## 2 Advances in Composite Materials for Sustainable Soundproofing Applications  
### Sound-Absorbing Composites Based on Natural Materials  
The increasing environmental concerns and stringent regulations on synthetic materials have spurred significant interest in developing sustainable alternatives for sound insulation and absorption. In this context, the use of natural materials and residues of natural materials is of significant interest both for environmental protection and for the creation of environmentally friendly new composite materials. Wood composites incorporating sustainable and recycled components from agricultural and industrial waste streams offer a viable alternative to traditional soundproofing materials. Hybridized natural fibers, such as pineapple, area, and ramie, reinforcedwith wood-based industrial tea waste and epoxy matrices, have demonstrated effective sound absorption [31]. One study [32] created panels from various wood waste species (pine, oak, mahogany, and olive tree pruning) using different assembly techniques and adhesives. These panels demonstrated thermal and acoustic performance comparable to commercial products, with oak panels exhibiting particularly good sound absorption (\(\alpha\) peak of 0.9) and insulation properties (transmission loss up to 11 dB). The incorporation of wood plastic composite (WPC) into structural wood walls has been shown to affect sound insulation performance [33]. This study found that WPC can be a viable substitute for traditional wood studs and that the type of external panel material significantly influences the overall sound insulation of the wall. Further improvements were achieved by using sound-absorbing materials as elastic strips. The utilization of stranded driftwood residues as a building material with thermo-acoustic properties is explored in another study [34]. The researchers investigate the thermal and acoustic characteristics of both unbound and mineralized (cement-based additive) samples. The results demonstrate that stranded driftwood residues, especially in their mineralized form, possess competitive thermo-acoustic properties, showing promise as a sustainable building material. Many recent studies explore the potential of natural fibers in acoustic applications. One study evaluated the acoustic and mechanical properties of coir (Cocos nucifera) and fique (Furcraea Agavaceae) fibers and panels, finding that fique generally exhibited superior performance in most tests, particularly in high-frequency sound absorption due to its fiber diameter [35]. Another study utilized water hyacinth and pineapple leaf fibers, combined with polylactic acid (PLA), in 3D-printed acoustic panels [36]. This research demonstrated high sound absorption coefficients (\(\alpha\)-max > 0.5 at high frequencies) at low fiber loadings, showcasing the potential for waste mitigation and sustainable material usage. The successful 3D printing of these biopolymer composites highlights the potential of additive manufacturing in creating lightweight, customizable acoustic panels. The study of [37] explored the use of waste flax fibers as the core material in multilayered panels, sandwiched between perlite layers of varying grain sizes. This approach resulted in panels with varying mechanical, thermal, and acoustic properties. The study found that perforation of the panels' surface further enhanced the sound absorption coefficients, achieving values up to 0.95 at specific frequencies. This highlights the potential for optimizing acoustic performance through material selection and panel design. Three-dimensional materials are of particular interest for light and effective sound-proofing. A study [38] investigated the sound insulation performance of 3D woven hybrid fabric-reinforced composites incorporating jute, E-glass, and biomass fillers such as coffee husk and waste palm fiber. These composites showed significantly enhanced sound insulation, with jute-based composites achieving noise reduction levels as high as 44.9 dB at 10,000 Hz.