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Porous materials for sound absorption

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Abstract

With the rapid urbanization and fast growth of transportation, noise pollution has become one of the most serious environmental problems in front of the people worldwide, it usually causes various disorders and greatly affects the work efficiency and living standards of human beings. Reducing noise by using sound absorption materials is an important approach to lessen the harm of noise pollution. As the most abundantly used materials, porous materials combine the properties of lightweight, wide absorption frequency range and highly sound absorption ability, and they hold great potential in the field of sound absorption. In this review, the recent progress in the design and fabrication of porous sound absorption materials is summarized and highlighted. This review covers the introduction of the sound absorption mechanism and evolution of prediction models for porous sound absorption materials, and the research and development of the design concepts and fabrication of sound absorption foams and fibrous sound absorption materials. The review concludes with some perspectives and outlook for the porous sound absorption materials.

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... Porous materials have become a desired type of materials in several fields, such as sound absorption and insulation [1][2][3][4][5] , heat exchange, chemical processing, bioengineering, and energy storage [3] , due to their specific properties, including lightweight, high specific surface area, low bulk density, and microstructure controllability [6,7] . The lattice is one of the three types of porous structural categories, which is an ordered and location-specific structure that repeats the unit cell in a certain manner. ...
... While the complex interior structure of engineered lattices makes it difficult to analyze their acoustic properties, there exist many mathematical models that aim to characterize the sound absorption or transmission properties of porous materials. These models include the classical Delany-Bazley (DB) model [8] , the Johnson-Champoux-Allard model [1,9] , the Biot theory [10,11] , and the transfer matrix method (TMM) [12,13] . Furthermore, the parameters for the above models may be obtained through measurements, analytical, or empirical methods [1,[14][15][16] . ...
... These models include the classical Delany-Bazley (DB) model [8] , the Johnson-Champoux-Allard model [1,9] , the Biot theory [10,11] , and the transfer matrix method (TMM) [12,13] . Furthermore, the parameters for the above models may be obtained through measurements, analytical, or empirical methods [1,[14][15][16] . For instance, research on the sound absorption efficiency of IN625 foams has been proposed by Zhai et al. [17] The classical DB model using a tetrakaidekahedral unit cell was employed to predict the performance of foams, which are fabricated through template replication processing. ...
Article
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Sound absorption is one of the important properties of porous materials such as foams and lattices. Many mathematical models in the literature are capable of modeling the acoustic properties of lattices. However, appropriate models need to be chosen for specific lattice structures on a case-by-case basis and require significant experience in acoustic modeling. This work aims to provide simplified insights into different mathematical models for the simple cubic lattice. The strut lengths and radii of the unit cells were varied, and the sound absorption properties were measured using an impedance tube. The sound absorption coefficients of the lattices generally increased and exhibited more resonant-like behavior as the strut radius increased. The Delany-Bazley (DB) model and the multi-layered micropore-cavity (MMC) model were used to simulate the acoustic properties of the lattices. The correction factors in the MMC were calculated based on empirical relations fitted using experimental data of the design geometry parameters. Results show that the DB model was able to model the sound absorption coefficients for lattice samples with porosities as low as 0.7, while the MMC with resonator theory is a more appropriate acoustics approach for lattices with porosities lower than 0.7. This work will be highly useful for materials researchers who are studying the acoustic properties of novel porous materials, as well as manufacturers of acoustic materials interested in the additive manufacturing of lattice structures for sound absorption and insulation applications.
... Scheme of inorganic and organic secondary constituent units [166][167][168] [191][192][193][194][195]. Although increasing the length of organic connectors increases the volume of cavities formed, in some cases this increase in length improves the penetration process of the networks [196][197][198][199][200][201]. Figure 8 shows an overview of the different classifications of metal-organic lattice frames. ...
... For five reasons, porous metal-organic frameworks are suitable materials for gas storage and separation. These reasons include [196][197][198][199]: ...
