Sound Absorbing Properties of Selected Green Material—A Review

: Noise pollution is often overlooked and invisible, but it signiﬁcantly impacts the quality of human life. One of the most straightforward solutions to mitigate noise pollution is by using sound-absorbing materials. Recently, research trends to develop sound absorbing green materials, typically derived from agricultural by-products, have witnessed an uptick. This paper summarizes the sound-absorbing properties of various green materials found in the literature, including coconut ﬁber, kenaf ﬁber, rice bran, rice husk, rice straw, Hanji (a traditional Korean paper), tea-leaf ﬁber, mandarin peel, pineapple-leaf ﬁber, corn husk, peanut shell, sugar palm trunk, yucca gloriosa ﬁber, fruit stones, wood barks, ﬂax ﬁber, and ne�le ﬁber. Natural ﬁbers can be made by compressing the raw material or manufacturing them into ﬁbrous materials or composites. The key variables that determine sound absorption performance are the thickness and density of the green material, as well as the presence of an air back cavity. Generally, thicker materials exhibit be�er sound absorption performance in the low-and mid-frequency range. Moreover, higher density is associated with be�er sound absorption performance at the same thickness. Additionally, increasing the distance between the sound-absorbing material and the air back cavity enhances sound absorption performance at low frequencies. Thus, these physical variables, rather than the speciﬁc materials used, primarily inﬂuence sound absorption capabilities. Therefore, various green materials, such as ﬁbers, granules, and porous materials, can be eﬀective sound absorbers if their thickness, density, and air back cavity are properly controlled.


Introduction
Currently, environmental pollution is a major problem in the world, and this affects air, water, and soil causing significant damage to nature and humanity.According to a recent study, up to 22% of diseases worldwide are caused by environmental pollution [1][2][3][4].
Noise is an invisible and overlooked type of pollution.The impacts of noise pollution on lives and cities decrease the quality of life.In Western Europe, it is regarded, after ambient particulate ma er, as the leading cause of worsening health [5,6].
Noise pollution can be reduced by incorporating sound insulation and sound absorption materials in buildings.Sound insulation is a method of blocking sound energy, thus preventing noise from passing through partitioning, whereas sound absorption is a method of absorbing sound energy to reduce sound reverberation in confined ambients [7].
Since the 1970s, synthetic fibrous materials (fibrous glass, asbestos, and mineral fibers) have found wide commercial sound absorbing applications due to their low cost and performance at mid and high frequencies [11][12][13].On the other hand, they have adverse health effects, including cancer, depending on the frequency of exposure [13][14][15].These effects extend to workers recycling these materials [16][17][18][19].Synthetic fibers also contain volatile organic compounds and formaldehyde, which cause direct harm to occupants, including sick building syndrome.
"Green materials" consist of resources such as natural fibers and wood that are sustainable and renewable [20].Natural fibers used as building materials can be produced on a mass scale with high quality and at competitive prices.Accordingly, they are a racting interest, due to their durability, nontoxicity, and excellent thermal insulation properties [12,[21][22][23].
Green sound-absorbing materials include coconut fiber, kenaf fiber, and rice husk and rice straw, which are frequently used and produced; Hanji (Korean traditional paper); and various agricultural by-products (tea leaf, mandarin peel, pineapple-leaf, corn husk, peanut shell, sugar palm trunk, Yucca gloriosa, fruit stones, and wood bark particles, flax fibers, ne le fiber, areca nut leaf, denim shoddy, and jute fiber).
Figure 1 compares the price of synthetic and natural fibers from 2011 to 2017, revealing that natural fibers were cheaper [36][37][38].

