In the context of physics, intensity is generally defined as the amount of energy transferred per unit area per unit time. It is a measure of the power (energy per unit time) received or transmitted through a surface, often associated with waves, such as light waves, sound waves, or other forms of electromagnetic radiation.
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Wave intensity is the time-averaged power carried through unit area perpendicular to the transport direction. It has SI units . For a uniform beam carrying power through a perpendicular area , ; a nonuniform beam instead requires integration of its local energy flux.
For a progressive plane acoustic wave in a lossless fluid,where is the peak pressure amplitude, its root mean square, the mean mass density and the speed of sound. Doubling the wave amplitude therefore quadruples the wave intensity in the same medium. A standing wave can have large local pressure amplitude with zero net averaged sound intensity, so the progressive-wave formula must not be applied to an arbitrary field.