... Six important factors in the process of separation of guest molecules in metal-organic frameworks are (1) Adsorption temperature (2) Adsorption pressure (3) Size, shape, and flexibility of cavities in the frame (4) van der Waals forces and dimensions of guest molecules (5) Energy Potential of walls of cavities or channels (7) Poor bonding between guest molecules and host framework such as hydrogen bonds [219][220][221][222][223]. Examining the enthalpy of adsorption of materials capable of storing gas molecules, it can be concluded that porous coordination polymers are the best option for adsorption and release of gas molecules at normal temperature and pressure. This is because the inner surface of these polymers is rich in hydrocarbons and aromatic groups [197][198][199]. Aromatic groups are one of the best attractions of guest molecules. ...
Article
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Nonporous materials have nano-sized pores. High specific surface area and size and shape selectivity (Size and Shape Selectivity) are the most important features of these materials that have led to their widespread use in various industries such as catalysts, water treatment and separation of pollutants. The development of properties and applications of these materials depends on the fabrication of nanoporous materials with optimal and controlled structures. In this paper, porous nanostructures and supermolecular chemistry are introduced in detail. Then, a number of common nanoporous materials such as activated carbon, metal-organic frameworks and zeolites, then various types of mineral and organic nanoporous materials as well as methods of synthesis, characterization and applications of these materials will be studied in detail.
... The more favorable acoustic properties of the 0.95 and 1.0 NCO/OH systems are probably due to the larger size of the pores in the structure. The mechanism of sound suppression by porous systems is based on the phenomenon that sound waves penetrating the cellular structure promote vibrations of cell walls and the air inside them [51]. Damping of the induced vibrations by the cellular structure is attributed to the micro-deformations of walls and the friction between walls and airflow. ...
... When the average sound absorption coefficient is higher than 0.2, the material can be considered a sound-absorbing material applied to the buildings' insulation [55]. All tested materials revealed higher αavg values in the frequency range of 100-6300 Hz, which allows classifying series with NCO index below 1.05 as materials with acoustic insulation potential [18,[51][52][53][54][55]. Table 4 presents the parameters describing the mechanical performance of prepared flexible PU foams and their variations with changes in the isocyanate index. ...
Article
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Polyurethane (PU) foams are versatile materials with a broad application range. Their performance is driven by the stoichiometry of polymerization reaction, which has been investigated in several works. However, the analysis was often limited only to selected properties and compared samples differing in apparent density, significantly influencing their performance. In the bigger picture , there is still a lack of comprehensive studies dealing with the stoichiometry impact on PU foams' performance. Herein, flexible PU foams with a similar apparent density but differing in the isocyanate index (IIso) (from 0.80 to 1.20) were prepared. The stoichiometry-structure-performance relationships were investigated considering cellular and chemical structure, as well as the static and dynamic mechanical properties, thermal stability, thermal insulation, and acoustic performance. For IIso of 1.00, the biggest cell diameters of 274 µm were noted, which was 21-25% higher compared to 0.80 and 1.20 values. Increasing IIso reduced open cell content from 83.1 to 22.4%, which, combined with stiffening of structure (rise of modulus from 63 to 2787 kPa) resulting from cross-linking, limited the sound suppression ability around five times. On the other hand, it significantly strengthened the material, increasing tensile and compressive strength 4 and 13 times, respectively. Changes in the foams' performance were also induced by the glass transition temperature shift from 6.1 to 31.7 °C, resulting from a greater extent of urethane groups' generation and additional isocya-nate reactions. Generally, the presented work provides important insights into preparing flexible PU foams and could be very useful for the future development of these materials.
... The main strategies of noise reduction are mainly classified into two categories: passive and active noise reduction methods. Passive noise reduction is mainly based on isolation of noise sources from its environment by using heavy barriers and reactive materials to hinder and block noises and suppress the structural vibrations (e.g., see (Cao et al., 2018;Wu, 2008;Jahanpour et al., 2016a;Jahanpour et al., 2016b;Soltani et al., 2020)). Sound absorption is another passive strategy to reduce the noise energy. ...
