# Advanced Sound Insulating Materials: An Analysis of Material Types and Properties
## 3 Results  
Five groups of publications were identified in this literature review by type of material (Table 1), namely, composites (28 articles), fibrous (41 articles), porous (22 articles), polymers (6 articles), and nanomaterials (11 articles). In the first group of identified publications, the authors developed hybrid fiber composites from banana and glass [1], sugarcane and coconut [2], pigeon pea stalks and cotton [3], and jute and coconut [4; 5]; hybrid composites reinforced with sisal fibers [6], palm fibers reinforced with epoxy resin [7], and reinforced composites in which polyester resin is the matrix [8]; biocomposites based on non-woven rice straw and polylactic acid [9]; a composite made from pea stalks and cotton waste [10]; an acoustic-thermal composite made from sugarcane bagasse and bamboo charcoal fibers [11]; a biocomposite made from polymers reinforced with banana fibers [12]; co-fiber composites combining natural fibers from drought-tolerant plants [13]; and additive manufacturing of acoustic panels made from polylactic acid reinforced with a wood fiber composite [14]. This characterized group of articles also analyzed the effect of chemical modification of bamboo fibers on acoustic properties [15]; the acoustic and mechanical properties of hybrid Fiber Reinforced Polymer (FRP) panels made of natural fibers [16] and those of hybrid biocomposites made of jute fabric and aloe mats [17]; the effect of sample thickness and fiber/resin ratio on the acoustic properties of biocomposites [18]; the properties of polymer composites reinforced with Cissus quadrangularis fibers (CQSF) and epoxy  
Figure 3: Number of publications classified by research methodology. polymers, with the addition of nanosilica [19]; the manufacture of polypropylene hybrid composites [20]; and the preparation of aluminum fiber-reinforced composite foams [21]. Polymer composites with 316 L stainless steel hollow spheres produced by casting [22] and the effect of different syntactic hollow spheres on the acoustic and mechanical properties of polyurethane composites were also investigated [23]. In two articles, the researchers highlighted the environmental and ecological aspects involved in the use of rubber granules from tires [24] and the use of recycled tires and textiles in epoxy composites [25]. Three articles had a review character and dealt with the status of basalt fiber (BF) research in China [26], the growing interest in the use of natural fibers as a sustainable alternative to traditional sound-absorbing materials in transport and construction [27], and the use of rice husk-filled polymer composites, particularly in the aerospace and marine industries [28]. Forty-one publications were assigned to the second group. The research carried out concerned the use of woolen fabrics [29; 30; 31; 32]; woolen needlesd and stitched nonwovens [33]; nonwovens made from cotton, polyester and bi-component fibers [34]; sound absorption of nonwovens based on cotton fiber wool and polyester fiber [35]; the synthesis of composites made from pea stalks and cotton fibers in different proportions [10]; nonwoven mats made from recycled cotton and polyester [36]; sugarcane and coconut fibers with a polyester matrix [2]; materials made from natural fibers [16; 37; 38; 39; 16]; polymer composites reinforced with natural fibers [19]; the production of polylactic acid acoustic panels reinforced with wood fibers [14]; and the acoustic properties of natural cellulose materials [40] and co-fiber composites combining natural fibers from drought-tolerant plants [13]. Researchers have also paid attention to environmental issues through the use of coffee husk waste and cotton spinning industry waste [41], cotton waste and pigeon pea stalks [3], peanut