I found this equation to calculate the sound power level of a rectangular sound source:

PWL = SPL + [10 log (a*b/r^2)] - 10 with the condition r > b/Pi

PWL = Sound power level

SPL = Sound pressure level

I used this equation in my analysis and found that the SPL I got was greater than the PWL. Is this possible? Shouldn't the SPL decrease as the sound travels farther away from the source?

I can't seem to find the reference for this equation so I can't check its validity. Does anyone here come across this equation? Is there any other equation to calculate the sound pressure level of a rectangular sound source? Or can anyone recommend some reference books for me to check up on? Thanks in advance.