USD/Ton
Sound waves interacting with a material or surface can be categorized as incident or reflected sound.Incident sound refers to the sound waves that directly strike the material, whereas reflected sound refers to the sound waves that rebound off the material.In the context of sound absorption, the effectiveness of a material is determined by its capacity to convert incident sound energy into alternative forms, such as heat or vibrations, rather than reflecting it into the surrounding environment.An efficient sound-absorbing material should minimize sound energy reflection and maximize sound energy absorption [9,28,39,40].
The sound absorption coefficient serves as a metric for the amount of sound energy absorbed by a material relative to the incident sound energy.It represents the proportion of sound energy that is absorbed rather than reflected by the material.A higher sound absorption coefficient signifies a greater capability of the material to absorb sound and decrease sound reflection [9,28,39,40].
Most studies have measured the sound absorption coefficient (α) using an impedance tube (Figure 2).Impedance tubes are primarily designed to measure the sound absorption coefficient due to normal incident sound.They are most effective in measuring the sound absorption properties of materials when sound waves are directly incident on the sample at a perpendicular angle.However, impedance tubes are limited in their ability to accurately measure the abilities of materials to absorb sounds from different directions or at different incident angles [39].The sound absorption coefficient is calculated using Equation (1) [41,42] where -absorbed sound intensity, -incident sound intensity, Pi-pressures of incident, and Pa-pressure of reflected waves.

NRC =
(2) According to ISO 11654 [43], the sound-absorbing performance is categorized using the NRC.The grading system is as follows: A, B, C, D, and E grades correspond to an NRC value ranging from 0.90 to 1.00, 0.80 to 0.85, 0.60 to 0.75, 0.30 to 0.55, and 0.15 to 0.25, respectively, and any performance below 0.10 does not receive a grade.This standardized grading system is used to assess and classify the sound absorption capabilities of materials, allowing for easy comparison and selection of suitable materials for specific applications [44].This study examined the sound absorption performance of various natural soundabsorbing materials presented in previous studies.Most of the references do not provide the raw data of sound absorption curves.Thus, this study extracted the raw data from the sound absorption curves in the references using Engauge Digitizer software (Version, 12.1) [46].From extracted the raw data, this study recalculated the NRC and optimal sound absorption coefficient (peak value).

Coconut Fiber
Coconut fiber is obtained from the outer coconut husk and primarily sourced from Southeast Asia.It is a marketable natural fiber because it is cheaper and lighter than synthetic fiber [30].Numerous studies utilizing coconut fiber as interior building materials [40,47,48] and as a natural sound absorber have been conducted.
Figure 3 illustrates a schematic of the impedance tube and sound absorption curve of a coconut fiber sound absorber according to thickness (20,35, and 50 mm) [49].The NRC values were 0.20 at 20 mm thickness, 0.32 at 35 mm thickness, and 0.43 at 50 mm thickness.The NRC increased as the thickness of the sound absorber increased.
In addition, at a thickness of 20 mm, the maximum sound absorption coefficient was 0.83 at a frequency of 3651 Hz; it was 0.76 at 2564 Hz and a thickness of 35 mm, and it reached 0.88 at 1435 Hz and a thick-ness of 50 mm.Thicker coconut fiber sound absorbers demonstrated improved sound absorption capabilities, particularly at lower frequencies.The sound absorption mechanism is a ributed to the air viscosity within the interconnected voids that separate the coconut fibers.Furthermore, the intricate void structure significantly influences sound energy absorption within the material.This behavior is a commonly observed in fibrous sound-absorbing materials [50,51].

Figure 3.
Sound absorption curve of coconut fiber sound absorber according to thickness [49].

