In-situ measurements of sound propagating over extensive green roofs

In this study, in-situ measurements of sound propagating over flat, extensive green roofs are presented for 5 cases. Measurements were performed just before and just after the placement of the green roof (under dry conditions) with an identical source-receiver configuration in both situations, allowing a direct estimate of the acoustical effect. Situations involving a single and double diffraction over the green roof were considered, for substrate thicknesses ranging from 20–30 mm to 180 mm, and for vegetation cover ranging from absence to 100%. The green roof acoustic effect was analyzed for propagation path lengths interacting with the roofs ranging from 2.5 m to 25 m. Measurements show that green roofs might lead to consistent and significant sound reduction at locations where only diffracted sound waves arrive relative to common, non-vegetated roofs. A single diffraction case with an acoustic green roof improvement exceeding 10 dB was found for sound frequencies between 400 Hz and 1250 Hz, although the green roof interaction path length was only 4.5 m. For less shielded receivers, a change in interference pattern might be observed, leading to positive or negative effects, relative to a non-vegetated roof top. For the double diffraction cases the green roof improvement is less frequency-dependent and a case with positive effects up to 10 dB was found.