Trees to improve noise barrier efficiency in wind
Trees as windbreaks
One could possibly improve the efficiency of a noise barrier by introducing an aerodynamic design. The effects reported so far of such measures are limited. Another solution, not focusing on the barrier itself, is investigated thoroughly. A row of trees behind the barrier can act as a windbreak. Such a measure is relatively inexpensive, it can be used to treat existing noise barrier configurations, and additional psycho-acoustic effects can be expected. Windbreaks actually reduce wind speeds; as a result, turbulent scattering into the acoustic shadow zone is counteracted as well.
The study consisted in a wind tunnel experiment at scale (Act. Ac. Ac. 88, 231-238, 2002), in a field monitoring (Act. Ac. Ac. 88, 869-878, 2002) and in numerical calculations to optimize parameters involved, by combining CFD analysis and FDTD calculations (Act. Ac. Ac. 89, 764-778, 2003). The use of a vegetation screen behind a noise barrier was considered to be a good solution to counteract screen-induced refraction of sound by wind.
The positive effect of a row of trees is not limited to the strict downwind direction. For wind directions up to 45 degrees away from the orthogonal direction, only less than 1 dBA of the net efficiency of the trees was lost in a realistic traffic situation. In case of upwind sound propagation, one does not obtain a significant worse situation.
The presence of a row of trees behind a noise barrier results in increased sound pressure levels at high frequencies due to scattering on the canopy of the trees. Typical traffic noise (i.e. main application of noise barriers) however produces only a small amount of acoustic energy in the high-frequency range relative to lower frequency bands. So the contribution of this scattered sound to the total A-weighted sound pressure level is small.
The porosity of the row of trees, wind speed, and the distance between the noise source and the barriers were shown to be important parameters to quantify the efficiency of the windbreaks. The position of the trees relative to the noise barriers was important as well. It was found e.g. that gaps between the top of the barrier and the canopy should be avoided.
|Wind field near a single noise barrier. The horizontal component of the wind velocity is shown (in m/s). The large gradients in the wind speed above the acoustic shadow zone of the barrier are clearly visible.|
|The positive effect of a row of trees behind a traffic noise barrier increases significantly with wind speed in case of downwind sound propagation. This was found based on a field monitoring (Act. Ac. Ac. 88, 869-878, 2002). A direct comparison of the sound pressure level between part of a noise barrier with (B) and without (A) a row of trees was made, at close distance. For a wind speed of 11 m/s, about 4dBA was gained by the presence of the trees.|