Kenaf Fiber
Kenaf fiber is obtained from the annual plant of the Malvaceae family originating from India and Africa, and it is cultivated in more than 20 countries [52].Kenaf has demonstrated good productivity, with a high growth rate and low crop rotation [53].Kenaf fiber is used as food protection packaging material, in textiles, and for filtration, and it performs excellently as a filler for composite material [54].
Figure 4a compares the sound absorption curves of kenaf fiber (density: 93.5kg/m 3 , thickness 40 and 45 mm, and rock wool sound absorber (density 100 kg/m 3 and thickness: 45 mm).From an acoustic point of view, compared to rock wool material, the sound absorption performance of Kenaf fiber could outperform that of synthetic rock wool material.However, to achieve this, its sound absorber thickness must be greater than that of mineral wool, because kenaf has a lower density [55].
Figure 4 presents the sound absorption curve of kenaf fiber as a function of the thickness (Figure 4b), air back cavity (Figure 4c), and density (Figure 4d) [55].Noticeably, the increase in the sound absorption performance corresponded to increasing material thickness.The absorption bandwidth improved significantly at lower frequencies; the thicker the absorber, the higher was the number of low-frequency components absorbed.Using an air back cavity between 0 and 30 mm increased the sound absorption performance at low frequencies, demonstrating an identical effect as that of a thick sound-absorbing material [55].
The overall sound absorption performance clearly improved as the bulk density of the sound absorber in-creased because the higher the sound absorber density, the be er is the absorber tortuosity, leading to greater sound energy loss [55][56][57].Husain et al. [58] investigated the sound absorption performance of needle-punched kenaf nonwoven fabrics for noise absorption in automotive applications through a webforming process.Overall, the sound absorption performance of the kenaf fiber sheet was determined by the thickness and density of the sample.The NRC values of kenaf nonwoven sheet were 0.14 for 2-ply (thickness: 2.74 mm, density: 410 g/m 2 ), 0.15 for 4-ply (4.52 mm, 994 g/m 2 ), 0.19 for 6-ply (7.52 mm, 2069 g/m 2 ), and 0.18 for 8-ply (8.22 mm, 2461 g/m 2 ).On the other hand, the NRC of butyl rubber sound absorber was only 0.07 (3.64 mm, 5013 g/m 2 ), and the NRC of a hybrid kenaf mixed with PET (4.71 mm 2507 g/m 2 ) was 0.21.
An increase in the sample thickness due to an increase in the layer numbers enhanced the sound absorption performance.These characteristics were common to fibrous soundabsorbing materials [55][56][57].Therefore, Kenaf fiber is expected to serve as an eco-friendly sound-absorbing material.

Rice Husks
Rice, along with wheat and corn, are one of the three staple crops worldwide, and it is the staple food in most Asian countries [59].The kernel of rice mainly consists of the endosperm, bran, germ, and husk, with the la er accounting for 20-21% of the total seed weight [60,61].These agricultural by-products have great potential in rice-producing countries; however, a decrease in rice consumption per capita has affected rice farmers, who have experienced increasing difficulty in generating profits [62].
Rice husks are usually incinerated or only recycled for low-cost applications such as thermal insulation materials, polymer composites, and concrete composites [60].The uses of these by-products to expand to overall building materials, it would not only create added value for domestic farmers but also contribute to improving competitiveness within the construction industry by supplying cheap, green building materials [60,[63][64][65][66]. From this perspective, several studies have been conducted to investigate the potential uses of rice husk as a sound-absorbing material.
Figure 5 shows the sound absorption curve of rice husk [67].When the thickness of the rice husk sound absorption disk increased, the sound absorption coefficient (α) at low frequencies increased.The NRC of the rice husk disk extracted from the graph was 0.11 at 10 mm, 0.18 at 20 mm, and 0.33 at 30 mm.Mahzan et al. [68] investigated the sound absorption performance of six types of rice husks mixed with polyurethane.The NRC was 0.36 for the polyurethane mixture with 15% rice husk and 0.65 for the mixture with 30% rice husk.Conclusively, as the rice husk content increased, the sound absorption performance improved.
Wang et al. [69] investigated the sound absorption performance of polyurethane composites based on the rice husk content and reported that a polyurethane-rice husk mixture comprising 5% chaff had the best sound absorption performance.Rice husk has a significant influence on the pore size and pore size distribution of polyurethane foam, which leads to a decrease in flow resistance and an improvement in sound absorption performance, suggesting that the improved sound absorption was a ributable to the addition of rice husk.The rice husk was advantageous as the thickness of the porous polyurethane material did not need to be increased for improved sound absorption performance at low frequencies.
António et al. [60] analyzed the sound absorption performance of two boards made of a rice husk composite: one with expanded cork granules and the other with recycled rubber granules.Regarding the expanded cork granules, the NRC was 0.15 for the composite comprising 50% rice husk and 0.25 for the 75% rice husk.For recycled rubber granules, the NRC was 0.35 and 0.45 for 50% and 75% rice husk, respectively.The sound absorption performance increased with rice husk content.
Marques et al. [70] investigated the sound absorption characteristics of polymerbased composite materials fabricated using rice husk and expanded cork by-products.When the rice husk content was low, the density of the composite increased and the sound absorption coefficient (α) decreased over the entire frequency range.
These studies suggest that rice husk has sound absorption performance, and it can be applied to construction systems, including walls and ceiling.Rice husk has demonstrated excellent sound absorption behavior independently, as well as in composites with a polymer, which suggests that rice husk has great potential for use as a green soundabsorbing material.

