In a free field environment, the sound level decreases 6 dB every doubling of distance. Of course this means it increases if you move closer as well, doubling at half the distance. It’s one of the few things pretty reliable here on Earth.

We can figure this stuff out pretty easily.

If we call the new sound level we want L₂ at distance r₂, we can figure out the difference from a reference level L₁ that is the case at another distance r₁. All we need is this sorcery:

L₂ = L₁ + 20 log₁₀(r₁/r₂) dB

Or just use this chart. This does not include proximity effect from directional microphones or buildup from small and resonant recording spaces, but it does tell you how much your level will generally change when moving a microphone closer to, or feather away from a sound source.

Change in distance	Resulting change in level
0.0625× (1/16) 0.070× 0.079× 0.088× 0.099× 0.111× 0.125× (1/8) 0.14× 0.157× 0.177× 0.198× 0.223× 0.25× (1/4) 0.281× 0.315× 0.354× 0.397× 0.445× 0.5× (half as far) 0.561× 0.630× 0.707× 0.794× 0.891× Same distance 1.122× 1.260× 1.414× 1.587× 1.782× 2× (twice as far) 2.245× 2.52× 2.828× 3.175× 3.564× 4× 4.490× 5.040× 5.657× 6.350× 7.127× 8× 8.980× 10.08× 11.31× 12.70× 14.25× 16×	+24dB +23dB +22dB +21dB +20dB +19dB +18dB +17dB +16dB +15dB +14dB +13dB +12dB +11dB +10dB +9dB +8dB +7dB +6dB +5dB +4dB +3dB +2dB +1dB Same level -1dB -2dB -3dB -4dB -5dB -6dB -7dB -8dB -9dB -10dB -11dB -12dB -13dB -14dB -15dB -16dB -17dB -18dB -19dB -20dB -21dB -22dB -23dB -24dB