Optimization of acoustic absorption by green walls made of foliage and substrate

Green walls may absorb sound and contribute to noise reduction in urban areas. Recent experiments demonstrate that a foliage layer placed above a substrate layer may lead to a significant increase of acoustic absorption coefficient in a broad frequency range. However, the physical origin of this improvement remains unclear. In this work, measurements are carried out in an impedance tube on foliage, substrate, and foliage/substrate samples using the three-microphone two-load method. Acoustic absorption coefficient and surface specific impedance are measured in rigid backing condition between 100 and 1000 Hz. Effective speed of sound and characteristic impedance are also experimentally determined for foliage and substrate. For foliage/substrate samples, a good agreement is obtained between measured acoustic absorption coefficients and calculated ones using the effective properties of foliage and substrate layer and matrix manipulations. Analysis of results reveals that absorption coefficient spectrum is mai...