What is the sound power level ?

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.