Road traffic noise shielding by vegetation belts of limited depth

Road traffic noise propagation through a vegetation belt of limited depth (15 m) containing periodically arranged trees along a road is numerically assessed by means of 3D finite-difference time-domain (FDTD) calculations. The computational cost is reduced by only modeling a representative strip of the planting scheme and assuming periodic extension by applying mirror planes. With increasing tree stem diameter and decreasing spacing, traffic noise insertion loss is predicted to be more pronounced for each planting scheme considered (simple cubic, rectangular, triangular and face-centered cubic). For rectangular schemes, the spacing parallel to the road axis is predicted to be the determining parameter for the acoustic performance. Significant noise reduction is predicted to occur for a tree spacing of less than 3 m and a tree stem diameter of more than 0.11 m. This positive effect comes on top of the increase in ground effect (near 3 dBA for a light vehicle at 70 km/h) when compared to sound propagation over grassland. The noise reducing effect of the forest floor and the optimized tree belt arrangement are found to be of similar importance in the calculations performed. The effect of shrubs with typical above-ground biomass is estimated to be at maximum 2 dBA in the uniform scattering approach applied for a light vehicle at 70 km/h. Downward scattering from tree crowns is predicted to be smaller than 1 dBA for a light vehicle at 70 km/h, for various distributions of scattering elements representing the tree crown. The effect of the presence of tree stems, shrubs and tree crowns is predicted to be approximately additive. Inducing some (pseudo)randomness in stem center location, tree diameter, and omitting a limited number of rows with trees seem to hardly affect the insertion loss. These predictions suggest that practically achievable vegetation belts can compete to the noise reducing performance of a classical thin noise barrier (on grassland) with a height of 1–1.5 m (in a non-refracting atmosphere).