ABSTRACT
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.