Using vegetation to tackle environmental noise problems - combining exposure level reduction and noise perception improvement

The use of vegetation in the (peri)-urban environment performs a myriad of ecosystem services, and improving the sound climate is one of them. However, well-thought out application is essential to benefit from both the physical noise level reduction and perception improvement green might bring. For surface transport noise sources, the interaction with ground/growing substrates, multiple scattering and absorption by above-ground plant material, and the impact on the micro-climatology are the main physical effects of practical concern. This contrasts with past research focusing largely on the effect of leaves only. The perception improvement by vegetation is often experienced by the population as very strong. In order to understand this, existing research has been scrutinized in view of three potentially explaining mechanisms namely source (in)visibility, the mere presence of visible green, and vegetation as a source of natural sounds. When following the restorative hypothesis of visible vegetation, most consistent conclusions could be drawn. In order to make such perception related improvements more tangible and useful during planning, equivalent sound pressure level reductions were derived for a main environmental noise indicator namely self-reported noise annoyance.