Audio calculator
SPL Distance Calculator
A point-source loudspeaker loses 6 dB of SPL every time the distance doubles: the inverse square law. Enter the SPL at a reference distance (spec sheets use 1 meter) and the listener distance, and this calculator returns the level at the listener.
Spec sheets state maximum SPL at 1 m. Use a measured level for real predictions.
Formulas
Inverse square law
Lp2 = Lp1 - 20 × log10(d2 / d1)- Lp1:
- SPL at the reference distance d1
- Lp2:
- SPL at the listener distance d2
How it works
Sound from a point source spreads spherically, so its intensity falls with the square of distance. In decibels that is 20 × log10 of the distance ratio: 6 dB per doubling, 20 dB per tenfold increase. A cabinet doing 130 dB at 1 m does about 100 dB at 32 m in free field.
Line arrays behave differently in their near field: a long coupled array approximates a cylindrical source and loses closer to 3 dB per doubling until the geometry stops supporting it, which is precisely why arrays throw. This calculator models point sources: a single cabinet, a front fill, a monitor wedge, or an array far enough away to act like a point.
Real venues add complications the law ignores: air absorption eats high frequencies over long throws, boundaries add energy, and wind and temperature gradients bend sound outdoors. Treat results as the geometric baseline, not a full prediction.
Worked example: A cabinet rated 134 dB max SPL at 1 m, audience at 40 m
- 1.Distance ratio: 40 / 1 = 40.
- 2.Loss: 20 × log10(40) = 32.0 dB.
- 3.Level: 134 - 32 = 102 dB SPL at the listener, before air absorption.
About 102 dB SPL at 40 m, and that is the physical ceiling, not the mixing level.
Point source loss vs distance (reference 1 m)
| Distance | Loss | Level if 130 dB at 1 m |
|---|---|---|
| 4 m / 13 ft | -12.0 dB | 118.0 dB |
| 8 m / 26 ft | -18.1 dB | 111.9 dB |
| 16 m / 52 ft | -24.1 dB | 105.9 dB |
| 32 m / 105 ft | -30.1 dB | 99.9 dB |
| 64 m / 210 ft | -36.1 dB | 93.9 dB |
Field notes
- For noise-limit compliance work, measure at the license point; do not argue geometry with an inspector holding a meter.
- Doubling amplifier power buys +3 dB; doubling the number of coupled cabinets buys up to +6 dB. Distance costs 6 dB per doubling. The math explains most PA sizing decisions.
Frequently asked questions
How much does sound drop per doubling of distance?
6 dB for a point source in free field. Long line arrays in their near field lose closer to 3 dB per doubling, which is the main reason arrays are used for long throws.
How loud will a speaker be at 100 feet?
Take the 1 m spec figure and subtract 20 × log10(30.5), about 29.7 dB. A 130 dB-at-1m cabinet lands near 100 dB SPL at 100 ft, before air absorption and any boundary effects.
Why does my measured level not match the calculation?
Spec-sheet max SPL is a burst figure into the cabinet at full rated power, not your show level. Reflections, array coupling, air absorption, and meter weighting (A vs Z, slow vs fast) all move the measured number.