How this calculator works
An ideal point source spreads its acoustic power across a sphere whose area grows with squared distance. The intensity falls as 1/r² and the pressure magnitude as 1/r, giving the same 20 log10 distance correction in decibels. The reference level must already be known at a nonzero reference distance. Indoor reflections, directivity, barriers and atmospheric absorption are excluded.
Formula
L2 = L1 - 20 log10(r2/r1); loss = L1-L2. r2 = r1 10^((L1-L2)/20).
Worked example
A source measured at 80 dB at 1 m gives 60 dB at 10 m. If 80 dB is measured at 2 m, the same 20 dB drop occurs at 20 m.
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Frequently asked questions
How much is lost when distance doubles?
About 6.02 dB under spherical free-field spreading.
Can attenuation be negative?
Yes. Moving closer than the reference distance gives a negative loss and a higher level.
Can I use zero distance?
No. The point-source expression diverges at the source and does not describe its near field.