High-frequency atmospheric absorption?

ISO 9613-1 at 20C/50%RH: 1.1 dB/km at 500 Hz, 50 dB/km at 8 kHz. Over 200 m: 1 dB at 2 kHz, 10 dB at 8 kHz. Distant sources sound bass-heavy.

Why farther at night?

Night inversions refract sound downward, trapping near surface. Increases levels 10-20 dB at 500+ m versus daytime upward refraction creating shadow zones.

Turbulent mic noise (use windscreen) plus gradient refraction. Downwind: louder. Upwind: shadow. ±10 dB beyond 200 m at moderate speeds.

When inverse square law fails?

Line sources (3 dB/doubling), near surfaces, indoors (reverberant field), large distances (atmospheric effects dominate). Use appropriate model per source type.

High-frequency atmospheric absorption?

ISO 9613-1 at 20C/50%RH: 1.1 dB/km at 500 Hz, 50 dB/km at 8 kHz. Over 200 m: 1 dB at 2 kHz, 10 dB at 8 kHz. Distant sources sound bass-heavy.

Why farther at night?

Night inversions refract sound downward, trapping near surface. Increases levels 10-20 dB at 500+ m versus daytime upward refraction creating shadow zones.

Turbulent mic noise (use windscreen) plus gradient refraction. Downwind: louder. Upwind: shadow. ±10 dB beyond 200 m at moderate speeds.

When inverse square law fails?

Line sources (3 dB/doubling), near surfaces, indoors (reverberant field), large distances (atmospheric effects dominate). Use appropriate model per source type.

Sound Propagation Outdoors — Atmospheric Effects

Sound Propagation Outdoors

Outdoor propagation involves geometric spreading, atmospheric absorption per ISO 9613-1, wind and temperature refraction, ground reflection, and barrier diffraction. These cause levels to differ significantly from free-field beyond 100 metres.

ISO 9613-1:1993ISO 9613-2:1996ISO 1996-2:2017

Geometric Spreading

Point: -6 dB per doubling. Line: -3 dB. Critical for traffic (line) vs machinery (point) predictions.

Atmospheric Absorption

ISO 9613-1 at 20 C, 50% RH: 1.1 dB/km at 500 Hz, 50 dB/km at 8 kHz. Distant thunder = low rumble.

Temperature/Wind

Normal: sound refracts upward (shadow). Inversions (night): downward (+10-20 dB at 500+ m). Wind: downwind louder, upwind shadow. ±10 dB beyond 200 m.

Ground

Hard surfaces: strong reflection. Porous ground absorbs. ISO 9613-2 correction factors.

Measurement

Per ISO 1996-2. Windscreen above 5 m/s. Calm overcast = most repeatable. Multiple sessions improve representativeness.

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