Introduction to Digital Audio, 2nd Edition

2

Some audio principles

2.1 The physics of sound

Sound is simply an airborne version of vibration. The air which carries sound is a mixture of gases. In gases, the molecules contain so much energy that they break free from their neighbours and rush around at high speed. As Figure 2.1(a) shows, the innumerable elastic collisions of these high-speed molecules produce pressure on the walls of any gas container. If left undisturbed in a container at a constant temperature, eventually the pressure throughout would be constant and uniform.

Sound disturbs this simple picture. Figure 2.1(b) shows that a solid object which moves against gas pressure increases the velocity of the rebounding molecules, whereas in (c) one moving with gas pressure reduces ...