# Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers
## 4 Conclusions  
The main objective of this work was to assess the efficiency of acoustic barriers incorporating a sound-absorbing material made of porous concrete in the railway context, defining adequate material models and testing different calculation methodologies.  
Figure 16: Insertion loss curves calculated with the BEM methodology, for reflective barriers (\(\alpha\) 0.01 and \(\alpha\) 0.01 L) and for barriers with an absorbing porous concrete layer (ISO 354 and ISO 354 L), with heights of: (**a**) 1.5 m and (**b**) 4 m.  
Figure 17: Insertion loss curves calculated with the BEM methodology, with the porous concrete sound absorption derived from ISO 354 experimental test, and London 1 and London 2 equations, for noise barriers with heights of: (**a**) 1.5 m and (**b**) 4 m. Dashed curves correspond to L-shaped cantilevered barrier, and continuous lines to a vertical barrier.  
For the definition of the material itself, the effect of the Aggregate/Cement ratio and thickness of the absorbing porous layer on the solution's acoustic behavior was first studied, and the obtained results indicated that, within moderate values of the cement content, the open porosity of the material is still effective. A proposal of an inversion technique to obtain macroscopic parameters required by the acoustic material model was presented, with an optimization algorithm which uses the first approximation macroscopic parameters measured in the laboratory with good results on several mixtures; cross-validation with samples with different thicknesses was performed, with acceptable results. Diffuse field sound-absorption curves computed from normal incidence coefficients using simple expressions proposed by [PERSON] allowed to obtain good correlation with large specimens measured in a reverberant room under diffuse field conditions.  
Several scenarios of noise barriers were simulated, taking into account different barrier shapes and with and without absorbing porous surfaces. Significant differences were found between the two calculation methods used here, namely the CNOSSOS-EU and BEM models in the frequency domain, with the first providing usually lower predictions of the insertion loss acoustic performance. The use of the BEM numerical method allowed for a more detailed analysis of the effect of acoustically absorbing materials. A relevant observation was related to the inclusion of absorbing material on the top surface of L-shaped cantilevered barriers, which seems to allow for a significant additional insertion loss acoustic performance of these noise barriers.  
Conceptualization, P.A.-M. and L.G.; Methodology, P.A.-M. and L.G.; Software, L.S., D.M.-G. and L.G.; Formal analysis, L.S., D.M.-G., J.M.B.-M., P.A.-M. and L.G.; Investigation, L.S., L.P. and D.R.; Data curation, L.S. and L.P.; Writing--original draft preparation, L.S., L.P. and L.G.; Writing--review and editing, D.R., D.M.-G., J.M.B.-M. and P.A.-M.; Funding Acquisition, P.A.-M. and L.G. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by FCT--Fundacao para a Ciencia e a Tecnologia, I.P., by Base Funding (UIDB/04029/2020) and Programmatic Funding (UIDP/04029/2020) of the research unit \"Institute for Sustainability and Innovation in Structural Engineering--ISISE\", and under the Associate Laboratory Advanced Production and Intelligent Systems ARISE under reference LA/P/0112/2020. This work was also funded by FEDER funds through the COMPETE 2020, Portugal 2020, under the projects POCI-01-0247-FEDER-033990 (iNBrazil) and POCI-01-0247-FEDER-046111 (FERROVIA 4.0), by national funds (PIDDAC) through FCT/MCTES under the project PTDC/ECI-EGC/3352/2021 (IntRAIL).  
The authors declare no conflict of interest.