... For instance, porous media dissipate the energy of the propagation of the sound waves by generating friction, vibration, and heat loss due to their large number of open and closed pores. The performance of the porous materials is inherently weak in low frequency range noise the thickness of the absorber is limited by the quarter working wavelength, making it difficult to achieve effective absorption of low-frequency sound waves in a limited space (Cao et al., 2018). ...
Article
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In this study, a computational optimization framework is presented to identify active noise control systems for vehicle enclosures. The adopted optimization approach determines optimum configuration of an array of secondary sound sources to suppress noises with low frequency (under 500 Hz) due to engine performance. The overall framework is conducted via a particle swarm optimization algorithm coupled with a modified modal interaction method developed to estimate sound field inside enclosures. The applicability of the proposed optimization approach has been examined with two enclosures of different size by considering 1-speaker and 2-speaker array options as a secondary sound source. The numerical investigation shows that, with the determined optimum configurations, a significant noise reduction may be obtained in vehicle enclosures.
... Poroelastic materials [1], as typified by fiber materials, are employed in different fields, including the automotive industry [2,3], owing to their substantial sound absorption performance in the mid and high-frequency range. The sound absorption performance of fiber materials improves with the surface area per unit volume for the same mass, and the surface area is inversely proportional to the fiber diameter. ...
... The ρ t in Equation (3) represents the equivalent fluid's effective mass. Here, in Equation (1), if the poroelastic material's bulk density is increased (ρ→∞, i.e., ρ t →∞), ρ(ω) → ρ (ω) , Equation (1) shows that the effective density converges to the value of effective density depicted in Equation (2). This illustrates that, as the density of the skeletal material's density increases, the skeleton's vibration can be neglected. ...
Article
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This study aimed to discover an easy and precise prediction model for the acoustic properties of nanofiber nonwoven fabrics. For this purpose, a prediction model focusing on the two dominant parameters in the Limp frame model—bulk density and flow resistivity—was suggested. The propagation constant and characteristic impedance was generated from the effective density and effective volume modulus generated by the predictive model and treated as a one-dimensional transfer matrix. The sound absorption coefficient was then estimated using the transfer matrix approach. The trend of the normal Incident sound absorption coefficient measured and the sound absorption coefficient obtained from the predictive model were consistent. Thus, it is suggested that the predictive model for the proposed nanofiber nonwoven composite sheet is valid.
... The major function of porous structures was to provide superior sound absorption performance by the friction behavior of the wall of the pores and the viscosity effect of the air in the pores [3,4]. Generally, porous sound-absorbing materials can be classified into organic fibrous materials, inorganic fibrous materials, foam sound absorption materials, metal sound absorption materials and cement-based sound absorption materials [5,6]. The fibrous and foam materials had the main disadvantage of poor strength and durability, and the cost of metal sound absorption materials was too expensive nowadays. ...
... The other was that acoustic waves rubbed with the pore wall of ceramsite. The above two main ways led to the sound energy being consumed according to the viscosity resistance of air and the friction effect of the wall of the pores [6,23]. Subsequently, the transmitted sound energy was formed when the acoustic wave passed through the layer X. Figure 10b shows the physical structure model of the double-layer specimen B. Unlike specimen A which had only one size of pores, specimen B included two sizes of pores due to the two layers with different mixture design. ...
