On the factory floor we can make sound pressure measurements and find out if the workers risk hearing damage. But once we have found this, we may well want to reduce the noise. To do this, we need to know how much noise is being radiated and by what machine. We therefore need to know the sound power of the individual machines and rank them in order of highest sound power. Once we have located the machine making most noise we may want to reduce the noise by locating the individual components radiating noise.

We can do all this with intensity measurements. Previously we could only measure pressure which is dependent on the sound field. Sound power can be related to sound pressure only under carefully controlled conditions where special assumptions are made about the sound field. Specially constructed rooms such as anechoic or reverberant chambers fulfill these requirements. Traditionally, to measure sound power, the noise source had to be placed in these rooms.

Sound intensity, however, can be measured in any sound field. No assumptions need to be made. This property allows all the measurements to be done directly in situ. And measurements on individual machines or individual components can be made even when all the others are radiating noise, because steady background noise makes no contribution to the sound power determined when measuring intensity.

Because sound intensity gives a measure of direction as well as magnitude it is also very useful when locating sources of sound. Therefore the radiation patterns of complex vibrating machinery can be studied in situ. (from Bruel & Kjar)