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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
the time-averaged energy transmitted by a sound wave through a unit area normal to the direction of wave propagation per unit time. In the case of periodic sound, averaging is done either over an interval of time, large in comparison with the period, or over an integral number of periods.
The sound intensity of a plane sinusoidal traveling wave is
I = pv/2 = p2/2pc
where p is the amplitude of sound pressure, v is the amplitude of sound velocity, p is the density of the medium, and c is the speed of sound in the medium. In a spherical traveling wave, the sound intensity is inversely proportional to the square of the distance from the source. In a standing wave, 1=0, that is, on the average there is no sound-energy flux.
Sound intensity is measured in the International System of Units in W/m2 and in the cgs system in erg/(sec • cm2) = 10-3 W/m2. Sound intensity is also evaluated by the intensity level according to the decibel scale; the number of decibels N = 10 log (I/I0), where I is the intensity of the given sound and I0 = 10-12 W/m2.
I. G. RUSAKOV