The latest research status of porous sound-absorbing materials
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Zhiting Feng
Abstract
With the rapid development of urbanization and transportation, noise pollution has become one of the most severe environmental problems for people. It usually causes various disorders and significantly affects human work efficiency and living standards. In the noise control field, using sound-absorbing materials to reduce noise is a critical way to reduce the harm of noise pollution. As the most widely used sound absorption material, porous materials are lightweight, have a wide absorption frequency range, and have strong sound absorption ability. They have great potential in the field of sound absorption. This paper first summarizes the sound absorption mechanism of porous sound absorption materials and the critical factors affecting the sound absorption of porous materials. Secondly, the latest research status of fiber, foam, and new porous sound absorption materials in recent years is reviewed, and the advantages and disadvantages of different porous sound absorption materials are expounded. Finally, the future development trend of porous sound-absorbing materials is prospected. With the continuous expansion of knowledge in this field, it is expected that porous sound-absorbing materials will continue to improve and find more practical applications in emerging fields in the future.
1 Introduction
Noise pollution has attracted widespread attention as a new environmental problem that seriously affects human physical and mental health. Worldwide, noise pollution is ranked as one of the environmental hazards, along with air pollution, water pollution, and solid waste pollution. 1 , 2 Therefore, developing effective acoustic materials is crucial for noise reduction and has been widely used in different fields. The main preventive measures against noise are the control of sound sources and the use of acoustic materials. Sound source control is mainly done by improving the structure of the equipment and the quality of processing and assembly to reduce the sound source’s radiant energy. 3 , 4 , 5 , 6 , 7 Adopting sound-absorbing materials is the most versatile and effective passive sound absorption and noise reduction method. With a wide range of materials and relatively simple manufacturing processes, porous materials have become the most widely used acoustic absorption materials. 8 , 9 , 10 , 11 , 12
Porous acoustic materials consist of channels, cracks, and cavities, which allow sound waves to enter the material. The dissipation of acoustic energy is mainly due to heat loss caused by friction of air molecules against the pore walls and viscous loss of airflow inside the material. These energy dissipations give porous materials a wide acoustic absorption band. 13 , 14 , 15 According to the principle of sound absorption, 16 in general, the high-frequency sound absorption performance of porous materials is much higher than the low-frequency sound absorption performance. The sound absorption coefficient of porous materials with a thickness of 30 mm is poor below 500 Hz, which is usually less than 0.4, which is because acoustic energy dissipation is a quadratic function of frequency in the sound absorption mechanism of porous materials. 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 Enhancing the sound absorption performance by increasing the pore thickness and adjusting the pore structure parameters, such as pore diameter and porosity, can slightly improve the low-frequency sound absorption coefficient. 27 , 28 , 29 However, increasing the thickness or porosity of the specimen takes up space or reduces its strength, limiting its wide application. Therefore, it is usually impractical to use a single conventional porous acoustic material to achieve significant noise reduction at low frequencies. Therefore, the development of a lightweight, simple, and low-cost acoustic material to improve the acoustic performance of acoustic materials is an urgent problem for researchers in various countries. 30 , 31
Porous sound absorption material is a kind of material with excellent functions and properties such as sound absorption, vibration reduction, noise reduction, lightweight, high stiffness, and high strength. They have attracted great attention in both scientific research and engineering applications. 32 , 33 , 34 , 35 In this paper, the latest research status of fiber acoustic materials is systematically reviewed. The acoustic characteristics of organic fibers and inorganic fibers are analyzed and compared. The advantages of new porous acoustic materials, such as cellulose aerogels, which exhibit green, high-efficiency, and degradability features, are highlighted. This provides fresh perspectives for the research and development of acoustic materials aimed at sustainable development. Secondly, regarding foam acoustic materials, not only are the acoustic characteristics and application fields of organic foam and inorganic foam summarized but also the research challenges associated with broadening the sound absorption frequency band and enhancing the controllability of internal structures are discussed in depth. By presenting a case study on the utilization of recycled resources to prepare novel acoustic materials, the innovative application potential of foam acoustic materials in the context of environmental protection and resource recycling is demonstrated. Furthermore, this paper focuses on the design and research progress of composite structures of porous acoustic materials. By introducing novel designs such as layered porous acoustic metamaterials, microperforated plates, and porous material composite structures, it is revealed that such composite structures offer significant advantages in improving low-frequency sound absorption performance and achieving broadband, high-efficiency sound absorption. These research findings not only address the shortcomings of traditional porous acoustic materials but also provide new theories and methodologies for the design and optimization of acoustic materials. Finally, based on a summary of the existing research results, the future development of porous acoustic materials is prospected. By proposing innovative ideas in material modification and structural design, this paper aims to provide a valuable reference for subsequent research and promote the widespread application of porous acoustic materials in the field of noise control.
2 Porous materials
2.1 Sound absorption mechanism of porous materials
For any wave, any point on the wave surface of the wavelet emitted from the wave source can be used as the wave source of the wavelet. The envelope surface of each wavelet source surface is the next new wave surface, namely the Huygens principle. The vibration of the sound source causes fluctuations. In a continuous medium, the fluctuation state at each point is affected by the fluctuation of all other points. 36 , 37 When sound waves hit a material, there are three conversions of sound energy: reflection, absorption, and transmission. The total acoustic energy can be regarded as the sum of the reflected, absorbed, and transmitted energy. 13 , 38 Figure 1(a) shows the material medium’s transmission path of sound waves.
In the formula, E i is the total incident sound energy, E α is the absorbed sound energy, E t is the transmitted sound energy, and E r is the reflected sound energy.
Sound absorption mechanism of porous sound absorbing materials. (a) Schematic diagram of the transmission path of sound waves in the material medium; (b) porous sound absorption material pore distribution diagram.
The sound absorption coefficient α is the ratio of the sound energy absorbed by the material E α to the total sound energy incident on the material E i , which is the main evaluation index to characterize the sound absorption performance of the material. 23 When α = 0, the incident sound wave is completely reflected on the material’s surface, and there is no sound absorption effect. When α = 1, the incident sound wave is completely absorbed by the material. 13 The α values of all materials are in the range of 0–1. The larger the α value of the material in this range, the better the sound absorption performance of the material, and vice versa.