Rice Straw
The majority of rice straw, according to the International Rice Research Institute, is produced in Asia [71], with Japan producing 8.2 million tons in 2019 [72] and Korea producing an estimated 4 million tons as of 2020 [73].Most rice straw is burned on site after harvest, which negatively impacts the atmosphere and public health because this combustion releases greenhouse gases and forms suspended particles [74][75][76].
Previous studies designed various methods for using rice straw to reduce environmental pollution, in particular, noise pollution, and numerous studies using rice straw as a building material, including for insulation, have been conducted [77][78][79][80].This article presents the use of rice straw as a sound absorbent.Figure 6 compares the sound absorption curve of rice straw with that of synthetic glass wool [21].The NRC was 0.33 for glass wool and 0.24 for rice straw.The two graphs are comparable, but the sound absorption performance of rice straw is slightly lower than that of glass wool.The overall sound absorption characteristics for all frequency bands between the two materials were similar, although the NRC of rice straw was lower than that of glass wool.
Kang et al. [81] investigated the sound absorption characteristics of rice straw particle mats.The sound absorption coefficients (α) of the two methods were relatively similar.When the mat thickness was increased from 20 mm to 100 mm, the sound absorption coefficient (α) was 0.1-0.65 below 1000 Hz and 0.1-0.95above 1000 Hz.These results demonstrate that the sound absorption coefficient (α) increased at low frequencies depending on the rice straw particle mat thickness.Rice straw has be er sound absorption properties when mixed with various materials than when used alone.Yang et al. [82] investigated the sound absorption characteristics of composite sound-absorbing materials mixed with rice straw and wood particles and observed that, as the rice straw content increased, the sound absorption coefficient (α) of rice straw composite board surpassed that of other wood-based materials (fiberboard and plywood).
These studies suggest that rice straw is more useful as a sound-absorbing composite material than other natural materials.Climate change has prompted increased interest in reducing greenhouse gases worldwide.Accordingly, interest in recycling agricultural byproducts is growing, and a method of utilizing rice straw as a natural sound-absorbing material has a very positive outcome.If the use of rice by-products expands to building materials, it would not only create added value for domestic farmers but also contribute to im-proving competitiveness within the construction industry by supplying cheap, green building materials [60,[63][64][65][66].

Hanji (Korean Traditional Paper)
Hanji is a Korean traditional paper fabricated from paper mulberry (Broussonetia kazinoki Sieb) [83] that is characterized by longer fiber length and higher molecular weight than wood fiber, imparting high tensile and tear strength, as well as excellent preservability [84].Hanji is a traditional building material used in Korean houses for indoor environment (heat, light, and air) and humidity control.It is also a green material, whose manufacture and disposal do not damage the environment [85][86][87].

Glass wool Rice straw
Hanji, being very thin paper, has almost no sound absorption performance, but Jang et al. [88] reported that, as the pore size of Hanji decreased, the sound absorption performance slightly increased.
Hanji would be much more effective as a membrane-type sound-absorbing material with an air back cavity than as a porous sound-absorbing material.Lee et al. [89] added an air back cavity of 0-12 cm to bilayer Hanji and observed no significant change as the NRC ranged between 0.31 and 0.33.However, as the air back cavity length increased, the sound absorption at low frequencies tended to increase (Figure 7).
When compared to ordinary copy paper, the sound absorption performance of Hanji was greater than that of copy paper at frequency ranges between 600 and 700 Hz and 2000 and 2100 Hz [90].Moreover, as the basis weight was increased, the degree of impregnation increased, resulting in more sound absorption.
Compared to other natural materials, the sound absorption performance of Hanji remains insufficient.However, further research on sound-absorbing materials using Hanji is needed to increase the use of Hanji as a natural material unique to Korea.

