Numerical evaluation of sound propagating over green roofs

Sound propagation over intensive and extensive green roofs was numerically studied by using the finite-difference time-domain method. The Zwikker and Kosten model was used to simulate sound propagation in the substrate layer itself. The presence of a green roof is mainly interesting in it street canyon configuration, and fits well in the concept of quiet sides. Positive effects of green roofs, relative to fully rigid roofs, are mainly observed at the octave bands with centre frequencies 500 and 1000 Hz. The source type was shown to be unimportant when considering this particular parameter. There is a linear relationship between the fraction of the roof covered with green and the decrease in sound pressure level at the nonexposed canyon. The slopes increase with octave band centre frequency. The width-height ratio of the street canyon configuration has only a limited effect. For extensive green roofs, a pronounced attenuation peak is found when varying the layer thickness, leading to a maximum reduction of up to 10dB, relative to an acoustically rigid roof, for the octave band of 1000 Hz. A good overall efficiency is observed near the maximum layer thickness (15-20 cm) for this type of green roof. For an intensive green roof with it substrate layer thickness exceeding 20cm, which is common, positive effects are not influenced anymore by substrate thickness. (c) 2008 Elsevier Ltd. All rights reserved.