In the formula, p r is the sound pressure of the reflected sound wave, p i is the sound pressure of the incident sound wave, and r is the reflection coefficient.
The interior of the porous sound-absorbing material has many small pores and coherent bubble holes, and the air is very easy to pass through. 39 When the sound wave reaches the surface of the porous sound absorption material, the sound energy is mainly dissipated in three ways: (1) The vibration of the sound wave causes the movement of the air in the pore, which causes friction between the air and the pore wall so that a part of the sound energy is converted into heat energy, resulting in the sound wave attenuation that can achieve the purpose of sound absorption. (2) The longitudinal acoustic wave periodically compresses and releases the air in the pores to dissipate acoustic energy during conversion. (3) High-frequency sound waves enter the pores of porous materials so that the vibration velocity of air particles between pores is accelerated, and the sound energy is dissipated into mechanical energy and thermal energy. 40 , 41 Figure 1(b) shows a porous sound absorption material pore distribution diagram.
2.2 Factors affecting sound absorption of porous materials
2.2.1 Effect of airflow resistance
In the case of a stable flow rate of the fluid, the ratio of the static pressure difference of the gas flowing through the two surfaces of the porous material to the gas flow rate is the flow resistance. 42 The expression of the flow resistance is as follows: 43
In the formula, r f represents the flow resistance (Pas/m), ∆P represents the static pressure difference between the two sides of the porous material, U represents the volume velocity of the air, A represents the cross-sectional area of the porous material, and u represent the linear velocity perpendicular to the surface of the material.
The flow resistance is a parameter used to evaluate the viscosity of sound-absorbing materials to air. If the flow resistance is too small, the damping caused by friction and viscous force is insufficient, and the sound absorption performance is poor. If the flow resistance is too large and the air penetration is too small, the sound absorption performance will decrease, and the normal ventilation system will be affected. 44 Therefore, there is an optimal flow resistance value for porous materials. Too high and too low flow resistance values cannot give the material good sound absorption performance. Generally, the sound absorption performance is better when the flow resistance is 100–1,000 (Pas/m). 45
2.2.2 Effect of porosity
Porosity refers to the ratio of the internal pore volume to the total volume of the material. The larger the porosity of the sound-absorbing material, the greater the tortuosity of the pores, the more complex the internal channels, and the more significant the internal friction and viscous resistance after the sound wave enters the pores. The porosity of materials with good sound absorption performance is generally between 70 % and 90 %. 46 , 47 The expression of porosity is as follows: 35
In the formula, ε p is the porosity, V α is the pore volume inside the porous material, and V m is the total volume of the porous material.
2.2.3 Effect of pore size
For porous materials with a certain porosity, pore morphology, and thickness, the smaller the pore size, the more pores are inside the material, and the more complex the overall network structure of the pores. The number of reflections and refractions generated by sound waves passing through the pores will increase. The contact area between the air inside the material and its adjacent pore walls increases, and the energy loss caused by friction and heat transfer caused by relative motion will also increase, improving the material’s sound absorption performance. If the pore size is too small, the sound wave will have difficulty entering the interior of the porous sound absorption material, but it will directly reflect on its surface, and the sound absorption performance will decrease. Therefore, the pore size of the material should not be too large or too small, and the pore size of the porous material with better sound absorption performance is generally 0.1 mm. 48 , 49
2.2.4 Effect of thickness
The low-frequency sound absorption performance of porous sound-absorbing materials is poor. As the thickness increases, the peak value of the sound absorption spectrum moves in a low-frequency direction, the low-frequency sound absorption coefficient increases, and the sound absorption frequency band becomes wider. 50 Mainly because in porous sound-absorbing materials, high-frequency sound waves are mainly absorbed on the material’s surface, and low-frequency sound waves are mainly absorbed inside the material. In the case of certain pore size and porosity, as the thickness increases, the acoustic wave becomes longer through the pore channel, the number of blocking times of the corresponding tortuous channel increases, and the acoustic energy loss increases, showing a trend that the low-frequency sound absorption coefficient increases with the increase of thickness. However, when the thickness is too large, it will waste space and material, so the thickness of the material should have a suitable range. The calculation formula for the thickness σ of the sound-absorbing material is as follows: 51
where f r is the first resonant frequency (Hz), s is the base area (m2).
2.2.5 Effect of material density
Increasing the density of sound-absorbing materials with the same thickness can improve the sound absorption coefficient of medium and low frequency, but the effect is smaller than that of increasing the thickness. If the thickness is not limited, the porous material should be loose in the case of the same material. With the increase in material density, the material is dense, which leads to an increase in flow resistance, a decrease in air permeability, and a decrease in sound absorption coefficient. With the same density, increasing the thickness does not change the flow resistance, and the sound absorption coefficient always increases. The improvement in sound absorption performance is insignificant when the thickness increases to a certain value. 52 The formula for material density is shown below:
where M is the mass (kg), and V t is its bulk volume (m3).
2.2.6 Effect of structure factors
The structure factor depends on the material’s nature and reflects the material’s internal microstructure. It is related to internal and external shapes, porosity, and material properties. It is usually in the range of 2–10 and sometimes can reach 25. The skeleton in the internal space of porous materials is called a meridian. The incident sound wave mainly propagates along the pores between the meridians, and only a small part propagates along the meridians, thus forming different sound absorption mechanisms. Generally speaking, the larger the structure factor, the more complex the pores in the porous material, the more times the sound wave reflects, the greater the sound wave loss, and the better the sound absorption performance. Assuming that the pore cross sections of the porous material are all circular, and the angle between the axial direction of the pore and the normal direction of the porous material is φ, the expression of the structure factor is: 50
In the formula, τ ∞ is a tortuosity factor, and φ is the angle between the axial direction of the pore and the normal direction of the porous material.