Article
In this work, ceramsite was utilized to fabricate the sound-absorbing boards, in which two types of structure were considered, specifically, single-layer board with homogenous structure and double-layer board with gradient structure. The physical, mechanical and acoustic properties of these prepared ceramsite sound absorbing boards were studied, including the bulk density, compressive strength, flexural strength, softening coefficient, sound absorption coefficient and sound reduction index. The results show that the double-layer board with appropriate mixture design exhibited almost identical bulk density and mechanical strength to the single-layer board. All ceramsite sound absorbing boards had compressive and flexural strengths of more than 3 MPa and 1 MPa, respectively, and also demonstrated good water resistance. In terms of sound absorption and sound insulation properties, the overall performance of the double-layer board with reasonable gradient structure was better than that of the single-layer board. In addition, the physical structure models of ceramsite sound absorbing boards were established to illustrate the variation of mechanical properties and disclose the mechanism of sound absorption and insulation in the material.
... Therefore, it is still an urgent problem to prepare high-efficiency sound-absorbing materials in all frequency range. In addition, oxide ceramic nanofibrous materials are generally hydrophilic and easy to absorb moisture, which brings great challenges to maintain the sound absorption properties of materials with long-term stability [173]. The proper solution of these problems will further enhance the practical application level of electrospun SNFs sound-absorbing materials. ...
Article
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One-dimensional (1D) SiO 2 nanofibers (SNFs), one of the most popular inorganic nanomaterials, have aroused widespread attention because of their excellent chemical stability, as well as unique optical and thermal characteristics. Electrospinning is a straightforward and versatile method to prepare 1D SNFs with programmable structures, manageable dimensions, and modifiable properties, which hold great potential in many cutting-edge applications including aerospace, nanodevice, and energy. In this review, substantial advances in the structural design, controllable synthesis, and multifunctional applications of electrospun SNFs are highlighted. We begin with a brief introduction to the fundamental principles, available raw materials, and typical apparatus of electrospun SNFs. We then discuss the strategies for preparing SNFs with diverse structures in detail, especially stressing the newly emerging three-dimensional SiO 2 nanofibrous aerogels. We continue with focus on major breakthroughs about brittleness-to-flexibility transition of SNFs and the means to achieve their mechanical reinforcement. In addition, we showcase recent applications enabled by electrospun SNFs, with particular emphasis on physical protection, health care and water treatment. In the end, we summarize this review and provide some perspectives on the future development direction of electrospun SNFs.
... Noise cancellation methods include passive noise cancellation (PNC) [12] and active noise cancellation (ANC) [13]. PNC is primarily implemented using design methods such as those for headphones [14,15] and the characteristics of sound-absorbing materials [16,17]. Previous studies have shown that a PNC method can be effective in reducing high-frequency (>2000 Hz) components of the noise spectrum; however, the processing of low-frequency components requires further improvements. ...
Article
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With the development of active noise cancellation (ANC) technology, ANC has been used to mitigate the effects of environmental noise on audiometric results. However, objective evaluation methods supporting the accuracy of audiometry for ANC exposure to different levels of noise have not been reported. Accordingly, the audio characteristics of three different ANC headphone models were quantified under different noise conditions and the feasibility of ANC in noisy environments was investigated. Steady (pink noise) and non-steady noise (cafeteria babble noise) were used to simulate noisy environments. We compared the integrity of pure-tone signals obtained from three different ANC headphone models after processing under different noise scenarios and analyzed the degree of ANC signal correlation based on the Pearson correlation coefficient compared to pure-tone signals in quiet. The objective signal correlation results were compared with audiometric screening results to confirm the correspondence. Results revealed that ANC helped mitigate the effects of environmental noise on the measured signal and the combined ANC headset model retained the highest signal integrity. The degree of signal correlation was used as a confidence indicator for the accuracy of hearing screening in noise results. It was found that the ANC technique can be further improved for more complex noisy environments.
... On the other hand, meltblown nonwovens have become a product of interest for the Hybrid nonwovens have also been studied in the literature for their acoustic efficiency [36,37]. A limited number of research works investigated the effect of the arrangement of the fabric layers in the multi-layered material on acoustic performance under the term 'layer sequencing'. ...