Tea-Leaf Fiber Waste
Tea leaf waste fiber is obtained by drying and removing leaves after tea leaf processing.In industrial tea production, a considerable amount of tea leaf waste fiber is generated when extracting instant and bever-age tea.Therefore, various industries are exploring alternative uses for this waste fiber [91].The sound absorption properties of tea leaf li er fibers described in the literature are presented herein.
Ersoy and Küçük [92] investigated the sound absorption characteristics of tea-leaffiber waste at 500-6300 Hz. Figure 8 shows the sound absorption curve of a tea-leaf-fiber sound absorber according to thickness.The sound absorption coefficient (α) of a 10 mm thick absorber was 0.26 at 6300 Hz, and the maximum sound absorption coefficient (α) at a thickness of 20 mm was 0.6 at 6300 Hz.The sound absorption coefficient (α) of a 30 mm thick absorber was 0.7 at 5600.
In addition, the sound absorption coefficient (α) significantly improved when a single layer of woven cloth was placed as a backing rather than pure tea-leaf-fiber itself.

Mandarin Peel
In 2020, the production of mandarin in the Jeju region of Korea approached 654,000 tons [93].However, mandarin peels are rarely used, except in some herbal medicines [94].
Kang et al. [95] investigated the sound absorption characteristics of a sawdust-mandarin peel composite particle board and observed that the sound absorption coefficient (α) increased in the high-frequency band rather than the low-frequency band.They also revealed the characteristics of a typical porous sound absorber.
Figure 9 presents the effect of the amount of mandarin peel on the sound absorption curve of a sawdust-mandarin peel composite particle board.The sound absorption coefficient (α) increased to 0.52, 0.65, and 0.76 at 2000-3000 Hz as the mandarin peel content was increased from 10% to 40%.The authors suggested that the sawdust-mandarin peel composite particle board could be used as a sound-absorbing ceiling material.

Figure 9.
Sound absorption curve of a sawdust-mandarin peel composite particle board depending on content of mandarin peel [95].

Pineapple Leaf Fiber
Pineapple leaves are a valuable source of premium natural fiber.It has numerous potential applications, including reinforcing plastics, providing thermal insulation, and conferring sound absorption properties [96].
Putra et al. [97] examined the sound absorption performance of extracted pineappleleaf fiber.Figure 10 presents the sound absorption curve of extracted pineapple-leaf fiber according to thickness.Noticeably, as the thickness increased, the sound absorption coefficient (α) increased.The sound waves in the sound-absorbing material experienced difficulty in moving through the fibers, and sound energy was lost due to friction between the sound waves and pineapple-leaf fibers.The complexity of the path between the fibers allowed sound waves to be captured and absorbed within the sample.The introduction of an air back cavity also improved sound absorption at low frequencies.By controlling the density of pineapple-leaf fibers or introducing an air cavity behind the sample, an average sound absorption of 0.9 above 1000 Hz could be achieved.The increase in the density of pineapple-leaf fibers correlated with increased air resistance and tortuosity.
Figure 10.Sound absorption curve of pineapple-leaf fiber depending on thickness [97].

Corn Husk
Corn is the most widely cultivated cereal crop globally, with a significant presence in numerous countries.Specifically, the United States and China collectively make up approximately 50% of the global corn market [98].The stover of a corn plant typically comprises 50% stem (stalk), 35% leaves and cobs, and 15% hull.However, a significant portion of corn stover is treated as waste [99].Only a small amount is utilized as household fuel or animal manure, whereas the majority of the residues are commonly incinerated.However, these residues can serve as a renewable resource [100].
Tang et al. [101] analyzed the effect of a multilayer structure and air back corn husk cavity.The average single-layer sound absorption was 0.158 at 100-2500 Hz, whereas that for a five-layer structure was 0.409 and increased with the number of layers added to the sample.Figure 11 presents the sound absorption curve of corn husk with respect to thickness.Researchers consider corn husk to be a green sound-absorbing material because it is thinner and lighter than commonly used porous materials.Increasing the air back activity also improved the sound absorption of corn husk at low frequencies.This study demonstrates that corn husk has good sound absorption performance and is suitable for noise reduction.

