Sound is a universal, ever-present and wonderful component of our environment. Human beings and many other animals use it for communication. It informs us about our environment: unlike our eyes, our ears can detect signals arriving from any direction and from hidden sources. Our auditory system provides us with a very sensitive warning system which, unlike our visual sense, is on guard even when we are asleep. Sound is the only form of wave that can travel over long distances under water and, as such, is vital to many forms of marine life, to the mapping and investigation of the seabed, to divers and fishermen and to the navies of the world. It informs physicians about the state of our cardiovascular and pulmonary systems and allows them to examine our inner structures non-invasively. It alerts operators to potential failure or malfunction of machinery and processes under their control. In the form of music, it has powerful aesthetic, psychological and emotive effects which have not yet been fully explained (Sachs 2007). Sound can propagate in both fluid (liquids and gases) and solid media. This book concentrates principally on sound in atmospheric air; but ‘structureborne’ sound, which means the propagation of structural vibration in the audio-frequency range, features in Chapter 5 on Noise Control.