Stage Monitor SPL Calculator
Sound spreads out from a source and gets quieter with distance. In an open or free field, the sound pressure level drops by about 6 decibels every time you double the distance, following the inverse-square law. This calculator takes a reference level measured at a known distance (a speaker's rated SPL at 1 metre, say) and finds the level at any other distance, useful for setting stage monitor levels, estimating front-of-house coverage, and judging how loud a wedge sounds at the performer's ears. It also reports the change in decibels and the distance ratio, so you can see exactly how much level you gain or lose.
Inverse-square law formula
Distance ratio = target distance / reference distance
Level change = -20 * log10(distance ratio) (dB)
SPL at target = reference SPL + level change
Distance doublings = log2(distance ratio)
Use the same unit for both distances; only the ratio matters. Each doubling of distance costs about 6 dB in a free field. Negative change means quieter; positive means closer and louder.
Stage sound context
- Free-field level falls about 6 dB per doubling of distance from a point source.
- Indoors, reflections slow the falloff beyond the critical distance.
- Speaker sensitivity is usually quoted as SPL at 1 metre for 1 watt input.
- Lower monitor levels reduce spill into open microphones and protect hearing.
- Check published occupational exposure limits; sustained high SPL risks hearing damage.
Stage monitor SPL: frequently asked questions
How does loudness fall with distance?
In a free field, sound pressure level drops by about 6 decibels each time the distance from the source doubles. This is the inverse-square law in decibel form: the level at a new distance equals the reference level minus 20 times the base-ten logarithm of the ratio of the new distance to the reference distance.
Why 6 decibels per doubling and not 3?
Sound intensity (power per area) falls as the inverse square of distance, which is a 6 decibel drop per doubling for a point source in free space. The 3 decibel figure applies to halving acoustic power, a different quantity. For distance, use 6 decibels per doubling.
Does a real room follow the inverse-square law?
Only near the source and outdoors. Indoors, reflections build a reverberant field where level falls more slowly beyond the critical distance. The inverse-square law is the free-field ideal; treat indoor results as an upper bound on how much level you lose with distance.
How do I use this for a stage monitor?
Enter the rated or measured SPL at a known reference distance, often 1 metre, then enter the distance from the wedge to the performer's ears. The result estimates the level they hear, helping you set monitor levels and judge spill onto open microphones.
Is hearing-safe loudness a concern on stage?
Yes. Sustained high levels risk hearing damage. Occupational limits in the US start around 85 to 90 dBA for an 8-hour shift, with shorter safe times as level rises. Use this calculator alongside published exposure limits and hearing protection, not as a safety guarantee.
Official sources
- UNSW School of Physics: Decibels, sound level and the inverse-square law.
- U.S. CDC NIOSH: Noise and hearing loss prevention.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.