# Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers
###### Abstract
Indeed, the BEM has proven to be an excellent tool to simulate the scattering of waves around a noise barrier, as it accurately accounts for geometrical details of the structure and allows including different effects, such as the sound absorption of its surface. For example, in [34], noise barriers with complex surface geometries are analyzed, including strong diffusion effects generated by a so-called \"s-QRD\" barrier profile. In a more recent paper, meshless methods, such as the Method of Fundamental Solutions [35], are also used with success. The present paper intends to contribute to the body of knowledge on this topic by analyzing noise barriers with absorbing materials, both from a point of view of the material itself and from the point of view of their simulation, not only as an obstacle, but also including the effect of the material. The specific case of barriers with an absorbing layer of porous concrete including expanded clay is addressed. A methodology is here first developed to derive adequate parameters to model this material using the equivalent fluid Horoshenkov-Swift model. This strategy is based on the measurement of the relevant macroscopic parameters in the laboratory, together with the sound absorption curve (using an impedance tube), and then, using an optimization algorithm to adjust the macroscopic parameter values in order to minimize the difference between the result obtained using the theoretical model and the experimental measurement. Although the strategy is inspired by other works, such as [36], here, the initial approximation is derived from the material characterization and not completely obtained by inversion; the optimization process only allows relatively small variations of those parameters (up to 10%) to allow a better match between the measured and estimated sound absorptions. Normal incidence sound absorption results are then used to predict the diffuse field behavior of the material using different approaches, and results are compared with measurements performed following ISO 354 [18]. An important question that arises is related to the sensitivity of the numerical and engineering models used in environmental noise simulation with respect to the sound absorption values ascribed to the noise barrier surface. Indeed, results obtained in a reverberant chamber can exhibit significant differences from those derived from normal incidence conditions by applying simple expressions such as the classical ones proposed by [PERSON] [19], but the relevance of these differences in simulation models is not really known. Here, a study considering both the CNOSOS-EU calculation method and BEM model are used to address this topic. Since this study has been developed within the scope of the development of railway noise barrier solutions, two types of barriers are analyzed, a classical noise barrier, positioned at a significant distance from the railway track, and a low-height barrier, positioned very close to the track and subsequently to the noise source. Two different shapes are analyzed for each case, corresponding to a vertical barrier and to an L-shaped barrier. For all situations, both purely reflective and absorbing barriers are simulated. The paper is organized as follows: first, the materials and methods are presented, including a description of the used materials and of the test methods adopted for its characterization; results of this characterization are then presented and discussed, considering normal incidence conditions; this is followed by an estimation of the absorption of the tested materials considering diffuse field conditions, using different models and comparing with values measured in a reverberant chamber; finally, numerical simulations considering the application of the porous concrete in low-height and traditional noise barriers are performed.