Engineering Acoustics

8Sound Intensity, Measurements and Determination of Sound Power, Noise Source Identification, and Transmission Loss

8.1 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 ...