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The sound intensity (in watts per m^2) is equal to the sound power W of the source divided by the area S of the wavefront. If we're talking about a point source (omnidirectional) in free-field conditions then it's emitting sound waves in a sphere around it, therefore at distance r from the source I = W/(4 . pi . r^2) This is often referred to as geometric spreading. Linear or planar sources do things slightly differently. The sound intensity *level* (in dB) is given by 10log10(I/I0) where I0 is the reference sound intensity of 1x10^-12 W/m^2