Influence of rainfall on the noise shielding by a green roof

The potential of a green roof to reduce diffracting sound waves towards a shielded side of a building has been assessed before by both in-situ and laboratory experiments, and by numerical simulations. However, like any porous material, the acoustic performance of the green roof's substrate could suffer from the presence of water. A 46-day lasting controlled sound propagation experiment was set up near the edge of a 6-m tall building equipped with an extensive green roof, having a substrate thickness of 7 cm. At selected moments, test signals were emitted, allowing to monitor the attenuation between a reference microphone and a microphone at low height positioned on the green roof (at 15.3 m from the roof's edge). Meteorological parameters and the green roof's substrate moisture content were continuously measured. Sound diffracting over a green roof showed to be sensitive to the substrate moisture content in a specific sound frequency range, more precisely between 250 Hz and 1250 Hz. The difference in noise attenuation between a rather dry state (0.1 m3/m3) of the substrate and the maximum observed volumetric water content (close to saturation, 0.33 m3/m3) could range up to 10 dB. However, calculations show that the impact of the water content in an extensive green roof substrate for the specific case of road traffic noise abatement is expected to be limited.