What Is an Inverse Square Law SPL Calculator?
Sound pressure level (SPL) drops predictably as you move away from a source, because sound energy spreads out over an ever-larger sphere as it radiates outward. This calculator applies the inverse-square law — the same physics that governs light and radio signal falloff — to predict SPL at a new distance from a known SPL measured at a reference distance.
It's a standard tool for live sound engineers positioning speakers, audiologists explaining noise exposure, and anyone estimating how loud a source will be from farther away.
How to Read Your Results
SPL at New Distance
The predicted sound pressure level at your new distance, assuming free-field (open-air, no reflections) propagation from a single point source.
Level Change
How many decibels the level rises or falls compared to your reference measurement — negative when moving farther away, positive when moving closer.
Distance Ratio
How many times farther (or closer) the new distance is compared to the reference distance.
Perceived Loudness Change
A rough plain-language read on the change: roughly every 10 dB drop is perceived as "about half as loud" to human hearing, while every 3 dB is the smallest change most people can just barely notice.
Real-World Example
A speaker measures 100 dB SPL at 1 meter. How loud is it 8 meters back, in the audience?
| Distance | SPL |
|---|---|
| 1 m (reference) | 100 dB |
| 2 m | 94 dB |
| 4 m | 88 dB |
| 8 m | 82 dB |
Each doubling of distance (1→2→4→8 m) drops the level by almost exactly 6 dB, which is the hallmark signature of inverse-square-law falloff — going from 100 dB at 1 m to 82 dB at 8 m, an 18 dB drop across three doublings of distance.
Tips for Using This Calculator
- This law assumes free field conditions — a single point source in open air. Indoor spaces with reflective walls lose sound more slowly than the formula predicts.
- Line sources (like a long line array) follow a different, gentler falloff rate than point sources.
- Use it to plan speaker placement so front-row and back-row listeners get a more even level.
- Use it for hearing-safety estimates — moving twice as far from a loud source roughly halves your perceived loudness exposure.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team