2.2.7 Effect of back cavity
The dissipation mechanism is mainly viscosity and heat loss when the porous material has no cavity. After adding a cavity behind the porous material, it can be used as a Helmholtz resonant cavity. When the sound wave is incident on the surface of the porous material and penetrates the material into the cavity, a resonance effect will be generated in the cavity. This resonance effect can consume acoustic energy, improving sound absorption. 53 Increasing the thickness and density of the material can improve the low-frequency sound absorption coefficient, and then the setting of the cavity can achieve a similar effect without increasing the thickness and density of the material. 54
3 The latest research status of porous acoustic materials
3.1 Fiber sound-absorbing materials
Porous fiber sound-absorbing materials are widely used because of their light weight, softness, simple processing technology, strong machinability, and low cost. 55 Fiber-based sound-absorbing materials can be divided into organic fibers and inorganic fibers according to chemical composition and raw material sources. 56 Organic fibers are fibers composed of organic polymer compounds, which can be divided into synthetic fibers and natural fibers according to the source of raw materials and production methods. Among them, synthetic fibers are generally prepared from petroleum derivatives, the most representative of which are polyethylene fibers and polypropylene fibers. These materials are widely used because of their small harm to the human body, excellent sound absorption performance, and simple process. However, due to their high degree of polymerization and stable chemical properties, they are generally difficult to degrade under natural conditions, and improper treatment may cause serious pollution to the environment. 56 Natural fibers mainly include wood fiber, cotton fiber, hemp fiber, animal fiber, and so on. The materials they prepare are degradable and have no risk of environmental pollution. However, these fibers are not fireproof and are not corrosion resistant, which greatly limits their development. 57 Inorganic fiber refers to fiber composed of inorganic substances (such as minerals), such as glass fiber, carbon fiber, etc. These fibers usually have excellent high-temperature resistance, high strength, and chemical stability.
3.1.1 Organic fibers
Synthetic fibers are durable, designable, and have regular micro-geometric shapes, which can be designed according to specific sound absorption requirements. They are synthetic materials that do not exist in nature and belong to artificial porous materials. 58 Synthetic fiber materials are widely used in the field of noise reduction in various industries due to their unique acoustic and mechanical properties. 59 Qian et al. 60 developed a new multilayer hybrid material composed of sound-absorbing nonwovens and electrospun polystyrene fiber membranes with different thicknesses. The electrospun polystyrene fiber membrane’s porous structure improves nonwoven sound absorption performance. When the thickness of the electrospun polystyrene fiber membrane is less than 0.8 mm, the medium and high-frequency sound absorption performance is improved. The low-frequency sound absorption ability is enhanced with the further thickness increase. Therefore, combining electrospun polystyrene fiber membranes and sound-absorbing nonwovens is an effective way to improve sound absorption performance. People can design electrospun polystyrene fiber membranes with different thicknesses according to the requirements of application scenarios for sound absorption in different bands. Figure 2(a) is the physical image of an electrospun polystyrene fiber membrane/nonwoven sound-absorbing material. Aiming at the problem of poor low-frequency sound absorption performance of traditional fiber sound-absorbing materials, Shao et al. 61 prepared double-layer and three-layer composite sound-absorbing materials by laminating polyvinyl butyral nanofiber membranes with polyester fiber felt and polyurethane films. As the thickness increases, the sound absorption peak gradually moves to the low-frequency direction for the double-layer structure, but the effective sound absorption band becomes narrower. The composite material has the best sound absorption performance in the low-frequency range for the three-layer structure, and the sound absorption coefficient can reach 0.78. Figure 2(b) is the preparation process of the nanofiber membrane multilayer composite material, and 2(c) is the schematic diagram of the sound-absorbing structure of different layers. Gliscinska et al. 62 used flax fibers as reinforcing material and polylactide fibers as a matrix material to prepare thermoplastic sound absorption composites. The sound absorption properties of nonwoven multilayer composites under different pressing conditions were studied. The results show that adding pre-pressed nonwovens to the composite material increases the sound absorption coefficient in the whole test frequency range, whether on the side or back of the incident sound wave.
Organic fiber sound absorbing material. (a) The physical image of electrospun polystyrene fiber membrane/nonwoven sound-absorbing material; 60 (b) the processes to prepare the nanofiber membrane multilayer composite material; 61 (c) the schematic diagram of the sound-absorbing structure of different layers; 61 (d), (e), and (f) are the cross-sectional SEM images of cotton, coconut, and sugarcane, respectively; 66 (g) the physical image of the luffa fiber/polyester fiber sound-absorbing composite; 67 (h) the preparation process of luffa fiber/polyester fiber sound absorption composite. 67
The advantages of synthetic fiber composites are high strength and mechanical properties, simple preparation process, and low cost. However, most synthetic fibers are made of nonrenewable resources, which have stable chemical properties and are difficult to degrade under natural conditions. Adhesives and foaming agents must be added during preparation, which harms the human body and the environment. In addition, synthetic fibers have poor air permeability compared with natural fibers, which may become an unfavorable factor in some application scenarios.
In recent years, natural fiber-reinforced composites have received extensive attention from researchers as an alternative to synthetic fiber-reinforced composites. They have relatively good mechanical and physical properties and can be used in various applications. Natural fiber is a biodegradable, pollution-free, harmless, lightweight, and renewable material. The most commonly used reinforcing natural fibers in composite materials are hemp, flax, loofah, banana, jute, sisal, sugarcane, ramie, betel nut, etc. 63 , 64 Various studies have shown that natural fibers such as tea fiber, rice straw fiber, and coconut fiber also have good sound absorption properties and are widely used to manufacture sound-absorbing panels in industry. 65 Hassan et al. 66 prepared three kinds of natural fiber-based composites by adding cotton, coconut, and sugarcane waste to epoxy resin. With the increase in fiber content, the sound absorption coefficient also increased. The sound absorption coefficient of coconut fiber-based composites is higher than that of other fiber-reinforced composites. Figure 2(d)–(f) show the cross-sectional SEM images of cotton, coconut, and sugarcane, respectively. Chen et al. 67 used luffa waste and environmentally friendly polyester fiber to prepare luffa fiber sound-absorbing composites by clean hot pressing technology. The average sound absorption coefficient of the composite material reaches 0.645, which is an efficient sound absorption material. In addition, compared with other plant fiber composites, luffa fiber composites show better moisture absorption and moisture dissipation properties and can be applied to sound-absorbing pads, buffer materials, and filling materials of sleep products. Figure 2(g) is the physical image of the luffa fiber/polyester fiber sound-absorbing composite, and 2(h) is the preparation process of the luffa fiber/polyester fiber sound-absorbing composite. Sakthivel et al. 52 prepared bagasse/bamboo charcoal, an environmentally friendly sound-absorbing material by compression molding process, can absorb more than 70 % of the incident noise. Under high humidity conditions, the acoustic properties of the natural fiber composite did not change significantly. This study improves the problem of sound absorption performance degradation of natural fiber sound-absorbing materials in humid environments. The acoustic properties of these natural fiber composites can be used as sound barriers, walls, pavements, auditoriums, halls, apartments, cars, aircraft, pipelines, noise equipment shells, and mechanical insulation materials.
