Outdoor noise control by natural/sustainable materials in urban areas

This study explores the effects of natural and sustainable materials including vegetation, green roof systems and green walls on outdoor noise control in urban areas. The concept of this study starts with a hypothesis that well-planned use of the natural materials on building and urban surfaces can achieve useful reductions in noise levels and reverberation in urban spaces. Firstly, this study examines random-incidence absorption and scattering coefficients of vegetation through a series of measurements in a reverberation chamber in order to characterise the effects of various designable factors such as soil depths, soil water content and vegetation densities. This data is used later in acoustic computer simulations. To quantify the scattering effect of trees and to allow including it in numerical predictions, a series of measurements are carried out for individual trees in an open field. Green roof systems are placed on a low profiled structure to examine sound transmission through the vegetated low barrier. To suggest noise abatement schemes in relatively small urban spaces, the acoustic effects of landscape designs using vegetation in a courtyard are studied through a case study. As a preliminary study on the noise reduction effect of vegetation in relatively large urban spaces, field measurements are carried out at outdoor spaces in high-rise apartment complexes. Based on the measurement results, the noise reduction effect of vegetation in apartment complexes is also predicted. The overall results for each research topic can be summarised as follows: It is shown that low-growing vegetation can be an effective measure for absorbing/scattering sound energy, especially at high frequencies. Results of field measurements show that tree reverberation exerts an influence only on frequencies above 1 kHz. At 4 kHz, RT (reverberation time) can be as long as 0.28 sec. Measurements made near the same deciduous tree with and without leaves indicate that leaves increase reverberation at 4 kHz by 0.08 sec. The results on sound transmission over the low barrier with green roof systems suggest that SPL attenuation increases with the increasing green roof areas. The extra SPL (sound pressure level) attenuation caused by green roof systems could be up to 9.5 dB at certain frequencies. With well-planned application of landscape designs in a courtyard, speech levels and RT at 500 Hz are decreased by 9.3 dBA and 81 %, respectively. In outdoor spaces of apartment complexes, RT is generally rather long, over 4 sec at 500 Hz, influenced by many factors such as openness, source-receiver distance and building height. In terms of SPL distribution, the measured SPL is up to 8 dB higher compared to the semi-free field situation. It is also found that vegetation at the apartment complex can be effective in reducing RT by 0.95 sec (46 % decrease).