Audio Engineering

Audio Principles

John Watkinson

1.1 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 neighbors and rush around at high speed. As Figure 1.1(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 1.1(b) shows that a solid object which moves
against gas pressure increases the velocity of the rebounding molecules, whereas in
Figure 1.1(c) one moving with gas pressure reduces that velocity. The average velocity
and the displacement of all the molecules in a layer of air near a moving body is the
same as the velocity and displacement of the body. Movement of the body results in a
local increase or decrease in pressure of some kind. Thus sound is both a pressure and a
velocity disturbance.

CHAPTER 1

Figure 1.1 : (a) The pressure exerted by a gas is due to countless elastic collisions between

gas molecules and the walls of the container. (b) If the wall moves against the gas pressure,

the rebound velocity increases. (c) Motion with the gas pressure reduces the particle velocity.

Pressure

(a) (b)

Rebound is

faster

(c)

Rebound is

slower