Sound intensity
A fundamental acoustic quantity which describes the rate of flow of acoustic energy through a unit of area perpendicular to the flow direction. The unit of sound intensity is watt per square meter. The intensity is calculated at a field point (x) as a product of acoustic pressure p and particle velocity u. Generally, both p and u are functions of time, and therefore an instantaneous intensity vector is defined by the equation below.
Many acoustic sources are stable at least over some time interval so that both the sound pressure and the particle velocity in the field of such a source can be represented in terms of their frequency spectra.
The applications of sound intensity were fully developed after a reliable technique for intensity measurement was perfected. Sound intensity measurement requires measuring both the sound pressure and the particle velocity. Very precise microphones for sound-pressure measurements are available.
An application of the intensity technique is the measurement of sound power radiated from sources. The knowledge of the radiated power makes it possible to classify, label, and compare the noise emissions from various pieces of equipment and products and to provide a reliable input into environmental design. See Sound
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