## 4 Results and Discussion  
The results of the tests described in the previous section are reported in Figures 9-13 showing the sound absorption coefficient \(\alpha\) and the transmission loss \(TL\) obtained by means of the impedance tube measurements. For each sample, the NRC has been calculated and reported in Table 2.  
Figure 9 shows that, for samples 03 and 22, the most interesting sound insulation performance was given by sample 22, with a \(TL\) following the mass law up to 800 Hz and then a behavior typical of double walls, with a coincidence frequency around 1800 Hz. Sample 03 was not able to reach the same performances. As regards the absorption coefficient, it was characterized by wide peaks at given frequencies due to holes in the egg-cardboard behaving like Helmholtz resonators. In the case of sample 03, the peak around 700 Hz was very wide due to the presence of sound absorbing polyester fibers inside the main volume. This result suggests that to reach a good \(TL\) it was important to arrange the egg boxes upside down, but they still had to be coupled with other materials to improve their performances. The presence of the holes improved the absorption characteristics. The \(TL\) curves just analyzed were very similar to the curves obtained for the second set of samples and reported in Figure 10. This behavior can be explained by the nature of the samples which were built with the same elements: the egg-box, the polyester foam and steel spheres. The only variables were the orientation of the samples and the presence of a cardboard disk. The TLs of samples 04 and 05 were characterized by the typical mass law behavior due to the single egg cardboard, except a dip around 1200 Hz. Samples 04 and 05 showed an interesting absorption coefficient, and sample 04 featured better performances  
Figure 11: Transmission loss (**a**) and absorption coefficient (**b**) of samples made of several layers of cardboard. Figure 12: Transmission loss (**a**) and absorption coefficient (**b**) of samples made of clothes (one fabric at a time). Figure 13: Transmission loss (**a**) and absorption coefficient (**b**) of samples made of egg-boxes and clothes. since it did not present valleys at high frequency. However, since these samples were permeable to air, the \(TL\) was very weak. Reducing the permeability, in samples 07 and 08 the additional cardboard layer caused an increase of the curve slope due to the mass-spring-mass behavior of the layers. Sample 08 exhibited very interesting performances at low frequencies and the highest \(TL\) of the group also at high frequencies. In sample 08 the additional perforated cardboard layer facing the sound source captured sound energy at some specific frequency bands. Part of the energy that passed through the egg-box was absorbed by the fibrous material. Finally, the final last cardboard reduced the transmitted energy. For these samples, the absorption coefficient was characterized by multiple peaks typical of the resonators featured in the cardboard portions. The width of the peaks depended again on the presence of polyester fibers. The best performance belonged to sample 08, characterized by a rather good sound absorption coefficient also at high frequency, with a maximum of 0.95 at 2 kHz. As regards the third set of samples, Figure 11 shows very similar \(TL\) curves for all the samples. This group of TL was the highest among the entire group of tested materials. Such behavior is due to the high density of the samples that made the samples similar to sandwich materials featuring non compressible cores. For this reason the trend was characterized by a mass-law behavior followed by the typical coincidence dip. For samples 13 and 14, the sound absorption coefficient was marked out by a maximum around 600 Hz due to the resonance of the Helmholtz resonators featured in the cardboard. For sample 17, the graph of the sound absorption was very similar to the one of sample 08 (having a very similar structure), with three peaks reaching a value of 0.8.