As a renewable agricultural resource, natural fibers have low density are environmentally friendly and have good sound absorption and sound insulation properties. Their acoustic properties are comparable to those of traditional synthetic materials. Natural fibers also have good mechanical strength and dimensional stability, so they are often used as reinforcing materials for polymer composites. However, the problems of natural fiber sound-absorbing materials, such as flammability, insect resistance, mildew, and short service life, have not been effectively solved.
Cellulose aerogel is a new type of porous sound-absorbing material with high efficiency, green, recyclability, and degradability, and it has broad application prospects in noise suppression. Ruan et al. 68 prepared a multifunctional sound-absorbing cellulose nanocrystals (CNC) aerogel by freeze-drying technology using calcium chloride as a green crosslinking agent. The sound absorption coefficient can reach 0.99, and the average sound absorption coefficient is 0.85. Lou et al. 69 prepared oriented cellulose aerogels with regular distribution and axial open porous structure, and the sound absorption coefficient reached 0.88. This oriented cellulose aerogel can solve the problems of irregular distribution of open and closed pore structures and the coexistence of open and closed pore structures in traditional cellulose aerogels (CAs). Sun et al. 70 used down feather fibers (DFF) as a reinforcing material to disperse down feather fibers in a carboxymethyl cellulose (CMC) matrix to prepare cellulose composite aerogel. Adding down feather fibers greatly improves the sound absorption performance of cellulose aerogels. In addition, down feather fibers as a reinforcing material also improve cellulose aerogels’ mechanical properties and thermal insulation properties. Figure 3(a) shows the SEM images of CMC/DFF composite aerogels, and Figure 3(b) shows the sound absorption properties of CMC/DFF composite aerogels. Niu et al. 71 prepared carbon nanofiber-reinforced graphene aerogels (CAF/GAs). The sound absorption coefficient is close to 0.9, and its efficient sound absorption ability is due to the long honeycomb structure and specific surface area of open pores with high porosity, which helps diffuse sound waves’ propagation path. Compared with cellulose aerogels, carbon nanofiber-reinforced graphene aerogels have excellent sound absorption properties and strain sensing properties, which can be used in automotive, aerospace, and electrical industries. Figure 3(c) shows the preparation process of CAF/GAs. Figure 3(d) and (e) are the SEM images of CAF/GAs.
As an emerging third-generation aerogel material, cellulose aerogel not only inherits the advantages of cellulose, such as wide source, environmental protection, and sustainability, 72 but also has superior adjustable porous structure, such as ultra-high open porosity (up to 98.4 %), ultra-low bulk density (as low as 0.047 g/cm3), 73 large specific surface area (356 m2/g), and pore volume (1.27 cm3/g). 74 This gives them a strong competitive advantage in noise suppression applications. However, cellulose aerogels’ low density and high porosity lead to low mechanical strength. 75 The researchers further improved the mechanical strength of cellulose aerogels by adding reinforcing materials. This multifunctional aerogel material not only paves the way for the development of sustainable and efficient sound absorption materials but also has great application potential in the fields of construction, transportation, environmental acoustics, and so on.
3.1.2 Inorganic fibers
Currently, the widely used inorganic fiber sound absorption materials mainly include glass fiber, metal fiber, carbon fiber, and ceramic fiber. Glass fiber has a high sound absorption effect and is lightweight, which reduces the load of the building structure and improves construction efficiency. Glass fiber sound-absorbing board is made of natural inorganic materials, which are environmentally friendly. It also has excellent durability, is not easily affected by humidity, temperature, and other factors, and has a long service life. 76 Glass fiber has good flame retardancy and can be used in high-temperature environments. Glass fiber is also often used as a reinforcing filler for other sound-absorbing materials. Gokulkumar et al. 77 added a small amount of glass fiber to epoxy resin to enhance the composites’ sound absorption and mechanical strength. Mondal et al. 78 discussed the sound absorption properties of composites composed of banana and glass fiber, and the sound absorption coefficient of 75 % banana and 25 % glass fiber composites was the largest. Glass fiber is widely used in transportation, construction, aerospace, and other fields. However, glass fiber is easy to break and fall off, resulting in fine fibers that irritate the skin and respiratory system, which greatly limits its application.
Metal fiber can still maintain good sound-absorbing performance after long-term use as a sound-absorbing material. Metal fiber materials also have excellent strength and stiffness and good fire and weather resistance. The used metal fibers can be recycled without causing environmental pollution and have electromagnetic shielding, thermal conductivity, and other versatility. 56 Therefore, metal fibers are more and more widely used in the field of sound absorption. This kind of material has excellent sound absorption performance in the high-frequency band. After forming a composite structure with the cavity, the sound absorption performance in the low-frequency band can be significantly improved. Wang et al. 79 studied the sound absorption properties of porous metal fiber materials with single-layer, double-layer, and three-layer gradient structures. The sound absorption coefficient of the multilayer gradient structure was found to be much higher than that of the single-layer structure at the same thickness. Zhang et al. 80 studied the sound absorption properties of acoustic labyrinth porous metamaterials (ALPM) at high temperatures. The metamaterial is constructed by perforation on a uniform metal fiber-based porous substrate with folded slits. ALPM exhibits excellent sound absorption performance in the frequency range of 20–600 Hz, especially at high temperatures. However, at some specific frequencies, the sound absorption effect of metal fiber sound-absorbing materials may not be as good as other specially designed sound-absorbing materials. Moreover, the metal fiber sound-absorbing material is more expensive than other traditional sound-absorbing materials, so the metal fiber is less commonly used.
