41.1 Introduction

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A careful examination of noise measurement data reveals that there exists a very wide variety of noise

sources. Each source is peculiar to its generation mechanism, which can be any of a wide range of

phenomena including fluid mechanics and the vibration of structures. However, in analysis, sources are

normally simplified to rather simple and typical models in their generation mechanism.

The vibration of a solid body, which may be in contact with the fluid medium, generates sound waves

or vibratory forces acting directly on a fluid, will result in the emission of acoustic energy in the medium.

In the next section, an expression for an idealized sound source is introduced. We will assume that the

fluid medium outside the source region is initially uniform and at rest. Also, we will concentrate on wave

propagation in an infinite medium.

Generally, acoustic waves sensed as a sound represent a very small energy density in the medium. Only

a very small fraction of the mechanical energy of a source body is converted into acoustic energy. The

conversion factor, defined as the ratio of sound power to the mechanical power of the source, is in the

order of 1027 to 1025. Some examples of estimated sound power conversion factor are given for typical

common noise sources.

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