## 3 Results
### Effect of Sample Thickness and Concrete Type  
Figure 5 shows the graphs of the four samples (A-D) for three different thicknesses of the single layers. Overall, the absorption spectra of panels A are uneven, and tend to provide poor absorption (<0.25) at frequencies lower than 630 Hz, while at higher frequencies, the sound absorption coefficients ranges between 0.40 and 0.70 for panels with either 40 mm or 60 mm thicknesses. The 20 mm thick panel features an absorption peak at 3150 Hz, which reaches the value of 0.90 and provides a poor absorption (<0.25) at frequencies lower than 2000 Hz. The 40 and 60 mm panels present a higher absorption coefficient with respect to the 20 mm panels in the 500-2000 Hz frequency range.  
The absorption spectra of samples B are uneven and tend to provide poor absorption (<0.25) at frequencies lower than 630 Hz for panels with either 40 mm or 60 mm thicknesses, while at higher frequencies the absorption coefficient ranges between 0.20 and 0.60. The 20 mm thick panel features an absorption peak at 2500 Hz of about 0.60 and provides poor absorption (<0.25) at frequencies lower than 1600 Hz. This might be due to the curing process of the 40 mm sample, which might have led to lower porosity of these samples.  
The absorption spectra of panels C are also slightly uneven and tend to provide poor absorption (<0.25) at frequencies lower than 630 Hz for panels with either 40 mm or 60 mm thicknesses, while at higher frequencies, the absorption ranges between 0.40 and 0.80. The 20 mm thick panel features an absorption peak around 4000 Hz, achieving a value of 0.90 and provides poor absorption (<0.25) at frequencies lower than 1600 Hz.  
Figure 5: Comparison of the absorption coefficients for samples (**A**–**D**) with different thicknesses obtained from multiple layer combinations (20, 40, 20 + 20, 60, and 20 + 40 mm). Sample (**A**): crushed normal weight aggregates 4–8 mm; Sample (**B**): round lightweight aggregates 4–8 mm; Sample (**C**): round lightweight aggregates 2–4 mm; Sample (**D**): round lightweight aggregates 0.5–1 mm.  
The absorption coefficient for this thickness becomes lower than 0.25 at frequencies below 2000 Hz. The 60 mm sample reaches significant high values of absorption coefficient (>0.40) at 800 Hz, while the 40 mm panel at 1250 Hz.  
The absorption spectra of panel D are more even than the other three typologies, and tend to provide significant absorption (>0.40) at frequencies higher than 630 Hz for panels with either 40 mm or 60 mm thicknesses, where the sound absorption ranges between 0.40 and 1. The 20 mm thick panel feature an absorption peak between 2500 Hz and 4000 Hz, achieving a value of 1.20; the peak is broader than those featured by 20 mm thick panels of types A-C. Values higher than 1 may occur in the measurements with finite sample size for materials with high absorption properties [39; 40]. The 20 mm sample of panel D achieves significant absorption (>0.40) above 1000 Hz, while for panels A-C, this occurred above 2500 Hz, 2000 Hz, and 3150 Hz, respectively. The sound absorbing performances of thicker panels are extended towards the lower frequencies, in the range below 1600 Hz. Indeed, for the thicker panels (40 mm and 60 mm), the significant absorption range is extended in a similar way down to 630 Hz.