Question 1

How does SPL decay differ between point sources and line sources?

Accepted Answer

For a point source (e.g., an isolated machine), SPL decreases by 6 dB every time the distance doubles — the inverse square law: ΔL = 20 log(r/r₀). For a line source (e.g., a long highway with continuous traffic flow), SPL decreases by only 3 dB per doubling of distance: ΔL = 10 log(r/r₀). Real road traffic often behaves intermediate between point and line source depending on traffic density and measurement distance.

Question 2

Under what conditions is atmospheric absorption most significant?

Accepted Answer

Atmospheric absorption (ISO 9613-1) increases with frequency and is largest at low temperatures. Relative humidity shows a peak absorption around 40–60%. At 4 kHz, absorption can reach several dB per 100 m, while at 125 Hz it is nearly negligible. For long propagation distances (>500 m), always include atmospheric absorption for high-frequency components in your noise model.

Question 3

How is the Maekawa barrier insertion loss calculated?

Accepted Answer

The Maekawa formula gives IL = 10 log(3 + 20N), where N is the Fresnel number N = 2δ/λ (δ = path length difference between diffracted and direct paths, λ = wavelength). For example, with δ = 0.5 m and f = 500 Hz: N ≈ 1.47, IL ≈ 15 dB. Taller barriers and receivers closer to the barrier give larger δ and thus greater insertion loss.

Question 4

When should I use more advanced models than ISO 9613-2?

Accepted Answer

ISO 9613-2 is designed for moderate distances (10–1000 m) over flat or gently rolling terrain under standard meteorological conditions. For long distances, complex terrain, wind and temperature gradient effects, or urban canyon acoustics, consider Nord2000, CNOSSOS-EU, or Harmonoise. For detailed near-field diffraction and room acoustics, FEM/BEM solvers in CAE software provide higher fidelity results.

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Theory Notes