TABLE OF CONTENTS Instantaneous sound intensity is a vector quantity, and is equal to the product of the sound pressure and the particle velocity. We are usually interested in measuring the time-averaged intensity in stationary sound fields; this vector quantity describes the net flow of sound energy passing through a unit area that lies normal to the measurement surface. This can be used to calculate the sound power radiated by various surfaces in rooms.
If we were to measure the intensity of a propagating plane wave in an anechoic environment, this could be done simply by using measurements of the time-averaged mean-square sound pressure (Eq. 1.19). In practice we want to measure sound intensity in a variety of different sound fields in and around buildings, often where the radiated sound is reflected back from other surfaces and where there are other sound sources present. For example, measurement of sound intensity radiated by a separating wall in a reverberant room where there is flanking transmission from the surrounding walls and floors. In these situations, a single microphone cannot be used to give the magnitude and direction of the intensity, it is necessary to determine both the sound pressure and the particle velocity.
To illustrate the issues pertaining to sound intensity measurement in the presence of reflected waves it is simplest to look at the superposition of two plane waves travelling in opposite directions (Fahy, 1989). In a one-dimensional interference field, the temporal and spatial variation of the sound pressure is described...
