TABLE OF CONTENTS Sound and air are closely related: it is common knowledge that the Moonians (the inhabitants of the Moon) have no ears! This means we will begin our study of sound with the physics of its traveling medium: air. Sounds that propagate through our atmosphere consist of a variation of the air's pressure p( x, y, z, t) according to position in space x, y and z and to the time t. It is these variations in pressure that our ears can perceive. In this chapter, we will first study how these sounds propagate as waves. We will then describe a few different types of sounds and various ways of representing them. Finally, we will explain the concept of filtering, which allows certain frequencies to be singled-out.
The propagation of a sound wave can occur in any direction, and depends on the obstacles in its path. We will essentially be focusing on plane waves, that is to say waves that only depend on one direction of space. We will assume that this direction is the x-axis, and therefore that the pressure p( x, y, z, t) is independent of y and z. Hence it can simply be denoted by p( x, t). This type of function represents a plane wave propagating through space, but also a sound wave inside a tube (see Figure 1.1), such as for...
