Microphone Distance SPL Calculator
See how much the sound level changes when you move a microphone nearer or farther from a source, using the inverse-square law.
How it works
ΔSPL = 20·log₁₀(d₁ ÷ d₂) · SPL₂ = SPL₁ + ΔSPL
In a free field, sound level follows the inverse-square law, so halving the distance to a source raises the level by 6 dB and doubling it drops 6 dB. Close-miking exploits this: a capsule 15 cm from a source hears it far louder than the room, which is why close mics reject bleed and reverb so well. The same law explains the proximity effect’s cousin — signal-to-noise improves dramatically as you move in.
Worked example
A source measured at 94 dB at 1 m, miked at 0.15 m: the change is 20·log₁₀(1 ÷ 0.15) = 20·log₁₀(6.67) ≈ +16.5 dB, so the capsule sees roughly 110.5 dB — plenty of level and strong rejection of the room.
Frequently asked questions
Why do close mics reject room sound so well?
The direct sound obeys the inverse-square law, but the reverberant room level is roughly constant everywhere. Moving from 1 m to 15 cm boosts the direct sound about 16 dB while the room stays put, so the direct-to-reverberant ratio improves by the same amount — cleaner, drier recordings.
Does this predict distortion or clipping?
It predicts the acoustic level at the capsule, not the electrical signal. A very high SPL can exceed a microphone’s maximum SPL rating and distort, or overload the preamp — check the mic’s spec and engage its pad if the calculated level is high.
Is the inverse-square law exact indoors?
Only near the source, within the "critical distance" where direct sound dominates. Beyond that, the reverberant field takes over and the level flattens out, so the calculator is most accurate for close-miking and outdoor or well-treated spaces.
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This calculator is for educational purposes. Double-check important results before acting on them.