## 2 Advances in Composite Materials for Sustainable Soundproofing Applications
### Sound-Absorbing Composites Based on Natural Materials
The study concluded that both coffee husk and palm fiber are effective and eco-friendly fillers. Another study [39] investigated the acoustic absorption of date palm midribs-based fabric acoustic panels. This research found that the panel thickness and density significantly influenced absorption, with optimal performance observed in certain configurations. These panels offer a sustainable and aesthetically pleasing alternative for interior design applications. Similarly, the potential of sunflower straw as a bio-based sound absorber was explored [40]. The study of [41] investigated the acoustic, mechanical, and thermal properties of epoxy composites reinforced with cotton, coconut, and sugarcane fibers. The results indicated that increasing the fiber content enhanced sound absorption, with coconut fiber composites exhibiting the highest sound absorption coefficient. Another study utilized pineapple leaf fiber and silica aerogel in mortars, demonstrating that the addition of pineapple leaf fiber offset the compressive strength reduction caused by silica aerogel [42]. Table 1 summarizes the acoustic properties of certain mineral, bio-based, and fiber composites. The reported sound absorption coefficients (\(\alpha\)) of various natural fiber composites reveal significant variations in performance, influenced by the material composition, structural design, and tested frequency ranges (Figure 1). Sheep wool demonstrates exceptional acoustic efficiency, achieving a peak \(\alpha\) of 0.903 at 524 Hz, likely due to its porous, fibrous structure that effectively dissipates sound energy. Similarly, epoxy-based composites hybridized with pineapple/areca/ramie fibers and tea waste exhibit strong broadband absorption (\(\alpha\) = 0.63 across 600-6300 Hz), attributed to their multi-scale porosity and reinforced polymer matrix. Paper sludge-clay composites and larch bark panels also perform well (\(\alpha\) = 0.55 and 0.5, respectively), with their moderate absorption linked to the particle density and air gap integration. In contrast, mycelium-based composites show limited low-frequency absorption (\(\alpha\) = 0.2 at 125-1000 Hz), suggesting a need for structural modifications (e.g., increased pore size or layered designs) to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Material** &
\begin{tabular}{c} **Referenced to** \\ **Frequency Range, Hz** \\ \end{tabular} & **Reference** \\ \hline
1 & Paper sludge and clay composites & 250–1600 & [43] \\ \hline
2 & Sheep wool & 524 & [44] \\ \hline
3 & Mycelium-based composites & 125–1000 & [45] \\ \hline  & Epoxy-based composites filled with & & \\
4 & pineapple/areca/ramie hybridized with & 600–6300 & [31] \\  & industrial tea leaf wastes/GFRP & & \\ \hline
5 & Sugarcane fiber-reinforced composites & 1600 & [41] \\ \hline
6 & Coconut fiber-reinforced composites & 1600 & [41] \\ \hline
7 & Cotton fiber-reinforced composites & 1600 & [41] \\ \hline
8 & Larch bark panels & \textgreater{}750 & [46] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Acoustic properties of selected mineral, bio-based, and fiber composites. Figure 1: Sound absorption for selected materials listed in Table 1.  
enhance performance. Notably, sugarcane, coconut, and cotton fiber composites exhibit lower absorption (\(\alpha\) < 0.2 at 1600 Hz), likely due to insufficient fiber density or untreated surface morphology, highlighting opportunities for chemical or mechanical fiber treatments to improve energy dissipation. Mycelium-based composites, produced through bio-based manufacturing processes and combined with wood fibers, represent a novel approach to creating sound-absorbing structures from waste paper substrates [45]. The acoustic performance of these composites is influenced by material properties, such as the particle size, orientation, and density, as demonstrated in studies on larch bark panels [46]. The inclusion of air gaps in paper sludge and clay composites with wood fibers has been shown to significantly enhance sound absorption [43].