A sonorous wave consists of two parts, in one of which the air is condensed, and in the other rarefied.
The motion of the sonorous wave must not be confounded with the motion of the particles which at any moment form the wave. During the passage of the wave every particle concerned in its transmission makes only a small excursion to and fro.
The length of this excursion is called the amplitude of the vibration.
Sound cannot pass through a vacuum.
A certain sharpness of shock, or rapidity of vibration, is needed for the production of sonorous waves in air. It is still more necessary in hydrogen, because the greater mobility of this light gas tends to prevent the formation of condensations and rarefactions.
Sound is in all respects reflected like light; it is also refracted like light; and it may, like light, be condensed by suitable lenses.
Sound is also diffracted, the sonorous wave bending round obstacles; such obstacles, however, in part shade off the sound.
Echoes are produced by the reflected waves of sound.
In regard to sound and the medium through which it passes, four distinct things are to be borne in mind—intensity, velocity, elasticity, and density.