| Themes > Science > Physics > Acoustics > General Properties of Sound > Intensity |
Intensity is defined as power per unit area. We can think of it as the rate of energy flow across a defined area of the beam. (see the diagram on the previous page.) It is expressed in watts per square meter (W/m2) or in milliwatts per square cm (mW/cm2). Usually the power and the intensity are directly related. Doubling one doubles the other, and vice versa. In contrast, pressure amplitude and intensity are related through a square dependence. Specifically, the intensity, I is proportional to the pressure amplitude, A2. Thus, doubling the amplitude quadruples the intensity, tripling the amplitude increases the intensity by 9, etc. (Such a square-law proportionality also exists for the area of a square carpet and the length of a side; doubling the length from 2 yards to 4 yards quadruples the area of the rug from 4 square yards to 16 square yards.) |
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