## 3 Results
### Effect of Sample Mounting with an Air Gap  
The graphs in Figure 7 show the four sample types (A-D) mounted with an air gap of 50 mm between the panel and the rigid backing, i.e., the SSRR floor. The graph of Sample A shows that the performance is enhanced at the lower frequencies when an air gap is left between the panel of 20 mm and the backing, while the sound absorption at high frequencies decreases. The maximum peak is shifted at lower frequencies, i.e., at \(\sim\)630 Hz, with an absorption coefficient of \(\sim\)0.60. The 40 mm layer seems to be less affected by the presence of the air gap and the maximum peaks remain unvaried in frequency for this thickness. However, a slight decrease is reported at high frequencies and an increase of about 0.10 is observed at the peak value corresponding to 1250 Hz. Sample B shows different trends for the 20 mm and 40 mm layers. However, when an air gap is left between the panels and the backing, the performance is enhanced at the lower frequencies for the 20 mm and 40 mm layers. The high frequency sound absorption decreases for the 20 mm layer when the air gap is added, while the maximum peak is shifted at lower frequencies, i.e., at \(\sim\)630 Hz, with an absorption coefficient of \(\sim\)0.60. The 40 mm  
Figure 6: Back surface for 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. Figure 7: Comparison of the absorption coefficients for samples (**A–D**) of different thicknesses (20 and 40 mm) mounted with an air gap of 50 mm. Sample (**A**): crushed 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. layer seems to be less affected by the presence of the air gap at high frequencies above 2000 Hz. Conversely, the air gap seems to decrease the absorption over the 630 to 2000 Hz range for the 40 mm layer. A peak value appears at the frequency of 200 Hz with a value of about 0.55 of the absorption coefficients. Sample C shows a decrease of the absorption coefficient at high frequencies for both 20 mm and 40 mm layers when an air gap is left between the panels and the backing. For the 20 mm panel, this is significant above 2500 Hz, while for the 40 mm panel, it is more evident in the 1000 to 2500 Hz range. The performances are slightly enhanced at the lower frequencies in the range 315 to 800 Hz for the 20 mm sample and in the range 315 to 2500 Hz for the 40 mm sample, with the maximum peaks that are shifted at 630 Hz and 800 Hz, respectively. The sound-absorbing performances of sample D show a decrease at high frequencies when an air gap is left between the panels and the backing above 1600 Hz and above 800 Hz for the 20 mm and 40 mm panels, respectively. Nevertheless, the sound absorption coefficients in those ranges result above 0.55. The performances are enhanced at the lower frequencies, where several peaks appear around 250 Hz, 400 Hz, and 800 Hz. The absorption coefficient increases for both thicknesses in the 160 to 630 Hz range when the air gap is added, showing a very similar trend for both 20 mm and 40 mm panels. Effect of Sample Mounting with an Air Gap Filled with Porous Material  
The previous sections showed that sample D presents the highest sound absorption coefficients extended over the broader range of frequencies. In order to further improve the performance of the combination of panel D with an air gap, another strategy has been used considering the air gap filled with porous material. The introduction of a rock wool layer in the air gap has been tested with the sample of 20 mm thickness only. Two thicknesses of the rock wool layer, that is, 30 and 50 mm, have been introduced in the air gap. Recall that the air gap considered here is of 50 mm.