Article
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Unlike the term sound insulation, which means reducing the penetration of noise into other areas, sound absorption means reducing the reflection and energy of the sound on the surface. It has become a highly noticed issue in recent years because the noise in our daily life is increasing day by day, and it causes some health and comfort disorders. In many areas, textiles have been used for acoustics control and noise absorption purposes. The purpose of this work is to determine the most effective media for sound absorption performance and its relation to thermal conductivity from needle-punched nonwoven, meltblown nonwoven and hybrid forms in different arrangements of these fabrics. To provide comparable samples, both needle-punched nonwoven and meltblown nonwoven samples were produced from 100% Polypropylene fibres. According to sound absorption tests, the hybrid-structured sample having a composition similar to the needle-punched nonwoven sample placed at the bottom of our study, while the meltblown nonwoven sample placed as a face layer outperformed the rest of the samples in terms of sound absorption and thermal conductivity. ‘Meltblown only’ samples had remarkably higher sound absorption efficiency than most of the samples, while the ‘needle-punched nonwoven only’ sample had the lowest sound absorption efficiency in all frequencies.
... Researchers have introduced several methods for optimizing design, including a range of work on sound barriers [1][2][3], automobiles [4][5][6], buildings [7,8] and mufflers [9,10]. At present, porous materials have been widely used in the field of noise control because of their excellent sound absorption characteristics [11][12][13][14][15]. Considering various constraints, including economy, laying porous materials in certain areas is an effective method. ...
Article
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In this work, an acoustic topology optimization method for structural surface design covered by porous materials is proposed. The analysis of acoustic problems is performed using the isogeometric boundary element method. Taking the element density of porous materials as the design variable, the volume of porous materials as the constraint, and the minimum sound pressure or maximum scattered sound power as the design goal, the topology optimization is carried out by solid isotropic material with penalization (SIMP) method. To get a limpid 0-1 distribution, a smoothing Heaviside-like function is proposed. To obtain the gradient value of the objective function, a sensitivity analysis method based on the adjoint variable method (AVM) is proposed. To find the optimal solution, the optimization problems are solved by the method of moving asymptotes (MMA) based on gradient information. Numerical examples verify the effectiveness of the proposed topology optimization method in the optimization process of two-dimensional acoustic problems. Furthermore, the optimal distribution of sound-absorbing materials is highly frequency-dependent and usually needs to be performed within a frequency band.
... Sound insulators are categorized into resonant and porous insulation materials addressing the above-mentioned issue by absorbing the sound energy during a noise propagation process. Despite the limited efficiency of resonant materials caused by their narrow absorption frequency bands and their heavy weights, porous materials can be applied in a wide range of sound absorption frequencies with a wide range of structural designs [104,106]. However, the high thicknesses of sound absorption materials prevent their efficiency in viscous-thermal acoustic energy practices. ...
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Waste tire textile fibers (WTTF), as a by-product (10–15% by weight of tires) of end-of-life tires (ELT) mechanical recycling (grinding), are classified as hazardous wastes and traditionally burnt (thermal recycling) or buried (landfilling), leading to several environmental and ecological issues. Thus, WTTF still represent an important challenge in today’s material recycling streams. It is vital to provide practical and economical solutions to convert WTTF into a source of inexpensive and valuable raw materials. In recent years, tire textile fibers have attracted significant attention to be used as a promising substitute to the commonly used natural/synthetic reinforcement fibers in geotechnical engineering applications, construction/civil structures, insulation materials, and polymer composites. However, the results available in the literature are limited, and practical aspects such as fiber contamination (~65% rubber particles) remain unsolved, limiting WTTF as an inexpensive reinforcement. This study provides a comprehensive review on WTTF treatments to separate rubber and impurities and discusses potential applications in expansive soils, cement and concrete, asphalt mixtures, rubber aerogels and polymer composites.
... Thus, this study focused on the use of solid hardwood as an eco-friendly sound-absorbing material by studying the pore structure of the transverse section. For a porous material to absorb noise, pores through which sound waves can penetrate must be well developed [5]. The sound-absorbing capability of hardwood is better than that of softwood, as the hardwood species, in which tyloses are not developed in the vessels, are more advantageous for sound absorption [6]. ...
