Decibel Distance Calculator
Calculate sound level at any distance using the inverse square law.
Convert dB at one distance to dB at another.
Decibel Distance Calculator uses the inverse square law of sound to predict how loud a sound source will be at any distance, given a known measurement at a reference distance.
The inverse square law: Sound spreads outward in all directions. As you double the distance from a source, the sound energy spreads over four times the area — so the intensity (power per area) drops to one quarter, which is a reduction of 6 dB.
Formula:
dB₂ = dB₁ + 20 × log₁₀(d₁ / d₂)
Where:
- dB₁ = measured sound level at the reference distance
- d₁ = reference distance
- dB₂ = sound level at the new distance
- d₂ = new distance
The 6 dB rule: Every time you double the distance, sound drops by ~6 dB. Every time you halve the distance, sound increases by ~6 dB.
| Distance Ratio (d₂/d₁) | Change in dB |
|---|---|
| ×0.5 (half distance) | +6 dB |
| ×1 (same distance) | 0 dB |
| ×2 (double distance) | −6 dB |
| ×4 | −12 dB |
| ×10 | −20 dB |
Common reference levels:
- Whisper at 1 m: ~30 dB
- Normal conversation at 1 m: ~60 dB
- Lawnmower at 1 m: ~90 dB
- Rock concert (near speaker): ~110–120 dB
- Jet engine at 30 m: ~130 dB
Important limitations: This formula assumes a point source in a free field (open air, no reflections). Indoors, sound reflects off walls and floors, so real levels drop more slowly. Directional sources (speakers, sirens) follow this law only when you are on-axis.
Practical example: A car horn measures 100 dB at 1 meter. At 20 meters: dB = 100 + 20 × log₁₀(1/20) = 100 − 26 = 74 dB — about normal office noise level.
The calculator supports meters and feet for distance input.