Tech Tools / SPL Drop-off
SPL Drop-off
Enter source SPL at a reference distance. See level at each seat row with inverse-square (−6 dB per doubling) or near-field line-array (−3 dB per doubling).
Use at your own risk. Real rooms have reflections, air absorption, and array geometry that change measured SPL. Inverse-square is a free-field point-source model. Line-array −3 dB/doubling applies only in the near/cylindrical zone and is a simplification. This tool is a planning aid only. Always verify against manufacturer documentation and on-site measurements before deploying to a show.
| Label | Distance | SPL | Δ from ref |
|---|
Sources
- Inverse-square law: SPL₂ = SPL₁ − 20·log₁₀(d₂/d₁) → −6.02 dB per distance doubling (point source, free field).
- Cylindrical/line near-field approximation: −10·log₁₀(d₂/d₁) → −3.01 dB per doubling (simplified line-array near zone).
- Air absorption: ISO 9613-1 qualitative note only in this tool.
What this SPL calculator does
It predicts level change with distance from a known SPL at a reference distance using point-source or simplified line-array models.
Formula
Point: L₂ = L₁ − 20 log₁₀(d₂/d₁). Line near: L₂ = L₁ − 10 log₁₀(d₂/d₁).
Worked example
100 dB SPL at 1 m → at 10 m (point) = 80 dB; line near-field model = 90 dB.
FAQ
Why −6 dB when distance doubles?
For a point source in free field, intensity falls with 1/d², which is −6.02 dB per doubling: ΔL = −20 log₁₀(d₂/d₁).
When is −3 dB per doubling used?
As a simplified near-field line-array / cylindrical model: ΔL = −10 log₁₀(d₂/d₁). Real arrays transition with distance.
Does air absorption matter?
Yes at high frequencies and long throws. This tool notes it qualitatively — measure on site for critical HF.
Tuning coverage?
Kynetic designs PA systems for corporate and medical shows.