Carbon fiber sound-absorbing materials rely on the torsional vibration of their tiny fibers and the frictional resistance between the fibers to reduce the sound’s propagation speed and vibration intensity, thereby achieving a good sound absorption effect. 81 Pakdel et al. 82 processed carbon fiber waste and polyamide fiber into nonwovens and found that the sound absorption performance of the nonwovens was greatly affected by the sample thickness and carbon fiber content. Increasing the number of layers creates a longer path for the incident sound wave to pass through the sample, significantly improving sound absorption performance. The sound absorption performance of nonwovens increases with the increase of carbon fiber content. Carbon fiber has a smaller diameter than other traditional fibers such as polyester, nylon, wool, and cotton. Therefore, the number of fibers per unit volume is higher at the same thickness, thereby increasing the contact area between the sound wave and the material. 83 Therefore, the probability of reflection and secondary reflection inside the material will also be greatly increased, making the sound wave propagation path more tortuous, which is conducive to the loss of sound energy. Carbon fiber also has lightweight, high strength, and high stiffness, making it suitable for space-limited environments without adding too much burden to the structure. It also has strong corrosion resistance and high-temperature resistance and is suitable for harsh environments. However, the production process of carbon fiber materials is complex, requiring high-temperature and high-pressure production equipment and other auxiliary processes, resulting in high production costs. In addition, although the carbon fiber material has strong tensile strength and impact resistance, it is relatively fragile and prone to fragmentation and wear. Once damaged, it is difficult to repair and process, and it usually needs to be replaced as a whole, which greatly limits its application.
Ceramic fiber has good sound absorption and sound insulation effects. When the sound wave is transmitted to the material’s interior, it will produce a viscous effect with the air in the pores of the fiber. At the same time, the sound wave will also produce a frictional resistance with the fiber, thereby losing the sound energy and converting it into heat energy. 84 Kang et al. 47 covered the surface of ceramic fiberboard (VCFB) with wood veneer to improve the sound absorption performance of VCFB and prevent dust generation. The results show that compared with the ceramic fiberboard, the average improvement rates of the sound absorption coefficient of VCFB are 124 % (500 Hz), 119 % (1,000 Hz), 143 % (2,000 Hz), and 68 % (4,000 Hz), respectively. As a new high-insulation building material, ceramic fiber can maintain good sound absorption performance in high-temperature environments. Ceramic fiber can withstand temperatures up to 1,800 °C and exhibit low thermal conductivity at high temperatures, so it has been applied in aerospace equipment. The density of ceramic fiber products is small, generally between 64 and 500 kg/m3, making it a lightweight sound absorption material that is easy to install and transport. In addition to strong alkali, fluorine, and phosphate, ceramic fibers are almost not eroded by chemicals and are suitable for various environments. Although the production cost of ceramic fiber is lower than before, its price is still relatively high compared with traditional materials. Ceramic fiber has a certain moisture absorption. Although it is not as easy to absorb moisture as glass fiber cotton, and rock wool, it may affect its performance in a humid environment. Ceramic fibers may exhibit certain brittleness, affecting their durability in specific applications. 85
3.2 Foam sound-absorbing materials
Foam porous sound-absorbing materials have a three-dimensional connected space network structure, high porosity, and large thickness. They also have excellent resilience, high compressive strength, and good secondary machinability. They have been used in various sound absorption and noise reduction occasions, such as construction, furniture, transportation, and other fields. 86 Foam sound-absorbing materials are divided into organic foam sound-absorbing materials and inorganic foam sound-absorbing materials according to their composition. Organic foam sound-absorbing materials are foam plastics made of resin as the base material. Polyurethane and melamine foam are widely used, and they are lightweight and waterproof. 87 Inorganic foam sound-absorbing materials include foam glass, foam ceramic, and foam metal. Foam glass, also known as porous glass, is a porous material made of glass powder by roasting, melting, foaming, and cooling after adding a foaming agent. It has the advantages of high strength, heat preservation, and moisture resistance. 88 Foam ceramics, also known as porous ceramics, are made by adding foaming agents and other chemical reagents to raw materials through gel casting. They have the advantages of rich raw materials, lightweight, and high porosity. 89 Foam metal is usually used for noise reduction in airports and highways and has the advantages of environmental protection, lightweight, and fire prevention. 90 However, foam sound-absorbing materials generally have problems such as narrow sound-absorbing frequency bands and poor internal structure controllability, making it difficult to improve their sound-absorbing performance in the low-frequency band.
3.2.1 Organic foams
Polymer foam is a kind of polymer matrix containing many pores. They have the advantages of low density, good thermal insulation, good sound absorption, and corrosion resistance. 91 However, the application of polymer foams is limited due to their low mechanical strength, poor surface quality, low thermal stability, and low dimensional stability. It can be improved by adding various types of fibers and nanoparticles. 39 Polymer foams with nanoparticles have unique structures and properties due to the customizable properties of nanoparticles. Shape, size, and surface chemistry can be easily customized to control the structure and properties of foams. 86 Nourmohammadi et al. 92 added nano-silica, nano-clay, and graphene nanoplatelets to the ethylene-vinyl acetate polymer foam, and the results showed that the sound absorption performance of the polymer foam was improved by 9.76 %. Figure 4(a) is the physical image of polymer composite foam, and 4(b) is the SEM image of polymer composite foam. Compared with traditional polymer foams, polymer nanocomposite foams’ strength, surface area, and damping properties have been improved. 86
Organic foam sound absorbing material. (a) The physical image of polymer composite foam; 92 (b) the SEM image of polymer composite foam; 92 (c) physical image of polyurethane foam and graphene oxide impregnated polyurethane foams; 95 (d) the optimization of sound absorption properties in multilayer polyurethane (PU) foams impregnated with graphene oxide (GO); 95 (e) the sound absorption coefficient of melamine particles with different content; 96 (f) sound absorption mechanism and sound absorption coefficient curves of PUF and D-GO/PUF. 97
Polyurethane foam is the most widely used sound absorption material in polymers because of its lightweight, good plasticity, and good sound absorption performance. In the sound absorption of polyurethane foam, air molecules vibrate, rub the hole wall, and convert acoustic energy into heat energy through resonance. 35 Therefore, the sound absorption performance of polyurethane foam is closely related to its cell morphology. By adjusting the formulation of polyurethane foam, such as changing the concentration of polyols, catalysts, crosslinking agents, water, isocyanates, and surfactants, the sound absorption performance of polyurethane foam can be improved. 93 In addition, adding various fillers will change the interfacial compatibility between the filler and the polyurethane matrix, which greatly affects the cavity size and pore type in the polyurethane foam. 93 , 94 Lee et al. 95 proposed a multilayer graphene oxide-impregnated polyurethane (GO-PU) foam structure. The sound absorption performance of each polyurethane layer was controlled by graphene oxide impregnation. Figure 4(c) is the physical image of polyurethane foam and graphene oxide-impregnated polyurethane foams, and 4(d) illustrates the optimization of sound absorption properties in multilayer polyurethane (PU) foams impregnated with graphene oxide (GO). The experimental results show that the impregnation effect of graphene is better, and the sound absorption coefficient is increased by 153 % on average compared with the original polyurethane. Yun et al. 96 added melamine particles to polyurethane foam. When the additional amount of melamine was 3 wt%, the sound absorption coefficient could reach a peak of 0.98. Figure 4(e) shows the sound absorption coefficient of melamine particles with different content. Zhang et al. 97 prepared phosphorus-containing graphene oxide composite polyurethane foam (D-GO/PUF) by vacuum impregnation technology. Compared with polyurethane foam (PUF), the sound absorption coefficient was increased by 245.45 %, and the flame retardancy of polyurethane foam was improved. Figure 4(f) is the sound absorption mechanism and sound absorption coefficient curves of PUF and D-GO/PUF. Polyurethane foam occupies the largest market in polymer foam and is mainly used in automotive, aerospace, construction, and other fields.
