Sound level at target distance
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The inverse square law
Sound from a small source spreads out in all directions. The same acoustic energy covers a larger area as you move away, so the sound pressure level drops by about 6 dB every time the distance doubles in open air. That's the inverse square law, and it's the basis of every speaker coverage, PA and noise calculation.
L₂ = L₁ − 20 × log10(d₂ ÷ d₁)
Example: a speaker producing 100 dB at 1 m gives about 94 dB at 2 m, 88 dB at 4 m and 80 dB at 10 m.
Indoors is different
In a room, sound reflects off walls, floor and ceiling. Close to the source the direct sound dominates and the level falls about 6 dB per doubling; further away the reflected (reverberant) sound takes over and the level stays almost constant. dBReach lets you choose a gentler slope for large or small reflective rooms to approximate this.
Adding sound sources
Decibels are logarithmic, so you can't add them directly. Two equal, uncorrelated sources add 3 dB (90 dB + 90 dB = 93 dB). Ten equal sources add 10 dB. When one source is much louder, the quieter one barely changes the total.
| Difference between two sources | Add to the louder one |
|---|
| 0 dB | +3.0 dB |
| 1 dB | +2.5 dB |
| 3 dB | +1.8 dB |
| 6 dB | +1.0 dB |
| 10 dB | +0.4 dB |
Everyday sound levels
| Source | Typical level |
|---|
| Quiet living room | 35 – 40 dB |
| Normal conversation at 1 m | 60 dB |
| Busy street | 75 – 80 dB |
| Lawn mower at operator | 90 dB |
| Rock concert, front of house | 100 – 110 dB |
| Fireworks nearby | 130 – 140 dB |
Using the results
- Choosing speakers: compare a speaker's maximum SPL at 1 m with the level you need at the back of the room.
- Noise complaints: estimate how loud music or equipment will be at a neighbour's property line.
- Hearing protection: find how far you need to stand from a loud source to stay below 85 dB.
Frequently asked questions
How much does sound decrease with distance?
About 6 dB each time the distance doubles outdoors (inverse square law), and less indoors because of reflections.
How loud is 100 dB at 10 meters?
A source measuring 100 dB at 1 m is about 80 dB at 10 m in open air.
What happens when you add two equal sound sources?
The total rises by 3 dB. Two 90 dB sources give 93 dB.
How many decibels is twice as loud?
About 10 dB is perceived as twice as loud, although 3 dB is double the acoustic power.
Is 85 dB loud?
85 dB is the level at which hearing protection is recommended for long exposures – similar to heavy city traffic.
About dBReach
dBReach is an independent, free acoustics calculator for musicians, sound engineers, event planners and anyone comparing loudspeakers or noise sources. It uses standard sound-propagation formulas. We are not affiliated with any audio brand.
Every formula on this page runs entirely in your web browser. The numbers you type are never uploaded, stored or shared. Our goal is simple: give you an honest starting point so you can compare product specifications with confidence and avoid paying for capacity you do not need – or buying something too small to do the job.
The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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Last updated: October 28, 2026
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The calculator and articles on this website are provided free of charge for general information and educational purposes only. Results are estimates based on typical values and simplifying assumptions; real-world performance depends on the specific product, installation, climate, wiring and how you use your equipment.
Nothing on this site is professional engineering, electrical or financial advice. Always follow the manufacturer's instructions and local electrical codes, and have permanent installations, transfer switches and grid-connected systems designed and installed by a licensed electrician or qualified installer. Never operate fuel-burning generators indoors or in enclosed spaces – carbon monoxide can kill within minutes.
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