## 4 Discussion  
Given the results herein, a few aspects can be highlighted with respect to previous findings presented in Section 1 and Appendix A. The sound-absorbing properties of the panels under examination (i.e., A-D) were generally found to be extended towards the lower frequencies with increasing thicknesses of the panels (i.e., 20 mm, 40 mm, or 60 mm). The result is coherent with the findings of previous studies, such as in [13; 18; 21]. However, panel B exhibits an unexpected behavior, as while the sound absorbing properties of the thicker panels are higher at lower frequencies compared to the 20 mm sample, as it can be seen in the 500 to 1600 Hz frequency range, the 20 mm thick sample outperforms the 40 mm thick one in the range of 1600 to 4000 Hz. Moreover, no peak shift towards the lower frequencies is reported for the 60 mm thick panel compared to the 40 mm thick one, as both present an absorption peak at 800 Hz. These two aspects may suggest that the superficial and inner porosity of panels B are not uniform among the different thicknesses. Moreover, it can be argued that for this typology that the thickness threshold is \(\sim\)40 mm, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Characteristics** & **A** & **B** & **C** & **D** \\ \hline
20 mm & 0.10 & 0.10 & 0.10 & 0.20 \\
40 mm & 0.15 & 0.25 & 0.15 & 0.30 \\
60 mm & 0.25 & 0.25 & 0.25 & 0.45 \\
20 + 20 mm & 0.15 & 0.25 & 0.15 & 0.30 \\
20 + 40 mm & 0.20 & 0.25 & 0.25 & 0.40 \\
20 mm + 50 mm air gap & 0.20 & 0.10 & 0.35 & 0.50 \\
40 mm + 50 mm air gap & 0.20 & 0.10 & 0.30 & 0.50 \\
20 mm + 50 mm air gap (rock wool 30 mm) & & & 0.70 \\
20 mm + 50 mm air gap (rock wool 50 mm) & & & 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of single acoustic indices related to the weighted absorption coefficient (\(\alpha_{w}\)) for the four concrete types (A–D). i.e., no further increase of the absorption coefficient below 800 Hz is obtained with the thickness increase from 40 to 60 mm [7]. Sample D outperforms the other typologies and confirms that its superficial and inner structures are made of many small and uniformly distributed pores and apertures connected with each other and with the outer surface [11]. When comparing samples with round lightweight aggregates, i.e., B-D, it can be observed that there is a decrease in the sound absorption when the aggregate size increases from 0.5-1 mm (sample D) to 4-8 mm (sample B). This is due to an increase in the median pore size when increasing aggregate size as shown in [21], which would reduce the tortuosity of the pore network, thus resulting in lower absorption values. Indeed, sample D has a lower water permeability (Table 1), which is inversely correlated to tortuosity [42]. Moreover, note that sample A with crushed normal weight aggregates results in higher values of the absorption coefficient when compared to sample B, which has similar void ratio (25%) and aggregate dimensions (4-8 mm) to sample A, but features different aggregate shapes and densities, i.e., round lightweight aggregates. This might be due to a higher tortuosity enabled by internal pores with varied size connected to the surface, which is coherent with the aspects highlighted in [11]. This kind of difference was not observed in previous studies, that is, the work in [22], where only slight differences between round-shape (lightweight) and irregular shape (normal weight) aggregates with the same gradation were found. When considering panels composed of two layers, the presented results have highlighted some discrepancies between the sound absorbing performances of multilayered panels and those of a single layer panel of the same thickness in case of panel samples A-C. Conversely, samples D in the multilayered and single layer solution of equal thickness exhibit similar performances. This behavior may be linked to the different degrees of uniformity in the pore distribution of the different panel samples.