Article
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As part of the search for useful wood species as an eco-friendly and sound-absorbing material, the sound-absorbing properties of the Chinese parasol tree (Firmiana simplex (L.) W.Wight) and Chinese tulip poplar (Liriodendron chinese) were investigated. The pore structure of their transverse sections and the porosity (through-pore porosity, blind-pore porosity, and closed-pore porosity) according to their physical pore shapes were investigated. In conclusion, the sound absorption performance of the Chinese parasol tree and Chinese tulip poplar showed a positive linear relationship with their through-pore porosity. In addition, compared with other porous or fibrous sound-absorbing materials, their sound-absorbing capacities were found to be satisfactory. Both of these species are expected to be valuable as eco-friendly sound-absorbing materials.
... There are multiple methods to control and attenuate noise, vibration, and harshness (NVH) in the promising application of automotive engineering sectors. In addition to manipulating topology parameters [1,2], using acoustic metamaterials [3,4], porous or absorbent materials [5,6], Helmholtz resonance [7,8], and perforated structures [9,10] are commonly used nowadays. An inlet duct of the turbocharger compressor is one of the primary sources of vehicle noise propagation [11]. ...
Conference Paper
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Sound absorptive materials have been widely utilized to control, manipulate, and mitigate the propagation of sound waves in the aerospace and automotive industries. This paper proposes that porous materials can be fitted as the coating liner to the inlet duct of the turbocharger compressor in order to absorb the incident excitation and hence, reduce the propagation of sound waves. As a result, three different types of absorptive materials are used as coating liners to determine the impact that varying material properties have on the acoustic absorption performance of the system. A finite element approach is conducted, and acoustic pressure and poroacoustics studies are carried out over the desired frequency range (1-20kHz) using COMSOL Multiphysics. Sound transmission loss (TL) and sound pressure level (SPL) are investigated for the models with and without liners. The results indicate that sound-absorbing materials can be employed to significantly diminish sound levels in the turbocharger compressor inlet duct. According to the results, absorptive liners would considerably increase the maximum TL and lessen the sharp drop in TL that appeared in the model without liners at around 4321Hz. In addition, the average TL has increased due to the replacement of valleys with peaks. By adding only 5mm of an absorptive liner, the average TL was increased by 3.3%. The present study intends to introduce a fundamental approach that can be applied to many promising fields such as aerospace and automotive engineering.
... The addition of both aerogel and hydrogen peroxide has significantly improved the sound absorption of FLCC at all frequency ranges. This is attributed to the porous structure of the FLCC that led to the dissipation of sound energy due to the friction of air molecules with the pore walls, as well as the viscous loss caused by airflow within the structure [50]. ...
Article
Noise pollution and thermal discomfort are major concerns in urban areas these days. The building sector consumes a large amount of total energy to fulfill the thermal comfort demands, which are rapidly increasing with modern advancements. The overwhelming energy demand and noise pollution prevalent in the building sector can be controlled using insulating cementitious composites. Therefore, this research investigates the properties of foamed lightweight cementitious composite (FLCC) which incorporates the co-addition of micro-sized aerogel and hydrogen peroxide. Different contents of aerogel (0.5%, 1%, and 1.5% by weight of cement) were used as additives along with various dosages of hydrogen peroxide (1%, 2%, and 3% by weight of cement) to produce the FLCC. The optimum content of the FLCC comprising 1% aerogel and 3% hydrogen peroxide, has a low oven-dry density of 380 kg/m³ with an adequate compressive strength (about 3 MPa) for application as an insulating cementitious composite. The high porosity characteristic (up to 76%) of the FLCC improved sound absorption and thermal conductivity (reduction by 88%). Overall, the results highlighted the promising role of aerogel and hydrogen peroxide incorporated FLCC as a useful insulating construction material.
