## 1 Introduction
Efficiency is defined in this paper as the ratio between its performance and thickness or weight. When evaluating the production process of a vegetable fiber material, the following need to be considered:  
* The use of land for cultivation;
* The use of resources for plant cultivation (water, any chemical products, agricultural processing with non-renewable resources);
* The industrial processes necessary to transform the plant (or parts of it) into fibers, with the possible addition of binders to create panels of a certain shape;
* Transportation to the installation site. Naturally, if the fibers are made from production waste (for example, wood processing waste or coconut waste), the first two points are not applicable (or rather, they fall on the primary products), while the last two are usually applicable. If the fibers of a vegetable material are already available in nature, the sustainability of this material is very high, and its possible use is of great interest; a material of this type is present on the beaches of the Mediterranean: the so-called Sea Balls or Neptune Balls (scientific name: Aegagropiles), which are the result of the unraveling of the leaves of the _Posidonia oceanica_. They are made up of an agglomeration of fibers bound together in an ellipsoid shape (Figure 1a,b). _Posidonia oceanica_ is an aquatic plant endemic to the Mediterranean Sea, and it belongs to the Posidoniaece family (Monocotyledonous angiosperms). _Posidonia oceanica_ meadows occupy a surface area between 2.5 and 5 million hectares [11] and perform important ecological functions in the Mediterranean Sea, not only for biodiversity conservation but also for the mitigation of climate change, thanks to their role in carbon sequestration [12; 13] and their ability to trap microplastics [14]. In the last few decades, there has been a widespread regression (from 13% to 50% [15]) of the meadows in Mediterranean coastal waters. Therefore, in view of their important ecological role, _Posidonia oceanica_ meadows have been protected by European legislation through the Habitats Directive 92/43/EU, classified as a Priority Habitat and included as of \"Least Concern\" on the Red List of the International Union for Conservation of Nature (IUCN) [16]. The leaves of _Posidonia oceanica_ are ribbon-shaped and can be up to one meter long. They are green in color when young and, as they age, turn brown and fall off due to the action of the waves. Research shows that Aegagropiles are formed by hydrodynamic movements and are made of different _Posidonia oceanica_ plant fibers and sand grains [17; 18; 19; 20]. The wave motion deposits them on the coast, where they are dried by atmospheric agents. These fibers are entirely processed by the sea, thanks to the action of the water and the waves. This frees the fibers from the vegetal material, creating ellipsoidal aggregates that end up on the beaches. During storms, large quantities of Posidonia leaves are torn off and  
Figure 1: Examples of Posidonia Balls (**a**,**b**) and “banquettes” on the beach (**c**). deposited in large masses along the coast. The clusters, called \"banquettes\" (Figure 1c)--a few centimeters to several meters in thickness--can reduce the beach erosion rate [21], but if present in large quantities they are unwelcome to beach users. For this reason, in areas that welcome high numbers of tourists, large quantities of these materials are removed from the beaches before the summer season. Where it is not possible to do otherwise, they are sent to landfills as waste. The alternative uses of Posidonia Balls currently being studied include recycling them for the creation of compost [22] or for the production of energy [23], after sieving out the sand present inside them, which is used for beach nourishment. From an acoustic point of view, to the author's knowledge there is only one article in the literature that presents an analysis on the physical and acoustic characteristics and on the modeling of regular packets of these spheres as metamaterials.