When a hammer strikes a piece of metal a noise is produced. The sound is caused by the particles of the two separate metals vibrating. The vibrations are transmitted through the air in a series of waves. The presence of the vibrations can be detected by pressing a stiff piece of cardboard against the surface or side of the metal when it is struck.

Sound possesses three properties - intensity or loudness, pitch, and quality - by which one sound may be distinguished from another. The intensity of a sound depends upon the density of the medium through which the sound is transmitted and upon the amplitude of the sound waves which reach the ear. The intensity varies inversely as the square of the distance. The waves become smaller and smaller as they leave the point at which the sound is produced, because the quantity of air through which the sound is conveyed becomes greater and greater. In other words, the intensity of sound decreases as the distance from the source of sound increases.

All forms of speaking tubes are based upon the principle that the sound waves set in motion in the tube are confined to the air space of the tube. Therefore the sound is transmitted without any decrease in intensity.