... Noise pollution can be minimized by using fibers' sound absorption substances. Acoustic comfort (for example, speech intelligibility) could also be improved by using sound absorption materials to control reverberation time in concert halls, exhibition halls, workplaces, opera houses, and some others [28,29]. ...
Article
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Increasing global environmental problems and awareness towards the utilization of eco-friendly resources enhanced the progress of research towards the development of next-generation biodegradable and environmentally friendly material. The development of natural-based composite material has led to various advantages such as a reduction in greenhouse gases and carbon footprints. In spite of the various advantages obtained from green materials, there are also a few disadvantages, such as poor interfacial compatibility between the polymer matrix and natural reinforcements and the high hydrophilicity of composites due to the reinforcement of hydrophilic natural fibers. This review focuses on various moisture-absorbing and sound-absorbing natural fiber polymer composites along with the synopsis of preparation methods of natural fiber polymer composites. It was stated in various studies that natural fibers are durable with a long life but their moisture absorption behavior depends on various factors. Such natural fibers possess different moisture absorption behavior rates and different moisture absorption behavior. The conversion of hydrophilic fibers into hydrophobic is deemed very important in improving the mechanical, thermal, and physical properties of the natural-fiber-reinforced polymer composites. One more physical property that requires the involvement of natural fibers in place of synthetic fibers is the sound absorption behavior. Various researchers have made experiments using natural-fiber-reinforced polymer composites as sound-absorbing materials. It was found from various studies that composites with higher thickness, porosity, and density behaved as better sound-absorbing materials.
... The resonance-based treatments attenuate sound energy by viscous dissipation due to the strong oscillation of the acoustic mass composed of air in the neck at the resonant frequency [3,4], resulting in a narrow effective frequency range [5,6]. By contrast, the sound damping effects of the porous materials originate from the viscous friction and thermalelastic loss inside the pores of the materials, which have the potential for broadband noise mitigation [7,8]. However, the effectiveness of the porous attenuators at low and middle frequencies usually requires large thickness and high density in the sound propagation path [9,10], limiting their practical applications under strict restraint for installation weight and space. ...
Article
Acoustic metasurfaces (AMs) used for reflected sound wavefront manipulation are generally designed based on the generalized Snell’s law (GSL) at a single frequency, suffering from inferior broadband properties. Herein, an acoustic meta-porous layer (AMPL) with periodic structures is designed to realize reflected wavefront manipulation and effective sound absorption over a wide frequency band. The AMPL is constructed by four periodically arranged units, each consisting of porous elements inserted by acoustically rigid partitions, forming a linear reflected phase-shifting within 0 to 2π maintaining in a target frequency range of [1000,3000]Hz. To predict the reflected response of the element of the AMPL, an analytical model is proposed based on the transfer matrix method and is numerically validated. The sound reflection and absorption performance of the AMPL is investigated numerically and experimentally. The scattering sound pressure fields under normally and obliquely incident sound waves at different frequencies demonstrate the broadband reflected wavefront manipulation capability, including negative reflection and surface wave conversion. Compared to a uniform porous foam, an effective absorption is achieved by the AMPL due to the surface wave excitation at an identical thickness of 40 mm, with an averaged absorption coefficient greater than 0.9 in [800,3000]Hz. The results suggest the proposed structure could be a promising candidate of broadband noise absorption metamaterial for practical applications.
... Generally, the more porous materials, the higher absorption effect (i.e., larger sound absorption coefficients) (Kalauni and Pawar, 2019). The fibrous-type materials with smaller pore size and higher porosity tend to exhibit larger sound absorption coefficients at low frequency range (Cao et al., 2018). Previous study found that Chinese Bahaba generates low frequency sounds (< 200 Hz) with swim bladder (Wei et al., 2021). ...
