Example data table
Example assumes point source, L1 = 90 dB at r1 = 10 m, no extra losses.
| r1 (m) | r2 (m) | L1 (dB) | Predicted L2 (dB) | Total attenuation (dB) |
|---|---|---|---|---|
| 10 | 20 | 90 | 83.98 | 6.02 |
| 10 | 50 | 90 | 76.02 | 13.98 |
| 10 | 100 | 90 | 70.00 | 20.00 |
Formula used
This tool models free-field distance loss with a spreading term, then adds optional losses:
- L2 = (L1 + DI) − [k · log10(r2/r1) + α · (r2 − r1) + Lground + Lbarrier + Lmisc]
- k = 20 for point sources, k = 10 for line sources, k = 0 for plane waves.
- DI = 10 · log10(Q) when using a directivity factor.
Real sites include reflections, wind, terrain, and frequency effects. Use this as a first-order estimate, then validate with measurements when accuracy matters.
How to use this calculator
- Enter the measured or specified L1 at distance r1.
- Enter your target distance r2, and pick the spreading model.
- If needed, add directivity using Q or DI.
- Optional: set absorption α and other loss terms.
- Press Calculate to view results, table, and exports.
Noise distance attenuation guide
1) What distance attenuation means
Distance attenuation is the reduction in sound level as a wave spreads away from the source. In open air, the same acoustic energy is distributed over a larger area, so the measured sound pressure level drops. This calculator treats that drop as a predictable geometric effect, then adds optional real‑world losses. It is ideal for early design studies and quick field estimates.
2) Point, line, and plane assumptions
A point source radiates roughly in all directions, so it follows spherical spreading. A long road, conveyor, or rail line can behave more like a line source, where energy spreads outward in a cylinder. Very large, flat wavefronts approximate plane waves. Choosing the right model prevents over‑ or under‑estimating.
3) Fast check rules you can remember
For quick checks, remember the common rule: for a point source, doubling distance reduces level by about 6.02 dB. Tripling distance reduces it by about 9.54 dB. For a line source, doubling distance reduces level by about 3.01 dB. Use these rules to sanity‑check outputs.
4) Keep L1 and r1 as a matched pair
The reference level L1 must match the reference distance r1. If you measured 85 dB at 1 m, keep r1 at 1 m. If your specification is 85 dB at 10 m, set r1 to 10 m. Mixing the pair shifts every result and can hide true changes from barriers or absorption.
5) Directivity matters in the main beam
Some sources are directional, like loudspeakers, exhaust outlets, or fans with housings. Directivity is captured using Q or DI and is applied before distance losses. If you are standing in the main beam, DI can raise the effective L1. Off‑axis measurements should use a lower DI to avoid optimism.
6) Atmospheric absorption adds “air loss”
Atmospheric absorption increases with path length and depends strongly on frequency, humidity, and temperature. In broadband planning, α is often small, but at long ranges or higher frequencies it can matter. Use α to model air loss beyond pure spreading, especially when r2 is far larger than r1.
7) Ground and barriers can help or hurt
Ground and terrain can either reduce or increase levels because of interference between direct and reflected paths. Soft ground can add extra attenuation at mid to high frequencies, while hard ground can reinforce sound. Barriers add insertion loss when they block line‑of‑sight. Use separate terms to test scenarios quickly.
8) Use the distance table for planning
Use the distance table to create a simple compliance map. Start with a conservative L1, pick the model, then add realistic barrier and ground values. Compare predicted L2 against targets like daytime limits, worker exposure, or neighborhood comfort. When stakes are high, confirm with a meter on site.
FAQs
Does noise always drop with distance?
In free‑field conditions it usually drops, but reflections, wind, terrain, and temperature layers can change the trend. Use the result as a baseline, then validate at critical points with measurements.
Why does doubling distance give about 6 dB loss?
For a point source, sound energy spreads over the surface of a sphere. Doubling distance increases area four times, so level drops by 10·log10(4) ≈ 6.02 dB.
When should I use the line source option?
Use it for long, continuous sources where the length is large compared with your distances, such as highways, busy rail lines, or long industrial conveyors. It often matches mid‑field behavior better than point spreading.
What if I only know Q or only DI?
Select the mode you have. Q and DI describe the same directivity: DI = 10·log10(Q). The calculator converts internally so you can work with whichever value is available in your datasheet.
How do I choose atmospheric absorption α?
If you do not have frequency‑based data, start with α = 0 for short ranges. For long ranges or high‑frequency noise, try a small value and see sensitivity. When possible, use a value from a standard model or measurement.
Can ground effect be negative?
Yes. Hard surfaces can reinforce certain frequencies and increase measured level at a receiver. Enter a negative ground effect to represent that reinforcement, and a positive value to represent extra attenuation over softer ground.
Is this the same as indoor sound reduction?
No. This tool focuses on outdoor distance loss and optional path effects. Indoor results depend on room reflections, absorption, and building transmission. For indoor problems, use a reverberation or transmission calculator instead.