3.2.2 Inorganic foams
Foam metal is the most widely used foam sound absorption material in inorganic foam materials. Foam metal has characteristics that are both metal and porous in structure. It has attracted wide attention due to its high mechanical strength, good chemical stability, fire resistance, long service life, and easy recovery. It can be used as an effective sound absorption material. 98 Figure 5(d) shows distinct applications of metallic foams. Among various metals, aluminum foam has been extensively studied. 99 Liang et al. 42 compared the sound absorption performance of aluminum foam, copper foam, and nickel foam and found that the sound absorption performance of aluminum foam was better than that of copper foam and nickel foam. Figure 5(a) is the schematic diagram of nickel foam, 5(b) is the schematic diagram of copper foam, and 5(c) is the schematic diagram of aluminum foam. The open-cell aluminum foam has good sound absorption ability because the viscous friction and heat between the air molecules and the pore wall consume the sound energy. Because air permeability is the basic requirement of sound absorption materials, the sound absorption performance of closed-cell aluminum foam is not as good as that of open-cell aluminum foam. Changing the surface and structure of closed-cell aluminum foam can significantly improve its sound absorption performance. 100 Figure 5(e) is the open-cell structure of aluminum foam, and 5(f) is the closed-cell structure of aluminum foam. Opiela et al. 101 designed different perforation and cavity types to change the macroscopic structure of the aluminum foam and established the propagation model of sound waves in different structures to improve its acoustic performance. The results show that the through-hole aluminum foam has excellent sound absorption performance and poor sound insulation performance, while the half-through-hole aluminum foam has good sound absorption and sound insulation performance at high frequencies. The single-layer structure of metal foam has problems with good high-frequency sound absorption performance, poor low-frequency sound absorption effect, and narrow sound absorption frequency band. 102 At present, the research on the sound absorption performance of foam metal single-layer structure is mainly to improve the sound absorption coefficient or achieve the effect of wide-band sound absorption by changing the thickness of the material, the depth of the back cavity, the density, the pore size, the porosity, and other factors. In the study, it is found that with the increase of the thickness of the material and the depth of the cavity behind the material, the low-frequency sound absorption effect of the material is improved, but the sound absorption coefficient of the medium and high-frequency will decrease, and the sound absorption frequency band will narrow. Wang et al. 103 prepared ZL104 alloy/aluminum fiber composites with porosity of 70 %–90 % by infiltration casting method and studied their sound absorption properties. Compared with the original foam metal, the sound absorption performance of the composite material is significantly improved, the sound absorption coefficient at low frequency is increased by 6 times, and the sound absorption frequency band is increased by 2 times. Feng et al. 104 used polydopamine (PDA) as a binder to attach graphene oxide to the surface of the nickel foam skeleton and modified graphene oxide by PDA to improve its surface roughness. This synergistic effect significantly improved the sound absorption performance of nickel foam. Figure 5(g) shows that the graphene adheres to a foam nickel skeleton. Therefore, by combining foam metal with other reinforcing materials, the low-frequency sound absorption performance can be effectively improved, and the sound absorption band can be broadened.
Inorganic foam sound absorbing material. (a) Schematic diagram of nickel foam; 42 (b) schematic diagram of copper foam; 42 (c) schematic diagram of aluminum foam; 42 (d) distinct applications of metallic foams; 98 (e) open-cell structure of aluminum foam; 100 (f) closed-cell structure of aluminum foam; 100 (g) the graphene adheres to a foam nickel skeleton. 104
3.3 Other new porous sound-absorbing materials
Global waste production continues to grow, and its treatment methods mainly include landfills, combustion, and recycling. The landfill method relies on natural decomposition, has a long cycle, and may adversely affect surrounding residents. The combustion method uses waste as fuel, but untreated combustion emissions release pollutants, increase air pollution, and can cause lung cancer, heart and lung disease. Relatively speaking, the recycling method reuses the waste through pretreatment, which not only significantly reduces environmental pollution but also reduces costs. Therefore, recycling waste resources to prepare new sound-absorbing materials has become a research hotspot. Peceno et al. 105 evaluated the use of alternative renewable resources (such as mollusk shell waste in aquaculture) to completely replace traditional natural coarse aggregates to produce recycled porous concrete and use it as a sound absorption barrier for road traffic. The results show that the sound absorption coefficient of specimens made of any kind of shellfish waste with a particle size of 2–7 mm is 40 % higher than that of porous concrete specimens made of pebbles. The increase in coffee consumption has led to an increase in coffee waste. Coffee powder is a porous material that can effectively absorb sound. Yun et al. 106 developed a new sound-absorbing board combining coffee waste and resin. It is found that the sound-absorbing board can be applied to cafes that produce coffee waste, which can effectively reduce the noise generated in the cafe. Figure 6(a) shows the recycling options for coffee waste. Hassani et al. 107 studied the adhesion of phenolic resin to recycled denim as a potential substitute for synthetic sound-absorbing materials. Abandoned denim was collected from a municipal waste sorting plant and crushed into fiber form. The solid phenolic resin and fiber were sent to the dryer. Then, the mixed structure was introduced into the oven to solidify the resin and form an integrated, flexible structure. Flexible nonwoven composites with different bulk density and resin content were prepared. The results show that the resin-bonded regenerated denim composite has good noise absorption performance and the advantages of environmental protection and economic feasibility. Figure 6(b) is postconsumer and discarded denim wastes, and 6(c) is denim shoddy fibers. The waste is recycled and used to prepare a new type of sound absorption material, which realizes the maximum utilization of resources and reduces the harm of waste treatment to the environment.
