By Timothy Van Renterghema, Maarten Hornikxb, Jens Forssenc, and Dick Botteldoorena
Belgium (March 2013) – According to new research out of the EU, trees and plants can play a role in helping to quiet areas in cities and towns by softening the urban environment and reducing noise. Researchers found, in particular, that green roofs have the potential to significantly reduce road traffic noise in the urban environment. The results suggest that greening of roofs and walls with materials suitable for growing plants softens the urban environment keeping sound levels low, whereas hard, man-made structures tend to amplify traffic noise.
In the study, “The Potential of Building Envelope Greening to Achieve Quietness,” researchers investigated what type of greening produced the greatest benefit in terms of reducing noise in places that were already of some value as ‘quiet areas’. They considered green roofs, green facade walls on the fronts of buildings and low, vegetated screens at the edges of flat roofs.
Researchers looked at road traffic noise propagation towards the traffic-free sides of inner-city buildings (courtyards), where preserving quietness has been shown to be beneficial for the health and well-being of citizens. They then simulated how sound made by cars traveling at different speeds would be transmitted to enclosed courtyards shielded from the road by buildings. These simulated models looked at how sound travels in both two- and three-dimensions.
Study results show that green roofs have the highest potential to enhance quietness in courtyards, and may be able to reduce noise by up to 7.5 decibels. Noise reduction was smaller for green façade walls. Favorable combinations of roof shape and green roofs have been identified. Vegetated façades are most efficient when applied to narrow city canyons with otherwise acoustically hard façade materials.
Greening of the upper level in the street and (full) façades in the courtyard itself is most efficient to achieve noise reduction. The combination of different greening measures results in a lower combined effect than when the separate effects would have been linearly added. The combination of green roofs or wall vegetation with roof screens seems most useful.
The noise reduction was smaller for green facade walls, and depended on the materials used in the adjacent street. For example, the harder the bricks in buildings on the street, the greater the reduction in noise in the roadside courtyard. The model also predicted that green facade walls would be best positioned high up on the walls surrounding the courtyard, unless the materials used for buildings in the nearby street are softer, in which case the facades would be better positioned around the courtyard itself.
According to the researchers, greening could be used to limit noise from other sources, such as air conditioning units, although the current study focuses solely on traffic noise. Trees and vegetation also have other important environmental benefits, such as absorbing carbon dioxide, improving air quality, reducing the urban heat island effect, increasing urban biodiversity and making streets and roofs look more attractive.