The Nature of Sound

From the physical standpoint, sound consists of a series of condensations and rarefactions produced in the transmitting medium by a vibrating body. A vibrating body such as a tuning fork or a column of air is required to produce the waves, and some elastic medium such as air is necessary for the transmission of the sound waves. It is sometimes desirable to consider sound waves as traveling only through air. In the broadest sense, sound waves consist of a series of condensations and rarefactions existing in any medium.

Sound waves serve to transfer a portion of the energy of the vibrating body to the telephone transmitter, the radio microphone, or the ear. As the vibrating object moves, it displaces the air particles and causes condensations and rarefactions in the air. These particles transfer the condensations and rarefactions from one part of the air to another.

The vibrations of the air particles are very small, and are quoted¹ by one author to be from 0.00000005 inch for barely audible sounds to 0.004 inch for loud sounds. The to-and-fro motion of the air particles from their position of rest occurs in a direction parallel to the progress of the sound wave. Sound waves are therefore longitudinal as distinguished from transverse waves in which the displacement is at right angles to the direction of wave propagation. This motion is continued as long as the sound continues, that is, until the energy imparted by the vibrating body to the air particles is dissipated as heat or is transmitted away.

Simple sounds such as those produced by tuning forks are sinusoidal in nature. Steady-state complex (or non-sinusoidal) sounds can be analyzed into a fundamental and a series of harmonics. This is accomplished by having the sound wave actuate a high-quality microphone that in turn drives an oscillograph from which a photograph can be made and analyzed by the Fourier series methods. Or, the microphone can be connected to a wave analyzer, and the analysis made electrically. Panoramic viewing of a complex steady-state sound wave also is possible with special apparatus.