## 4 Results and Discussion
The results of the fourth set of test samples, which were made of fabrics, are shown in Figure 12. The absorption curves were similar for all the samples and typical of porous materials. The best performance was given by sample 30 having the highest density of the group. Additionally, sample 26 showed a good performance but since it was 100 mm thick it could not be directly compared with the other samples. However, this result shows that better performances could be achieved by increasing the thickness of the panels. Additionally, in this case, the sound absorption coefficient had a shape typical for porous materials, featuring an \(S\) shape, with low values at low frequencies and values approaching 1 at high frequencies. By comparing the \(NRC\) measured for samples 26 and 27 and reported in Table 2, it emerged that the operator skills affected the acoustic performances of the panels. Indeed, even if the samples were made of the same material and the same \(NRC\) should be obtained, a higher value of \(NRC\) was measured for the thinner sample (26) and this is probably due to the assembling mode. The fifth and last set of samples was a combination of fabrics, egg-boxes and metal parts. As can be observed in Figure 13, all the \(TL\) curves had very similar trends. If compared to the \(TL\) in Figure 12, for samples 32-36, values were generally higher at low frequency due to the higher mass per unit area of the samples. Sample 36 had the best performances for this group and this is probably due to the fact that the metal element could vibrate because it was not in contact with the rigid egg-carton. As concerns the sound absorption coefficient, the behavior was dominated by the Helmholtz resonator featuring a peak around 400 Hz followed by an increase of the coefficient due to the presence of the tissues. 25-30 dB in the range 100-300 Hz, and 30-40 dB in the range 300-2000 Hz. To reach interesting performances from both the insulation and the absorption point of view, it is necessary to couple egg-cartons, cardboard, polyester and metal elements. For this configuration, \(NRC\) is higher than 0.54 and the \(TL\) varies between 25 and 40 dB in the range 250-2000 Hz. Since in this study only the performance related to normal waves has been analyzed, in future investigation the most performing configurations will be tested in a reverberation room. Since the acoustic panels will be realized by not-skilled personnel and the manual skills affect the panels acoustic performance, it will be necessary to provide courses and guidelines for illustrating how the panels must be realized and installed. All authors conceived the presented idea. [PERSON] and [PERSON] designed the experimental campaign and wrote the paper with input from all authors. All authors have read and agreed to the published version of the manuscript. The authors would like thank the Department of Mechanical and Industrial Engineering of University of Brescia for funding the research through the MetATer PRD project. All the authors would like to thank the Applied Acoustics Laboratory of University of Brescia and [PERSON] for the opportunity to carry out the experimental tests and for the support in designing the experimental campaign. The authors declare no conflict of interest.