Experimental Characterization of Foliage and Substrate Samples by the Three-Microphone Two-Load Method

In addition to their aesthetic aspect, green walls can help decreasing noise pollution in the environment due to their absorption properties. Green walls mainly contain two components: the substrate which feeds the plants and the foliage. Some authors have shown that, for a given leaf density on a specific substrate, acoustic absorption can be improved with respect to the bare substrate. Although acoustic measurements have already been carried out on leaves and substrates, few studies have given information about the effective characteristics (bulk modulus, celerity, characteristic impedance, etc.) and transmission loss of substrate and foliage. Here, we propose to determine those characteristics from measurements performed in an impedance tube, with the three-microphone two-load method. This method consists in measuring transfer functions between three microphones, two of them located upstream of the substrate and the third one downstream, mounted on a movable hard termination. From the measured transfer functions, it is possible to establish the transfer matrix of the sample, which allows to determine its effective properties.The first part of this paper presents the results obtained on foliage as a function of frequency, sample thickness and foliar density, and the second part presents the results obtained on different substrates.