# Advanced Sound Insulating Materials: An Analysis of Material Types and Properties
## 5 Conclusions  
The purpose of this article was to analyze the current state of research and to identify trends and potential research gaps. Analysis of the publications assigned to the five types of materials made it possible, in summary, to identify the main gaps, trends, and recommendations for future research areas:  
1. Composites--research included the effect of selected parameters (e.g., type of fiber [1; 2; 3; 4; 5; 6] or resin addition [7; 8]), but was conducted under different test conditions. This makes it difficult to compare the results. Thus, it is worth conducting comparative studies with standardized measurement procedures for acoustic properties, e.g., according to ISO 11654 (for the classification of sound-absorbing materials) [96]. The second area worth pointing out is the insufficient amount of research in the context of environmental specs. Only a few publications have considered recycled materials (e.g., tire granulate [24], recycled textiles [25]), and there is a lack of a broader environmental assessment of composite materials. This is particularly important when promoting biocomposites as a sustainable and environmentally friendly alternative to other materials. 2. Fibrous--the category to which the most publications were assigned. The studies carried out were on different types of fibers (cotton [10; 33; 34; 36], wool [29; 30; 31; 32], jute [18], kapok [48], bamboo [15], luffa [18; 49], biduri [46], and banana [12]). However, a comparative analysis conducted under the same conditions is lacking, making it difficult to draw conclusions about the effectiveness of different types of fibers in terms of sound absorption. As with other materials, research has focused mainly on properties under laboratory conditions. Missing, for example, are analyses of the effects of atmospheric conditions, resistance to mechanical wear, or the behavior of a given material over a long period of use. On the basis of the analysis, it was also diagnosed that some of the work focused on modern concepts, but little of it translates into concrete prototypes of products ready to be implemented in practice. 3. Porous--the analyzed publications presented, among other things, results on specific material properties (e.g., [55; 56; 57; 58]). However, there is a lack of a consistent set of standards and test methods, which makes direct comparisons and utilization in practical applications difficult. Despite the innovation and high performance of some materials (e.g., aerogels with a hierarchical cell structure [59]), there is a lack of analyses of economic viability or the availability of raw materials and energy intensity of production processes. 4. Polymers--this group of materials was the least represented among the publications analyzed. Previous research has focused mainly on the acoustic properties of various forms of polyurethane foam [22; 72] and its modifications [23; 65; 70; 71]. In-depth analyses of the chemical, mechanical, and thermal properties of these materials, especially in the context of their durability or resistance, are lacking. It is worth extending the research to biodegradable polymers, especially in the context of sustainable development and environmental protection. In the context of future research, it is proposed, among other things, to compare biodegradable polymers with traditional polymers in terms of environmental impact, energy intensity, and CO\({}_{2}\) emissions. Further research may also aim to analyze increasing their performance without increasing costs by designing cellular and porous structures (e.g., PLA or PHA foams) for improved sound attenuation. In addition, research into the effects of various types of additives, such as natural fibers, on the acoustic properties of polymer composites is a promising direction of development. 5. Nanomaterials--in the reviewed publications, nanocomposites with enhanced mechanical properties [76], hardness [19], and elasticity [60], as well as compressive, flexural, and tensile strength [75], were analyzed.