Peanut Shell
Generally, 230-300 g of shells are produced per kilogram of peanuts, and the global peanut shell waste reached 10.7-14 million tons in 2017 and 2018 [102].Peanut shells have been extensively studied for industrial applications, such as biofuel production [103].
Malawade and Jadhav [104] investigated the sound absorption coefficient (α) of peanut shells.The thickness of the peanut shells ranged between 10 and 40 mm, and yellow dextrin powder was used as a binder.They considered weight ratios of the peanut shells to binder of 55:45, 60:40, 65:35, 70:30, and 75:25.
As shown in Figure 12, the sound absorption coefficient (α) increases as the thickness increased.The optimal sound absorption rate was obtained at a material-to-binder weight ratio of 70:30.
Jang (2022) [51] investigated the sound absorption performance of pure peanut shells.Peanut shells were packed in impedance tubes to a height of 3, 6, and 9 cm.The NRC was 0.23 at a 3 cm height, 0.43 at a 6 cm height, and 0.54 at a 9 cm height.Therefore, peanut shells are considered promising green sound absorbers.

Sugar Palm Trunk Fibers
The sugar palm is a species similar to sugar cane and is a naturally occurring species in the humid regions of Southeast Asia [105].Sugar palm fibers naturally envelop the entirety of the sugar palm trunk, extending from the uppermost part of the tree to its bottom [106].Sugar palm trunk fiber is a type of bast fiber known for its high cellulose concentration, which contributes to its excellent tensile strength.This fiber has both durability and resistance to seawater, making it suitable for various applications [107].
Prabowo et al. [108] investigated the sound absorption characteristics of sugar palm trunk fibers.As the thickness of the fibers was adjusted and the bulk density was changed, the sound absorption coefficient (α) increased as the bulk density increased.The increased bulk density resulted in significant sound absorption as sound energy was dissipated due to friction between the sound waves and fibers.
Figure 13 shows the sound absorption curves of sugar palm trunk fibers (40 mm and 30 mm) with a 10 mm air back cavity.Sugar palm trunk fibers (30 mm) with a 10 mm air back cavity exhibited similar sound absorption as a material with a 40 mm thickness in the frequency range of 1000-2500 Hz.Creating an air back cavity in sound absorption materials can result in identical sound absorption performance even if the sample volume is reduced.

Yucca gloriosa Fiber
Yucca gloriosa fiber, also referred to as Spanish dagger fiber, is obtained from the foliage of the Yucca gloriosa plant.An indigenous plant to the southeastern region of the United States, it is classified under the Agavaceae botanical family.Its notable features include a rose e arrangement of elongated, blade-like leaves [109].The mechanical properties of yucca gloriosa fibers have made them a preferred choice as fillers in hybrid composites.These fibers are known for their ability to enhance the reinforcing characteristics of the composites [110].
Soltani et al. [109] investigated the sound absorption characteristics of yucca gloriosa fiber.They demonstrated that the 15 mm-thick sample had an NRC of 0.18, but that of the 30 mm sample was 0.40.Thick samples had be er sound absorption properties than thin samples.
Figure 14 presents the effects of the air back cavity distance on the sound absorption curves.When the air back cavity is large, the sound absorption coefficient (α) decreased at high frequencies, and it increased at low frequencies.
Yucca gloriosa fiber demonstrated similar sound absorption characteristics as other natural fibers.

Fruit Stone Wastes (Cherry, Apricot, Peach, Olive)
Fruit stone waste did not receive much a ention in the past, but many studies have recently revealed that it can be used as a biomass feedstock for biofuel production [111].
Borrell et al. [112] investigated the sound absorption coefficient (α) of four types of fruit stone wastes (cherry, apricot, peach, and olive).The sound absorption coefficient (α) was dependent on the shape and size of the fruit stones.An increase in sample thickness correlated with an increase in sound absorption coefficient (α) at a low frequency.Apricot and cherry stones demonstrated the best sound absorption at low frequencies (Figure 15).
Fruit stones are effective sound-absorbing materials for reducing noise in buildings.The study suggested that expanding to other types of fruit stone and crop waste could create high-value-added products in the future.

