amount and suddenness of the change in pressure, and the note or pitch on the number of complete to and fro motions of the particles per second.
FIG. 125.—Wavy line made by a bristle attached to a tuning fork prong in vibration when passed over smoked glass.
The timbre of a sound or the quality that distinguishes the note of a violin from that of a piano depends upon the smoothness or abruptness of the changes in pressure. Therein lies the difficulty of the production of sound by means of a phonograph or telephone, for the sound waves must resemble each other in every detail in order that the result may be like the original.
FIG. 126.—Illustrating the action of air waves.
The mechanism with which we speak or sing is composed of two flexible membranes, stretched side by side across a small cylindrical box located at the top of the windpipe. The membranes are called the vocal chords, and the box the larynx. The chords are so arranged and controlled by muscles that their tension may be changed at will. In breathing, the air to and from the lungs passes freely between the chords. When the controlling muscles are tightened, so as to stretch the chords, the edges are also brought parallel and quite close to each other. If the breath from the lungs is then forced through the narrow slit between them, they vibrate like the reed of a musical instrument, and produce the sounds of the voice. The multitude of sounds which it is possible for a human being to produce are the result of various degrees of stretching of the vocal chords, together with the movements of the mouth, lips and tongue.
FIG. 127.—The vocal cords in position for making a sound.
