Introduction

Sound intensity is a measure of the magnitude and direction of the flow of sound energy. Although acousticians have attempted to measure sound intensity as long ago as the early 1870s, the first reliable measurement of sound intensity did not occur until over one hundred years later in the late 1970s. Then the convergence of theoretical and experimental advances, including the derivation of the cross‐spectral formulation for sound intensity and developments in digital signal processing, propelled sound intensity measurements from the laboratory into practical use. Most modern measurements of sound intensity are made using the simultaneous measurement of the sound pressure with two closely‐spaced microphones and this approach will receive the most attention in this chapter.

Sound intensity I is a vector quantity and is defined as the time average of the net flow of sound energy through a unit area in a direction perpendicular to the area. The dimensions of the intensity are sound energy per unit time per unit area (watts per square meter). For sound energy to be conserved, the sound power generated by a source must be equal to the normal component of the sound intensity integrated over any surface that completely encloses the source. This holds even in the presence of other sources outside the surface. A central point in noise control engineering is to determine the sound power radiated by sources. The value and relevance of determining the sound power radiated by a source is due to this quantity being largely independent of the surroundings of the source in the audible frequency range. Also, it can be used to predict the sound pressure level at a distance from a source once physical details of the surroundings are known. Sound intensity measurements make it possible to determine the sound power of sources without the use of costly special facilities such as anechoic and reverberation rooms and in the presence of steady background noise.

Sound intensity measurements seem to be most useful for the determination of the sound power of large machinery in situ, for noise source identification, and for measurement of the sound transmission loss of partitions. Care must be taken to sample the sound intensity in the sound field appropriately in order to reduce errors. Calibration should be undertaken as recommended by the manufacturers and standards bodies.