Work out noise barrier height instantly with clear inputs, formula shown and shareable results.
A noise barrier works by forcing sound to diffract over its top, lengthening the path. The Fresnel number, based on the extra path length, determines insertion loss. Breaking the line of sight gives about 5 dB; each doubling of the effective height above the sight line adds roughly 3 dB, with 10-15 dB a practical maximum.
Path difference
delta = source-to-top + top-to-receiver - direct distance
Fresnel number and insertion loss
N = 2 delta / wavelength; IL = 5 + 20 log10(sqrt(2 pi N) / tanh(sqrt(2 pi N)))
Noise assessment for planning or permitting must follow the applicable standard with measured or modelled source levels. Barrier design should be verified by an acoustic consultant.
Because sound diffracts around the ends of the barrier and reflects off other surfaces, so beyond about 15 dB those flanking paths dominate and further height gains nothing.
Only that it is sufficiently massive and has no gaps — typically 10 kg/m2 or more. Absorptive facing helps only where reflections between parallel barriers are a problem.