Experimental evaluation of the influence of water content on the acoustic properties of natural materials

Recently several studies have demonstrated that materials of natural origin have thermal and acoustic performance comparable to those of conventional materials, and, in addition, they generate a much lower environmental footprint. However, all natural materials, and in particular materials of vegetal origin, have a major enemy, which is water: environments with high moisture lead to the creation of moulds and fungi and to the deterioration of the materials by means of biological decomposition. Whereas thermal properties of natural materials have already been investigated in some researches, the authors are not aware of any study analysing the influence of water content on the acoustic properties of natural materials. Therefore, the present paper reports the results of an experimental campaign devoted to the measurement of the sound absorption coefficient of two natural materials, namely straw and reed, in order to understand how the water content of these materials may affect their acoustic performance. Normal incidence sound absorption coefficient was measured by means of an impedance tube. Loose straw and two configurations of reed, one with the stems parallel and one perpendicular to the direction of the sound waves inside the tube, were investigated, each forming samples with thickness of 5 and 7 centimetres. All the samples were conditioned using an environmental chamber and the water content was measured using the gravimetric method. The results show that for straw and for the longitudinal configuration of reeds there is a good correlation between the water content and the value of the maxima of the sound absorption, even if the global change in sound absorption change is limited. On the contrary, in the case of the perpendicular configuration of reed, the change of water content leads to substantial variations of the trend of sound absorption coefficient with the frequency.