Effect of a Row of Trees Behind Noise Barriers in Wind

The effect of a row of trees (in leaf) behind a noise barrier in wind is investigated. An experiment was set up along a highway. Measurements at a location with and without a row of trees behind a noise barrier were compared. This continuously monitoring lasted from the middle of the summer till the middle of fall. It is shown that for downwind sound propagation for an orthogonal incident wind, the efficiency of the noise barrier with trees becomes increasingly better compared to the noise barrier without trees, with increasing wind speed. The improvement by the trees is only slightly affected if the wind direction is not perfectly orthogonal to the barrier. Upwind sound propagation is affected only to a small degree by placing trees. Diffraction on the canopy of trees does not result in an increased total A-weighted sound pressure level due to the typical low-frequency spectrum of traffic noise. The contribution of wind-induced vegetation noise to the recorded noise levels can be neglected for highways with common dense traffic.