ABSTRACT
This chapter introduces the physical quantities of sound and the laws by which they are related. The basic laws are illustrated by considering simple forms of sound wave, namely the plane wave and the spherical wave. The chapter provides a short description of the properties of human hearing and of the main sorts of sound and sound sources that we are concerned with in room acoustics. In any sound wave, the particles of the medium undergo vibrations about their mean positions. Therefore, a wave can be described completely by indicating the instantaneous displacements of those particles. A sound wave is associated with variations in the pressure within a medium and with vibrations of the particles of which it consists. Both imply an increase in the mechanical energy, which the sound wave carries away from the source. The strength of this energy flow is characterised by the ‘intensity’, sometimes also called the ‘energy flux density’.
