ACOUSTIC ABSORPTION OF GREEN WALLS MADE OF FOLIAGE AND SOIL SUBSTRATE LAYERS

摘要

Recent works on acoustics of green walls show that their sound absorption may contribute to noise reduction and soundscape shaping in urban areas. In particular, it has been observed that a foliage layer placed above a substrate layer may lead to a significant increase of acoustic absorption coefficient in a broad frequency range. This result has been correlated to leaf area density and leaf orientation but the underlying physical phenomenon remains unclear. To tackle this problem, measurements are carried out in the 100-1000 Hz frequency range on different foliage (spindle or bay) and soil substrate (perlite, coco fiber or coco peat) samples using the three-microphone two-load method to determine: i) acoustic absorption coefficient and surface specific impedance in rigid backing condition; ii) speed of sound and characteristic impedance of the equivalent homogeneous acoustic fluid medium. Former measurements are also performed on composite samples consisting of a foliage layer and a soil substrate layer for various sample thicknesses and foliage densities. For composite layers, a good agreement is found between measured acoustic absorption coefficients and calculated ones using equivalent properties of each individual layers. Analysis of results obtained for different composite samples reveals that the variation of the absorption coefficient versus frequency is mainly governed by i) the thickness resonances of the composite sample and ii) the impedance matching between air and soil substrate provided by foliage which may operate as a quarter-wave transformer.