Any piece of machinery that vibrates radiates acoustical energy. Sound power is the rate at which energy is radiated (energy per unit time). Sound intensity describes the rate of energy flow through a unit area. In the SI system of units the unit area is 1 m2. And hence the units for sound intensity are Watts per square metre.

Sound intensity also gives a measure of direction as there will be energy flow in some directions but not in others. Therefore sound intensity is a vector quantity as it has both magnitude and direction. On the other hand pressure is a scalar quantity as it has magnitude only. Usually we measure the intensity in a direction normal (at 90 degree) to a specified unit area through which the sound energy is flowing.

We also need to state that sound intensity is the time-averaged
rate of energy flow per unit area. In some cases energy may be travelling back and forth. This will not be measured; if there is no net energy flow there will be no net intensity.

In the diagram opposite the sound source is radiating energy. All this energy must pass through an area enclosing the source. Since intensity is the power per area, we can easily measure the normal spatial-averaged intensity over an area which encloses the source and then multiply it by the area to find the sound power. Note that intensity (and pressure) follows the inverse square law for free field propagation. This can be seen in the diagram, at a distance 2r from the source the area enclosing the source is 4 times as large as the area at a distance r. Yet the power radiated must be the same whatever the distance and consequently the intensity, the power per area, must decrease. (from Bruel & Kjar)