# Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers
###### Abstract  
Noise barriers are common noise mitigation measures usually implemented near roads or railways, with proven efficiency. This work presents a study of a porous concrete material incorporating expanded clay as aggregate, to be used on the sound-absorption layer of noise barriers. A theoretical material model is calibrated using experimental data and then used to estimate the diffuse field sound absorption from the normal incidence sound absorption estimation/measurement. Validation of such estimation is performed by comparing to reverberant room measurements. Numerical simulations are carried out using the boundary element method (BEM) and the CNOSOS-EU calculation method to assess the performance of different types of barriers incorporating this material. L-shaped and vertical barriers are tested, as well as low-height and conventional (taller) barriers, employed in the context of a railway noise scenario. Different results are obtained by the two methods, mainly due to the different underlying physical principles. Good insertion loss values may be obtained using both conventional and low-height noise barriers together with the porous concrete material if a careful choice of its location within the barrier is made. noise barriers; porous concrete; absorption coefficient; insertion loss; boundary elements; CNOSOS-EU +
Footnote †: journal:  
rolling noise is velocity dependent (V) at a rate of 30 log\({}_{10}\) V (typically for speeds between 60-280 km/h), which means that the sound level increases with increasing velocity [6]. Mitigation of this noise source is an important concern to reduce environmental noise levels and indeed to allow the railway infrastructure to operate to its full potential, and thus one of the most relevant mitigation measures consists of the application of noise barriers next to the railway track. Noise barriers can be made of different types of materials, and can have different geometries or heights, among other variable parameters [7; 8; 9; 10; 11; 12]. Absorbing barriers, as well as diffuser systems, can have an active contribution to solving two particular problems of noise barriers, double reflections from high-sided vehicles and barriers' reflections on one side of the way [13]. The addition of absorbing systems in a noise barrier has been proven to be very effective. [PERSON] et al. [14] studied noise barriers for the railway to mitigate sounds whose sources are in the lower part of the locomotive. According to this work, the insertion loss values for noise barriers with an absorbing surface were about 6 to 10 dB higher than similar barriers without an absorbing surface. The application of absorbing areas in rigid barriers increases the insertion loss within a range between 3 and 6 dB. Consequently, the least efficient barrier was a corrugated barrier with a rigid surface. In contrast, the best-performing barriers were the flat and curved barriers with absorbing surfaces and a multi-edge barrier containing a partly surface absorbing. Due to the relevance of using an absorbing layer, in this work, porous materials were studied to be incorporated on the face of the acoustic barrier exposed to railway noise. Excellent qualities, such as high workability, low density, and sufficient mechanical resistance, may be found in porous concrete with light aggregates and an adequate amount of air. It can also be used to make architectural elements with appropriate acoustic and thermal behavior [15]. A porous material is made up of a solid structure containing cavities, tunnels or pores filled with air, and the interaction of sound waves with these two phases causes sound energy dissipation [13; 16]. Consequently, a possible approach to study sound propagation in a rigid-frame porous material is to model the fluid-filled medium as a homogeneous fluid with complex, frequency-dependent characteristic impedance and propagation constant. Several experimental and theoretical approaches may be used to study porous materials.