## 5 Conclusions  
The present study has been carried out in order to characterize the sound absorbing performances of a set of porous concrete panels varying in concrete mix design (A-D), thickness and mounting method. The measurements have been conducted in the 1:5 scale reverberation room of the Politecnico di Torino, in accordance with the ISO 354-1:2003 standard. The sound absorbing performances of the different panels have been described as 1/3 octave band and as weighted sound absorption coefficient \(\alpha_{w}\). The following conclusions have been drawn. 1. The mix design with the smallest round lightweight aggregate dimensions (0.5-1 mm), referred to as panel D, gave the most effective sound absorption coefficient for all the three sample thicknesses as well as for the mounting condition with an air gap at their back. It was shown that the worst performing mix design feature round lightweight aggregate with the greater dimensions (4-8 mm) referred to as panel B. 2. Samples with crushed normal weight aggregates of 4-8 mm, referred to as panel A, showed higher values of absorption coefficients compared to samples with round lightweight aggregates of the same dimensions. The performance is comparable to that of the sample with round lightweight aggregates of smaller gradation (2-4 mm) referred to as panel C. This was attributed to the pores dimensions and inner distribution which affects the pores tortuosity. 3. The sound-absorbing performances of the porous concrete panels tend to increase at low frequencies for greater panel thicknesses. In most cases, solutions with single panels or double-layered panels of an equivalent thickness provide similar performances. This mounting solution could be useful in practice for modular solutions and to limit the need for different formwork thicknesses. 4. The mounting method is shown to greatly influence the sound-absorbing performances. For all porous concrete types considered, the sound absorption performances are enhanced for lower frequencies by leaving a 50 mm air gap behind the panel. However, the air gap significantly lowers the high frequencies performance of the thinnest samples (20 mm). * The frequency dependent absorption coefficient and the weighted absorption coefficient \(\alpha_{w}\) comparisons showed that, depending on the mounting method, the performance of the concrete samples with aggregate dimensions of 0.5-1 mm, i.e., panel D, can be further improved. The \(\alpha_{w}\) reaches values 0.50 and 0.75 for the condition with an empty air gap of 50 mm and air gap completely filled with a rock wool layer, respectively. These values are comparable to those of most used conventional porous materials. Note that the mix design mechanical properties remain a crucial aspect that need to be considered when the applicability of such materials is discussed. It was observed that the material with higher acoustic performance (round lightweight aggregate of 0.5-1 mm) presents poor performances related to wear resistance, which makes the application of such panels in actual scenario impractical. Therefore, we endorse further testing in the attempt to find the most performing solution balancing sound absorption with wear resistance performances. Alternatively, when wear resistance is required, it is possible to use panels of type B and C, which, when coupled in layers of 20 + 40 mm, reach sound-absorbing performances close to those of the same configuration of panels type D for frequencies higher than 800 Hz. Further research could be conducted along this line of research to explore other mix design and mounting method strategies to hopefully increase awareness about the potential benefits of the application of sound absorbing porous concrete in the frame of the architectural and urban design strategies.