Inverse Square Law
This calculation will give you the amount of attenuation, in decibels, you can expect with a change in receiver distance, in a free field (outdoors).
For example if you were standing 20 feet from a loudspeaker, and were to move to 40 feet away from that loudspeaker, you would expect to see a drop in level of 6 dB. Sound that is radiated from a point source drops in level at 6 dB per doubling of distance.
Equation used to calculate the data:
Snew=Sref + (20 * Log (Dref / Dnew)
Dref = Reference Distance
Dnew = New Distance
Sref = Reference Sound Level
Snew = New Sound Level