Wood Bark
Various studies have examined the sound absorption capabilities of wood and woodbased materials.Wood cross-sections are naturally porous sound absorbers [113][114][115][116]. Additionally, when a hole is drilled in a wood panel and an air back cavity is introduced, it can act as a resonant sound absorber [117].
Kang et al. [118] analyzed the sound absorption performance of five spice coniferous barks and one spice hardwood bark.The specimens were prepared by crushing bark into different thicknesses and densities.The sound absorption coefficient (α) increased as the thickness increased.Figure 16 presents the NRC of wood bark particles (filled thickness: 100 mm) as the packing density was adjusted from 0.12 to 0.16.The overall NRC increased as the packing density of wood bark particles increased.The bark particles had varying shapes according to the tree species, resulting in different sound absorption performances.Wood bark may be used as an environment-friendly road barrier material.

Ne le Fibers
Ne le, a versatile plant that can be grown in various regions of Asia, including nonagricultural lands, requires minimal resources.Ne le fiber is a natural fiber with excellent thermal and mechanical properties [119].
Recently, ne le fibers have been studied for use as a natural sound absorbing material.Raj et al. [120] investigated the NRC of ne le fiber 8-80 mm thick.The relationship between thickness and the increase in NRC seems to follow a logarithmic pa ern.The optimal NRC (0.7) corresponded to a sound-absorbing sample thickness of 56 mm (Figure 17).Therefore, ne le fibers can be considered useful eco-friendly sound absorbers.

Conclusions
Green materials are low in toxicity and safe for human use, making them a preferred option for various applications.Their biodegradability reduces environmental impacts and promotes sustainability.Therefore, green materials are highly desirable as sound absorbing materials.This study reviewed a variety of green material sound absorbers: coconut fiber, kenaf fiber, rice bran, rice husk, and rice straw, Hanji, tea-leaf fiber, sawdust- mandarin peel, pineapple-leaf fiber, corn husk, peanut shell, sugar palm trunk, yucca gloriosa fiber, fruit stones, wood barks, flax fiber, and ne le fiber.For a majority of green sound-absorbing materials, as the thickness of the material increases, the density increases, or the air back cavity widens, the sound absorption coefficient at low frequencies is enhanced.Thick sound-absorbing material provides many spaces for sound energy to penetrate, effectively improving noise reduction.When an air back cavity is utilized, incident sound energy causes resonance, thereby absorbing sound.This result indicates that applying an air back cavity is much more efficient than using an absorbent with continuous thickness, thus saving material.
This review suggests that, in addition to the specific materials discussed, a wide range of granular, fibrous, and porous eco-friendly materials can be used as sound-absorbing materials.By carefully controlling the physical properties, such as thickness, density, and the presence of an air back cavity, these mate-rials can effectively absorb sound.This implies that numerous possibilities for exploring and utilizing different eco-friendly materials for their sound-absorbing capabilities exist, expanding the range of options available for addressing noise pollution.However, the commercial use of green materials as sound absorbers requires their constant supply.In this context, rice husk and rice straw, which have sustainable production capacity, may have the most commercial value.In terms of continuous productivity, further study is needed to improve manufacturing cost, mechanical strength, heat resistance, and insulation.
Notably, natural materials have to overcome problems such as moisture fragility, decay, and low fire safety.If these limitations can be overcome, the potential for developing green sound-absorbing materials can be greatly increased to include the replacement of synthetic sound-absorbing materials.

Figure 6 .
Figure 6.Comparison of sound absorption curves of rice straw and synthetic glass wool [21].

Figure 12 .
Figure 12.Effect of thickness on the sound absorption curve of peanut shells with binder (weight ratio of 70:30) [104].

Figure 13 .
Figure 13.Sound absorption curve of sugar palm trunk fibers of 40 mm thickness and 30 mm thickness with a 10 mm air back cavity [108].

Figure 14 .
Figure 14.Sound absorption curve of Yucca gloriosa fiber depending on air back cavity distance.

Figure 17 .
Figure 17.NRC of ne le fibers depending on the thickness.