What it does
A wall, fence or earth bank between a road (or machine) and a house works by making sound travel over the top instead of straight through. The extra distance it has to go, the path difference δ, decides how much quieter it gets. Low notes have long wavelengths and bend over a barrier easily; high notes are screened much better. So the tool works band by band.
For each octave band from 63 Hz to 8 kHz it works out the wavelength λ, the Fresnel number N = 2δ/λ and the insertion loss (how much quieter the band gets). It then adds up the A-weighted bands to give the overall level in dB(A) with and without the barrier.
How to use it
- Cross-section. Heights of the source, barrier top and receiver, measured from the same ground level, and the distances along the ground from source to barrier and barrier to receiver. For a road use the middle of the near lane, about 0.5 m up for tyre noise. For a window, use its height.
- Air temperature. Sets the speed of sound (343 m/s at 20 °C), which changes the wavelengths slightly.
- Method. Kurze–Anderson is a closed-form fit to Maekawa’s measured chart; it gives 5 dB when the barrier just touches the line of sight and a little screening even when the line of sight clears the top by a small amount. Simple uses 10·lg(3 + 20δ/λ), the form found in outdoor sound propagation methods.
- Cap. Real barriers rarely give more than about 20 dB, because sound also bends round the ends and is scattered by the air. The loss in any band is limited to this value.
- Spectrum. The sound level in each octave band at the receiver without the barrier, unweighted. Leave out a band you have no data for.
Worked example
The made-up layout: a source 0.5 m high, 10 m from a 4 m wall; a receiver 1.5 m high, 30 m beyond it. The sound over the top travels 686 mm further than the straight line. At 15 °C, the Fresnel number at 500 Hz is 2.02. With the Kurze–Anderson method the insertion loss runs from 8.4 dB at 63 Hz to the 20 dB cap from 2 kHz up. The made-up road-like spectrum adds up to 76.5 dB(A) without the wall and 59.4 dB(A) with it: the barrier cuts 17.2 dB(A). The simple method gives a 15.2 dB(A) cut.
FAQs
Why does a higher barrier help less than I expected? The loss grows with the logarithm of the path difference, so each extra metre adds less. Moving the barrier closer to the source or the receiver often helps more.
What about the ground? Over soft ground, such as grass, sound near the ground is already quieter, and a barrier takes some of that ground effect away. That can make the real benefit smaller than shown here. The tool assumes the ground effect is the same with and without the barrier.
Does the barrier material matter? Not for diffraction, as long as little sound goes through it: a rule of thumb is a surface mass of at least about 10 kg/m² and no gaps.
Is anything sent to a server? No. It all runs in your browser.
For planning or legal decisions, use the method and limits your local authority requires.
Where the numbers come from
- Path difference and Fresnel number N = 2δ/λ: diffraction over a thin screen, standard acoustics. Barrier attenuation by the Kurze–Anderson closed-form fit to Maekawa's curve (5 + 20·lg(√(2πN)/tanh√(2πN))), or by the simpler 10·lg(3 + 20δ/λ) form used in outdoor propagation methods. Written by fibub from the formulas.
- A-weighting at the octave band centres as usually tabulated from the A-weighting curve, rounded to 0.1 dB.
- Checked in the tests against an independent NumPy implementation. The example spectrum and layout are made up.
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Last updated 2026-11-09.