## 1 Introduction  
Research on sustainable sound-absorbing materials is being increasingly developed under the objectives of the Ecological Transition Plan, which aims to progressively reduce CO2 emissions on a global level. The assessment of the environmental sustainability of these materials includes the use of renewable resources for their production, the processes that reduce the release of polluting particles into the environment and, potentially, the ability of these materials to absorb polluting particles (for example CO2 during the growth of plants). We can therefore identify two main aspects concerning the environmental sustainability of sound-absorbing materials: the constituent material and the production process that allows the final product to be obtained. Analysis relating to the end-of-life of the product must also be added to these aspects, together with the possibility of recycling the product to create a new material. An objective analysis can be obtained with LCA (Life Cycle Assessment) methodology, which measures the environmental impact of a product by analyzing all phases of the product life in analytical form. It is thus possible to obtain comparative tables of the different types of materials in relation to their environmental sustainability. Sound-absorbing materials can be divided into different categories based on the origin of the material they are made of:  
* Vegetal origin: mainly materials with a fibrous structure, obtained from specially cultivated plants (cotton, hep, flax) or from production waste from other processes (wood fibers, coconut fibers); in some cases (e.g., cork) the materials can also have a cellular structure;
* Animal origin: for example, sheep's wool or goose feathers;
* Mineral origin: the production of these materials is based on the availability of natural mineral resources, which by definition are non-renewable; they can have a fibrous structure, such as mineral wool (amorphous silicate obtained from a mixture of rocks and other inert materials) and glass wool (amorphous silicate obtained from a mixture of glass and sand), or cellular structure such as pumice or perlite;
* Synthetic origin: materials that require chemical processes (typically polymerization) to obtain open-cell foams such as polyurethane, polyethylene, melamine, or fibrous materials such as polyester. To produce synthetic materials, it is necessary to use non-renewable fossil resources; therefore, environmental sustainability is lower than the first two categories unless recycled materials are used in the production process. More generally, a different category may include recycled materials obtained from end-of-life materials, regardless of their nature. This category includes felts obtained from textile fibers (recycled clothes), granulates obtained from recycled tires, and polyester fibers obtained from recycled plastics (PET). If the analysis is narrowed down to materials of plant origin, there are hundreds of articles presenting acoustic results from sound-absorbing materials obtained from the most disparate types of plants: an exhaustive list can be found in several review articles, such as [1; 2; 3]. The natural fibers investigated can be obtained from processing waste from agricultural production (e.g., green tea residues [4], corn husk [5] and pineapple-leaf [6]), from specifically cultivated plants (e.g., hemp [7] and jute [8]), or from naturally occurring invasive plants that have no other applications (e.g., broom [9] and Yucca gloriosa [10]). For most of these plants, lignocellulosic fibers are obtained through chemical and mechanical processes, including maceration, water retting, carding, and alkaline treatment. The acoustic results are often reported in terms of the sound absorption coefficient and therefore depend not only on the intrinsic properties of the fibrous material but also on its thickness and density. It would be very difficult to compare the data between different publications, and, in any case, this would not allow the real efficiency of the material to be determined.