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Predict how a measured noise level falls off with distance — 6 dB per doubling for a point source, 3 dB per doubling for a line source — using the ISO 9613-2 geometric divergence term. Add an optional excess-attenuation allowance to approximate ground, barriers, and air.
Enter a known level and the distance it was observed at, then the distance you want to predict. Distances just need to share a unit (ft or m) — they cancel in the ratio.
Point source · spherical spreading · 6 dB per doubling
Predicted level at multiples of the reference distance
| Distance | Level |
|---|---|
| 2× ref (100) | 84.0 dBA |
| 4× ref (200) | 78.0 dBA |
| 8× ref (400) | 71.9 dBA |
Worked example: A point source reads 90 dBA at 50 ft. At 200 ft the distance ratio is 200 / 50 = 4, so ΔL = −20 · log₁₀(4) = −20 · 0.602 = −12.0 dB, giving a predicted level of 90 − 12.0 = 78.0 dBA. Each doubling of distance (50 → 100 → 200 ft) removes another 6 dB. A line source over the same span would lose only 3 dB per doubling (−6.0 dB total). This estimate covers spreading loss alone — a modeled ISO 9613-2 prediction adds ground, barrier, and atmospheric terms.
For an ideal point source radiating into free space, sound pressure level drops 6 dB for every doubling of distance — the inverse-square law written in decibels. A line source such as a busy road, a long pipe rack, or a row of equipment drops about 3 dB per doubling. This sound level distance calculator applies the ISO 9613-2 geometric divergence term for both cases, so you can estimate noise attenuation over distance from a single known measurement.
It comes straight from the point-source term −20 · log₁₀(d₂ / d₁). Doubling the distance makes d₂ / d₁ = 2, and −20 · log₁₀(2) = −6.02 dB. So every time you move twice as far from a point source, the level falls by roughly 6 dB. For a line source the coefficient is 10 instead of 20, which produces the 3 dB per doubling figure.
No — it models geometric divergence only. Real noise attenuation over distance is also shaped by ground effect, barriers and screening, atmospheric absorption, and wind and temperature gradients, which the full ISO 9613-2 procedure adds as separate terms and which can shift the result by several decibels either way. Use the excess-attenuation field for a rough allowance, and contact CSTI for a fully modeled octave-band prediction.