Sound propagation through forests and tree belts

The potential use of forests or narrow belts of trees alongside surface transport corridors to reduce noise is often dismissed. This may be a consequence of conflicting experimental evidence and incomplete understanding of the various attenuation mechanisms involved. Important mechanisms include (a) destructive interference between sound travelling directly between source and receiver and reflected from the ‘acoustically-soft’ ground formed by decaying leaf litter, (b) the influence on this interference of loss of coherence due to the reverberant scattering by trunks and branches and (c) visco-thermal scattering by foliage. First the paper lists experimental evidence of significant attenuation due to forests and tree belts. Subsequently models for predicting the various contributions to overall attenuation are outlined. Predictions of the ‘soft’ ground effect are made using physically admissible ground impedance models. Incoherence due to trunk and branch scattering is modelled as enhanced turbulence. An empirical formula involving leaf area density and mean leaf size is used to predict foliage attenuation. Predictions that sum these contributions are compared with data. Regular or near-regular tree planting can cause ‘sonic crystal’ effects but the relatively sparse distributions of scatterers in realistic tree planting schemes means that the first band gap due to the periodic structure is weak. On the other hand the first pass band may be reduced and the second band gap can be enhanced by perturbing the tree locations with respect to periodic spacing. Finally results of numerical simulations showing the potential for traffic noise reduction by narrow tree belts are outlined