This article is from the Acoustics FAQ, by Andrew Silverman with numerous contributions by others.
Sound power level, Lw, is often quoted on machinery to indicate the total sound energy radiated per second. The reference power is taken as 1pW.
For example, a lawn mower with sound power level 88dB(A) will produce a sound level of about 60dB(A) at a distance of 10 metres. If the sound power level was 78dB(A) then the lawn mower sound level would be only 50dB(A) at the same distance.
 
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