How Distance Attenuation Works
Any wave or radiation spreading out from a source loses strength with distance simply because the same energy is spread over a larger area — no absorption required. For a small ("point") source radiating equally in all directions, that area grows with the square of the distance, so sound pressure level, light intensity, or radio-signal power all fall off following the same inverse-square law. This calculator converts a known level at a reference distance into the level at any other distance, using the standard decibel form of that law, and lets you add a source type and an extra loss term for more realistic outdoor scenarios.
The inverse-square law and the decibel formula
For a point source in a free field (no reflecting surfaces), intensity I is proportional to 1/d². Because sound pressure level is defined as Lp = 20·log₁₀(p/p₀), and pressure is proportional to √I, the level at distance d₂ relative to a known level at d₁ works out to Lp₂ = Lp₁ − 20·log₁₀(d₂/d₁). Doubling the distance (d₂ = 2d₁) gives a drop of 20·log₁₀(2) ≈ 6.02 dB — the well-known "6 dB rule." The same inverse-square math applies to any point-source quantity, including illuminance (lux) and radiated power density (W/m²).
Choosing point vs. line source
Not every source is a point. A long, continuous source — a busy highway, a pipeline, or a conveyor line — radiates more like a cylinder than a sphere, so its intensity falls off as 1/d instead of 1/d². That gives Lp₂ = Lp₁ − 10·log₁₀(d₂/d₁), a drop of only 10·log₁₀(2) ≈ 3.01 dB per doubling of distance — line sources "carry" much farther than point sources for the same source strength. Pick "line source" only when the source is genuinely long relative to your measurement distance; otherwise the point-source formula is the correct default.
Adding real-world losses
The formulas above capture geometric spreading only — the loss you'd see in an open field with no wind, humidity, or obstacles. Real outdoor propagation also loses energy to atmospheric (air) absorption, which grows with distance, frequency, and dryness; to ground effect from reflecting or absorbing terrain; and to barriers, walls, or vegetation. Standards such as ISO 9613-2 give detailed methods for estimating each term. Rather than model each one individually, this calculator's "Additional attenuation" field lets you add a single lump-sum estimate (in dB) for all of those extra losses combined, on top of the geometric-spreading result.