Laboratory study of the effects of green roof systems on noise reduction at street levels for diffracted sound

Along with the strong movement towards sustainable urban environments, there is an increasing use of green roof systems, including the semi-extensive green roofs, which are composed of lightweight layers of freedraining material that support low-growing, tough, and drought-resistant vegetation. Various kinds of green roof systems are also used at street levels, on the top of underground car parks, for example. To predict the sound fields in street canyons, it is important to demonstrate the acoustic effect of such green roof systems, especially considering diffracted sound waves. In this study, therefore, a series of measurements of sound pressure level were carried out in a semi-anechoic chamber using 20 green roof trays which consist of the Zinco substrate with a depth of 100mm and low-growing vegetation. To consider a low-profiled structure in urban areas, a box with a height of 1200mm was located on the ground, and the trays were installed on the top of the box. As experimental parameters to verify the effects of the green roof system on noise abatement for diffracted sound waves, the structure, area and position of the green roof system, and the type of vegetation were considered. The experimental results demonstrate that the green roof system at street levels can be effectively used to mitigate noise in urban spaces for diffracted sound waves.