Extensive green roofs for noise abatement: Combined acoustic effects of substrate and vegetation in a 3D environment

While it is known that the substrates of green roofs (GRs) can contribute to noise reduction, the combined acoustic effects of substrates and vegetation are underexplored. In this study, by comparing the noise levels we measured with a bare roof and an extensive green roof (EGR) in a 3D environment, the noise abatement of EGR was evaluated. The impacts of vegetation characteristics and soil moisture on the acoustic performance of EGR were also analyzed. It was observed that the noise of visitor activities on and around the EGR could be reduced by 2.1 to 4.3 dBA and 0.6 to 1.2 dBA respectively, and there was no significant reduction at the inside of the covered EGR area. After establishing a quadratic function (R2 = 0.93) to determine the optimal radius representing the effective range of EGR at a specific receiver point, a prediction model, which can achieve strong predictive performances (R2 = 0.90, RMSE = 0.76, MSE = 0.57, and MAR = 0.30), was developed by incorporating vegetation characteristics, receiver locations, and soil moisture contents within impact areas. Permutation feature importance and structural equation modeling revealed comparable contributions to noise abatement from the vegetation layer (0.18) and the 10 cm substrate layer (0.19), and the gap plays a critical role with a path coefficient of 0.79. Therefore, we suggest a mixed vegetation planting strategy with drought-tolerant plants to minimize vegetation gaps and increase soil porosities to enhance the noise reduction potential. These findings can provide valuable insights for urban planners and policymakers in strategically implementing EGRs to reduce urban noise.