New porous sound-absorbing materials. (a) Coffee waste recycling options; 106 (b) postconsumer and discarded denim wastes; 107 (c) denim shoddy fibers; 107 (d) the schematic diagram of the composite structure composed of porous materials and Helmholtz absorbers and the sound absorption curve of porous acoustic metamaterials. 108
Different sound absorption materials have different properties and application conditions. 109 Porous sound absorption materials have a good sound absorption effect on high-frequency noise, but the sound absorption effect on low-frequency noise is not good. A single material has limitations and cannot play a good sound absorption effect in a wide frequency band. Therefore, the development of new composite sound-absorbing structures has become the mainstream of sound-absorbing materials. Guo et al. 108 proposed a novel design method of hierarchical-porous acoustic metamaterials (HPAM) to improve the synergistic performance of Helmholtz resonators (HRs) and porous materials. This synergistic method achieves continuous and efficient broadband sound absorption, which is far beyond the ability of a single material, and the average absorption coefficient is as high as 0.77. Figure 6(d) shows the schematic diagram of the composite structure composed of porous materials and Helmholtz absorbers and the sound absorption curve of porous acoustic metamaterials. Yuan et al. 110 designed a tunable acoustic composite metasurface with a porous material and a modified microperforated panel (MPP) system. The broadband sound absorption performance of the composite metasurface in the frequency range of 200–3,500 Hz was evaluated by theoretical analysis, numerical simulation, and experiment. It is found that the designed composite metasurface has significant advantages over uniform porous materials in sound absorption performance, especially in the low-frequency range. Liu et al. 111 proposed a new type of porous metamaterial structure using a porous material matrix containing periodically perforated cylindrical holes. These cylindrical holes are arranged in a triangular mesh structure, and an additional interlayer of another porous material is introduced around these holes. The results show that the proposed new porous metamaterial structure can effectively improve sound absorption performance in high-temperature environments and low-frequency ranges. Wu et al. 112 proposed a flexible wedge-knitted composite (WKC) with a porous sound absorption mechanism and wedge structure resonance mechanism. In order to study the influence mechanism of coating and wedge structure on sound absorption performance, experiments, and finite element analysis were carried out. The porosity distribution is improved by adding viscoelasticity, and the sound absorption coefficient is increased by 239 %. The combination of viscoelastic coating and wedge structure can effectively promote sound absorption, especially at low frequencies. This work provides guidance for the structural design of new flexible sound-absorbing materials. It has broad application prospects in the fields of indoor curtain interiors, wall decorations, and automotive interiors. By combining porous materials with Helmholtz resonators, microperforated plates, and wedge-shaped structures, researchers have effectively improved the sound absorption performance of porous sound-absorbing material composite structures at low frequencies, making porous material composite structures have significant advantages in practical applications. 113 , 114 , 115
In the structural design of sound-absorbing materials, combining the advantages of porous materials with acoustic functional fillers with special structures to improve the sound absorption performance of porous materials at low and medium frequencies is challenging. Zhang et al. 116 proposed a directional antagonistic acoustic fabric prepared by a single-sided coating as a sound-absorbing material. By controlling the distribution of fillers on the porous material frame, it is found that the absorber exhibits a double gradient structure. The sound absorber exhibits significant anisotropic sound absorption under two paths. By adjusting the braided structure and thickness of the acoustic filler and substrate, the sound absorber exhibits the expected anisotropic sound absorption effect. It shows that the new sound absorber is lightweight, soft, highly efficient, and has broadband sound absorption characteristics, making it suitable for lightweight sound absorption applications. Li et al. 117 prepared a custom-built graphene aerogel (cGA) sound-absorbing material with a sound absorption coefficient of nearly 100 % by custom-designed controllable mechanical foaming and atmospheric drying technique, which can efficiently absorb noise within a specified frequency (2,000–6,000 Hz range) according to individual needs. Graphene aerogels expand through simple superposition from single-frequency to multifrequency efficient absorption. More importantly, a single aerogel has different electrical signal responses to sounds of different frequencies, which makes it possible to construct an acoustic detector that can convert noise signals into electrical signals to achieve the goal of sound fingerprint detection. Figure 7 is the fabrication process of graphene aerogel.
Illustration of the fabrication process of the cGA. 117 (a) Schematic diagram of P-rGO hydrogel synthesis; (b) the process of compressing custom-built cGA; (c) SEM image of the axial cross-section of cGA1, and (d) enlarged SEM images of cGA1 wall; (e–h) SEM images of the axial cross-section of cGA2, cGA4, cGA8, and cGA16; (i) the elliptic structure unit of cGAs and calculated ellipticity are abstracted from Figure 7(c–h).
