How this calculator works
The SPL Distance Calculator estimates how sound level changes when you move farther from a source—or when you add identical coherent sources. Live sound engineers use it for quick level checks at FOH, fill positions, and safety planning without opening a spreadsheet.
Point sources follow inverse-square law in free field (−6 dB per doubling). Line-source segments approximate cylindrical spreading (−3 dB per doubling). Rooms add reflections and absorption, so results are guides; measure with an SPL meter when compliance or gain structure matters.
Formula used
Level change (dB) = 10 × n × log₁₀(new distance / reference distance), where n = 2 for a point source and n = 1 for a line source model. Multiple coherent sources add 10×log₁₀(count) dB.
Example calculation
100 dB at 1 m, point source, new distance 10 m: ratio 10 → loss = 10 × 2 × log₁₀(10) = 20 dB → about 80 dB at 10 m. Two identical coherent sources add ≈ 3 dB → about 83 dB.
Key terms
- SPL — sound pressure level in dB re 20 µPa.
- Point source — spherical spreading; −6 dB per distance doubling in free field.
- Line source — cylindrical spreading model; −3 dB per doubling.
- Coherent summation — identical signals arriving in phase add in pressure.
Frequently asked questions
- How much level do you lose when distance doubles? About 6 dB for a point source in free field, or about 3 dB for the line-source model used here.
- When should I use point vs line source? Use point for a single box or short array; use line for a long, continuous line array segment where cylindrical spreading dominates.
- Do reflections change SPL loss over distance? Yes. Indoors, level often falls slower than inverse square because of room gain and standing waves.
- How are multiple speakers summed? Identical coherent sources add 10×log₁₀(n) dB. Real arrays include phase, splay, and filtering—not just power sum.