Environmental Noise Assessment and Modeling
This domain tests environmental descriptors, propagation modelling and the assumptions that quietly determine a predicted level.
Core rules
- ISO 1996 descriptors: LAeq,T, plus Lden, which adds 5 dB to the evening period (19:00–23:00) and 10 dB to the night period (23:00–07:00), and Lnight.
- Statistical levels L10, L50 and L90 describe the level exceeded for that percentage of the time; L90 is often taken as the residual or background level.
- ISO 9613-2 computes octave-band attenuation as the sum of geometric divergence, atmospheric absorption (ISO 9613-1), ground effect, barriers and other terms, with a meteorological correction applied afterwards.
- Divergence is 6 dB per doubling of distance for a point source and 3 dB per doubling for a coherent line source such as a road; real roads transition between the two.
- Barrier insertion loss follows Maekawa: Fresnel number N = 2δ/λ, where δ is the path difference; IL = 10 log10(3 + 20N) for N > 0.
Worked example
A barrier gives a path difference of 0.5 m at 500 Hz. Then λ = 343/500 = 0.686 m, N = 2(0.5)/0.686 = 1.46, and IL = 10 log10(3 + 29.2) = 10 log10(32.2) = 15.1 dB. That is an idealised value; diffraction around the ends and transmission through the panel reduce it in practice.
Traps that cost marks
- Omitting the evening and night weightings in Lden, or comparing an Lden value with an LAeq limit.
- Quoting a barrier design that does not break the source–receiver line of sight.
- Ignoring a tonality or impulsivity penalty where the standard requires one.
- Treating a modelled level as measured, or extrapolating beyond a model's validated range.