In the sound absorption mechanism of porous materials, sound energy dissipation is a quadratic function of frequency. The low-frequency sound absorption coefficient can be slightly improved by increasing the thickness and adjusting the pore structure parameters, such as pore size and porosity, to enhance the sound absorption. However, increasing the sample’s thickness or porosity will cause it to occupy space or reduce its strength, limiting its wide application of the sample. Therefore, it is a challenging research topic to improve the low-frequency sound absorption performance of conventional sound-absorbing materials under certain thickness conditions. He et al. 118 prepared a new type of reduced graphene oxide/polyvinyl alcohol (RGO/PVA) porous composite ceramic by freeze-drying method and used graphene and polyvinyl alcohol to optimize the design of the internal connecting pores of the porous ceramic matrix to make it have a network structure. Compared with the original porous ceramics, the prepared porous structure significantly enhances the low-frequency sound absorption band, and the average sound absorption coefficient is increased by 127.2 %. He et al. 89 in situ grew magnesium borate whiskers in the prepared porous sound-absorbing ceramic matrix. With the increase of whisker content, the sound absorption curve moves to a low frequency, and the sound absorption coefficient increases gradually. The sound absorption coefficient of medium and low-frequency increases by 28.9 %, and the compressive strength increases by 5 times. Due to the ceramic system’s high thermal and chemical stability, this method provides a new possibility for the wide application of porous sound-absorbing ceramics in sound absorption.
In order to explore the basic manufacturing principles of various porous materials and their applications in noise control, a lot of work has been done in the design and manufacture of various porous materials. The researchers significantly improved the low-frequency sound absorption performance of porous sound-absorbing materials by adding reinforcement materials, process optimization, and combining them with other structures. The appearance of new sound-absorbing composite materials makes up for the deficiency of traditional sound-absorbing materials to a certain extent.
4 Conclusion and prospect
The research and application of sound absorption materials are significant for treating noise pollution. Porous material is the most widely used sound absorption material at present. Traditional porous sound-absorbing materials face irreparable defects under the requirements of multifunction and high energy efficiencies, such as low acoustic absorption coefficient, short service life, easy deliquescence, secondary pollution, complex manufacturing process, and difficult control of design parameters. This paper mainly reviews the related literature on material modification and structural design from both macro and micro aspects. By comparing and analyzing different types of porous materials, we found that they have their own advantages and disadvantages in sound absorption, environmental protection, and durability. Researchers have greatly promoted the engineering application of porous sound absorption materials in different environments by adding different types of fillers, changing the structural design (gradient porous structure, perforated structure, multilayer composite structure, etc.), improving preparation methods, increasing cavity thickness, combining different pore sizes, and developing new composite materials. At present, the research on porous sound-absorbing materials has made great achievements, which has filled the defects of traditional porous sound-absorbing materials and improved the low-frequency sound-absorbing performance of porous sound-absorbing materials.
In future development, porous sound-absorbing materials will continue to move toward high performance, environmental protection and sustainability, multifunction, and intelligence.
High performance: The sound absorption performance of porous materials was continuously improved through raw material improvement, structural design, and process optimization. According to the specific requirements of different frequency sound absorption performance in practical application scenarios, an efficient porous sound absorption material is designed to meet the application requirements.
Environmental protection and sustainability: Renewable resources, natural waste, or recyclable materials are used as raw materials to reduce the impact of materials on the environment. At the same time, actively promote the research and development of biodegradable porous materials and strive to reduce the burden on the environment after the waste of materials to achieve real sustainable development.
Multifunction: In addition to their excellent sound absorption properties, porous sound absorption materials will be endowed with more practical functions, such as flame retardant, anti-ultraviolet, heat preservation, and antibacterial. This multifunctional integration will enable porous materials to play a wider role in construction, transportation, aerospace, and other fields to meet diverse market demands.
Intelligence: Exploring porous sound-absorbing materials with adaptive ability so that they can automatically adjust the sound absorption performance according to environmental changes. The intelligent management of porous materials is realized through the integration of sensors and advanced control systems to show higher applicability in complex and changeable environments.
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Research ethics: Not applicable.
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Informed consent: Not applicable.
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Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission. Zhiting Feng: conceptualization, data curation, formal analysis, methodology, writing-original draft, investigation. Yuanjun Liu: methodology, supervision, validation, funding acquisition, writing-review & editing.
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Use of Large Language Models, AI and Machine Learning Tools: None declared.
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Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Research funding: This work was supported by the Chinese Shandong Postdoctoral Innovation Project (SDCX-ZG-202400284); the Key Laboratory of High Performance Fibers & Products, Ministry of Education (2232024G-02); the Open Project Program of Ministry of Education Key Laboratory for Advanced Textile Composite Materials, Tiangong University (ATC2024).
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Data availability: Not applicable.
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© 2024 Walter de Gruyter GmbH, Berlin/Boston
Articles in the same Issue
- Frontmatter
- Material Properties
- The latest research status of porous sound-absorbing materials
- Thermal annealing and microwave irradiation in enhancing the mechanical performance of 3D printing CF/PA12 composite
- Investigation into the crystallization of poly-lactic acid following the application of a novel high molecular weight, high epoxy functionality polymer chain extender
- Effect of AO 4426 on damping properties of PVA/CPE-AO 2246
- Preparation and Assembly
- Curcumin-encapsulated Pluronic micelles in chitosan/PEO nanofibers: a controlled release strategy for wound healing applications
- Photocatalytic g-C3N4/poly(2-acrylamido-2-methylpropane sulfonic acid) composite hydrogel triggering the synergetic effect for long-lasting sustainable purifying organic wastewater
- Engineering and Processing
- A dynamic pressure strategy to minimize void formation in vacuum infusion
- Design and application of soft robot grippers using low-viscosity silicone by lost core injection molding manufacturing method
Articles in the same Issue
- Frontmatter
- Material Properties
- The latest research status of porous sound-absorbing materials
- Thermal annealing and microwave irradiation in enhancing the mechanical performance of 3D printing CF/PA12 composite
- Investigation into the crystallization of poly-lactic acid following the application of a novel high molecular weight, high epoxy functionality polymer chain extender
- Effect of AO 4426 on damping properties of PVA/CPE-AO 2246
- Preparation and Assembly
- Curcumin-encapsulated Pluronic micelles in chitosan/PEO nanofibers: a controlled release strategy for wound healing applications
- Photocatalytic g-C3N4/poly(2-acrylamido-2-methylpropane sulfonic acid) composite hydrogel triggering the synergetic effect for long-lasting sustainable purifying organic wastewater
- Engineering and Processing
- A dynamic pressure strategy to minimize void formation in vacuum infusion
- Design and application of soft robot grippers using low-viscosity silicone by lost core injection molding manufacturing method
