Acoustic effects of green roof systems on a low-profiled structure at street level

Green roof systems have become commonly used in urban spaces due to numerous ecological and environmental benefits. Various kinds of green roof systems can also be used at street level, on the top of underground car parks, for example. In this situation, it is important to systematically examine the acoustic effects of designable parameters, especially for diffracted sound waves. In this study, therefore, a series of measurements were carried out in a semi-anechoic chamber using green roof systems which consist of the Zinco and limestone-based substrates. They were placed on a box with a height of 1200 mm. Numerical simulations were also carried out for selected cases. Studied parameters included the area, depth, type and position of the green roof system, and the type of vegetation. The results show that such green roof systems can reduce SPL effectively at the receiver side of the boxes. Within the ranges of the parameters considered, the effect of the depth and type of substrates is relatively small compared to that of the overall configurations of the system. By adding pruned leaves on the green roof there is only a small noise reduction above 4 kHz but optimised absorption treatment could bring up to 4 dB(A) noise reduction for traffic noise. The position of the green roof system affects the pattern of SPL reduction differently at different frequency ranges.