## 4 Discussion  
Aegagropiles are very common plant materials on the Mediterranean coasts, which represent a potential problem--particularly during the tourist season--as their disposal can constitute a cost for local public administrations. However, they could be a useful resource for the production of sound-absorbing materials, particularly as the Aegagropiles fibers are found on the beach ready for use since they are naturally cleaned of any non-cellulosic components by the action of the waves and sea water. Only a mechanical treatment is  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Thickness** & **Density** & **Porosity** & **Airflow Resistivity** & **Tortuosity** & **VCL** & **TCL** \\
**[mm]** & **[kg/m\({}^{3}\)]** & **[-]** & **[Ns/m\({}^{4}\)]** & **[-]** & **[\(\mu\)m]** & **[\(\mu\)m]** \\ \hline
40 & 157 & 0.90 & 7867 & 1.11 & 52 & 171 \\ \hline
60 & 105 & 0.93 & 3871 & 1.07 & 88 & 232 \\ \hline
80 & 79 & 0.95 & 2455 & 1.05 & 128 & 287 \\ \hline
120 & 52 & 0.97 & 1356 & 1.03 & 217 & 388 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical parameters of the JCA model calculated with the proposed analytical relationships.  
Figure 9: Absorption coefficient of Posidonia fibers as density varies (solid lines) and of two traditional sound-absorbing materials (dots: polyester fiber and mineral wool).  
necessary to separate the intertwined fibers of the Aegaropiles and to sieve out any sand trapped inside.  
This research has shown that Posidonia fibers have potential applications in the acoustic field and that they can achieve absorption coefficients comparable with those of traditional materials such as mineral wool and polyester fibers. Depending on the degree of compaction of the fibers (and therefore on the density), it is possible to control the airflow resistivity of the material and consequently calculate the absorption curve as a function of the thickness of the material.  
The analytical model presented showed a good correlation with the experimental acoustic measurements and constitutes a design tool for acoustic panels made with this sustainable material; this is of great importance since there are no acoustic models related to the density of this sustainable material in the literature. It should also be underlined that the proposed characterization method, a hybrid of experimental measurements and the numerical inversion of acoustic measurements, proved to be very efficient in obtaining an analytical model of the acoustic propagation in the material.  
The large size of the fibers determines the need to use fiber densities higher than 100 kg/m\({}^{3}\), which is significantly higher than other traditional materials such as mineral wool or polyester fibers. The use of these materials is therefore recommended for applications where the weight of the panel is not a determining factor (such as on trains, cars, or airplanes), where a reduced environmental impact is sought. The lower acoustic efficiency of the material can be compensated for by its high sustainability and the reduced need for mechanical and chemical treatments, in addition to the fact that the waste material is reused--creating a circular economy.  
The research, currently limited to fibers from Aegagropiles, could be extended to Posidonia leaves deposited on the beaches in the form of \"banquettes\", which represent a greater problem since they are larger in quantity. In this case, it would be necessary to study an industrial treatment to obtain the fibers from the leaves swept up onto the beaches.  
This research was funded by MINISTERO DELLISTRUZIONE DELLUINIVERSITA E DELLA RICERCA (ITALY), within the framework of the project PRIN 2017, grant number 2017T88 BH9: \"Theoretical modelling and experimental characterization of sustainable porous materials and acoustic metamaterials for noise control\".  
The authors declare no conflict of interest.