Part 4.2: Elementary acoustic loads of a loudspeaker

The figure below shows a distribution of an active acoustic intencity^[1] in the vicinity of a vibrating piston. It is very high (red) on its surface, with a decay going from the center towards the edges (because of the tangential velocity at the edge). At a greater distance, the intensity decreases according to a profile which rapidly becomes spherical, and decreases with the distance tending towards zero (in blue).

Radiated power is conserved during propagation. At long distance ("far" field), the spatial dependence of the field tends to a sphere of radius r ;

the acoustic intensity is then proportional to   (r being the distance between source and receiver )

and the sound pressure is proportional to ;