A field tool for live sound engineers placing PA systems, acoustic consultants assessing noise, and AV designers checking coverage. Enter a known level and distance and read the predicted level anywhere else.
How it works
A point source radiating into free space obeys the inverse square law: the sound intensity falls with the square of distance. Because the decibel is a logarithmic ratio, this becomes a clean subtraction:
SPL₂ = SPL₁ − 20 · log₁₀(d₂ ÷ d₁)
The 20 · log₁₀ form applies because dB SPL is defined on the pressure ratio. Every time the distance doubles, the term equals 20 · log₁₀(2) ≈ 6.02 dB of attenuation — the familiar “6 dB per doubling” rule.
Worked example: a loudspeaker rated 100 dB SPL at 1 m. At 4 m the distance has doubled twice, so the level drops about 2 × 6 = 12 dB to roughly 88 dB. The tool computes the exact figure for any distance ratio, not just whole doublings.
When the simple rule breaks down
- Indoors, reflections raise the level beyond the critical distance, so the real SPL flattens out rather than continuing to fall 6 dB per doubling.
- Line arrays behave more like line sources in their near field, dropping closer to 3 dB per doubling until they transition to point-source behaviour far away.
- Outdoors over ground, absorption, barriers, wind and temperature gradients all modify the result; the inverse-square figure is the best-case baseline.
Tips and notes
- Use 1 metre as the reference distance for any speaker quoted with a sensitivity spec, since that is the standard measurement distance.
- A positive level change means the target is closer than the reference; a negative change means it is further away.
- For noise complaints, the free-field number is a conservative planning estimate — always verify with an on-site measurement.
Every calculation runs locally in your browser; nothing is sent to any server.