The urban fabric largely consists of acoustically rigid materials. This not only affects sound pressure levels in streets, but also how sound propagates towards the back side of a building or to connected urban canyons. A green roof is a practical solution to have roof absorption, mitigating diffracting sound waves. Flat green roofs were shown to provide roughly 3 dBA urban road traffic noise reduction relative to a common flat rigid roof. Although already relevant, it has been numerically studied in this work how the green roof insertion loss of flat roofs can be further increased. Solar panels on green roofs were found to significantly decrease sound pressure levels at the shielded building facade, up to 5 dBA on top of the insertion loss of granular substrates. Polyurethane foam slabs as green roof substrates provide relevant shielding when placed on a series of hollow trapezium-like cores of sufficient height.
KEYWORDSSound propagation, building skin, building envelope greening, green roofs, sustainable buildings, quiet side
1.INTRODUCTIONStreet surfaces and building facades most often consist of acoustically rigid materials like glass, concrete, tiles, bricks, pavements, and so on. As a consequence, there is often a strong increase in sound pressure level due to the multiple reflections in between opposing building facades and on the street surface. Heutschi 1 proposed a simplified model for the so-called "building correction" for typical street geometries based on a ray-tracing numerical model. Parameters that were identified as relevant were the height of the facades, width of the street (canyon), the absorption coefficient of the facades, the degree of diffusion and source/receiver positioning. Thomas et al. 2 proposed the "reflection ratio", an impulse response based indicator, which was measured in 99 real streets in a city centre. The main influencing parameters were street width, facade height and facade roughness. The large number of reflections inside the street clearly affect the sound intensity incident on the directly exposed facade of a building. Although increasing the acoustic insulation of a facade might be helpful to mitigate noise issues, window opening habits might strongly deteriorate the overall facade insulation and consequently increases the noise levels to which dwellers are exposed indoors 3,4 . Although one might opt for windows that cannot be opened or try to minimize their surfaces, such choices can conflict with preferences and a lower overall perceived comfort. There is a bulk of research [5][6][7][8][9] showing that there are various ways to reduce this street amplification. There are two major directions to follow namely increasing the diffusivity of the facades or increasing the absorption rate of the materials used (in a fixed geometry), similar to what is commonly done in room acoustic design.