Sound is related to vibration, and is described as a propagating perturbation through a fluid, which is air

or water in most cases. A very wide variety of noise sources exists. Each source is peculiar to its generation

mechanism, which may cover a wide range of phenomena including fluid mechanics and the vibration of

structures. Sound is perceived by the ear of the listener as a pressure wave superimposed upon the

ambient air pressure. The sound pressure is the incremental variation about the ambient atmospheric

pressure. Generally, it is detected by a microphone and expressed as oscillatory electric signal output from

an audio measurement instrument. We shall present a mathematical description of these pressure waves

that are known as sound. The field of acoustics concerns sound and vibration, and is treated in Chapter

12 to Chapter 20 of this book.