Article
As a critically endangered (CR) fish species, Chinese Bahaba is a unique “Giant Panda” fish species in China and has been listed among the national first-class wildlife protection animals and China's top 10 genetic resources of aquatic products since 2021. This fish species is of high commercial value because its swim bladder is commonly used in traditional Chinese medicine. Its otoliths are the sensory organs immersed in the endolymph for maintaining its balance and hearing. However, rare information has been reported on the sound absorption structure and chambers of otoliths of such “Giant Panda” fish. The big “C” groove was found in the fish’s front sagittal otolith with the crystal cluster in the back sagittal otolith, the former of which is a 3D layered structure, that is constructed by elongated prismatic crystals. Besides, there are numerous small holes and adhesion material in this 3D layered structure, where many chambers were also found, indicating that some specific sounds may be captured by this structure and these chambers may then amplify such sounds at a certain wavelength. This finding could be of great importance for protecting and conserving this critically endangered species.
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A compact structure comprising the gradient perforated porous material and the coiled-up channel (GPPC) is proposed. The sound absorption performance is investigated by a theoretical model based on the transfer matrix method (TMM) and simulation with the finite element method (FEM), and the simulations are experimentally validated. Broadband absorption of porous material can be shifted to a lower frequency and enhanced by coiled-up channel of GPPC, which are due to the resonances of the meso-pore with coiled-up channel and the coupling effect of multiscale porous material and coiled-up channel, and the energy trapping between the meso-pore and porous material matrix. The effects of the geometry parameters of the meso-pore and coiled-up channel on the absorption properties are also discussed. Moreover, the absorption enhancement of GPPC relative to homogeneous porous relies on a proper porous material matrix. Finally, the GPPCs with a width and thickness of 70 × 100 mm is designed by parallel coupling, and the absorption coefficient exceeds 0.7 at [240, 3000] Hz, which is promising in engineering noise control and provides some preliminary understanding of the combination of the gradient perforated porous absorber and the coiled resonator.
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This study proposes a general half-analytical method to predict the sound absorption of multiple inhomogeneous resonators inspired by Sellers’ method with small calculation cost. In this method, the sound absorption coefficient of single units is calculated by the finite element method (FEM), and superposition is used to predict the sound absorption coefficient of the overall structure. Unlike existing fully analytical methods that have difficulties with complicated or novel constructions, we combine FEM and the analytical method called the half-analytical method (HAE), which predicts sound absorption performance with excellent results. Two example structures are tested and the absorption coefficients from the analytical method, FEM, present method, and experiment show excellent agreement. The novel HAE method is promising to accurately predict the sound absorption coefficient of multiple inhomogeneous structures.
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Aviation noise pollution has become a significant public health problem, especially with the endless improvement of flight speed and loading capacity. Existing aviation noise absorbers have fatal defects of large weight, weak high-temperature stability, and difficulty to achieve both good low-frequency (<1000 Hz) and high-frequency (up to 6000 Hz) noise absorption simultaneously. Herein, we report a robust strategy to create flexible ceramic nanofiber aerogels with cascaded resonant cavities by the air bubbles-assisted freeze-casting technology. The stable hinged resonance cavity structures coassembled by flexible ceramic nanofibers, soft montmorillonite nanosheets, and silica sol glue endow the aerogels with temperature-invariant compressibility (from -196 to 1100 °C) and bendability. Moreover, the comprehensive advantages of cascaded resonance cavities and interconnected fibrous networks enable flexible ceramic nanofiber aerogels to have temperature-invariant full-frequency noise absorption performance (noise reduction coefficient up to 0.66 in 63-6300 Hz). The synthesis of this flexible ceramic nanofiber aerogel provides a versatile platform for the design of high-efficiency noise-absorbing material for various fields.
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A kind of porous sound absorber was studied to control the thermoacoustic oscillation of liquid mist flame, which combined natural fibrous materials and perforated plate. It could optimize the sound absorption characteristics of fibrous materials by adding perforated plate and air cavity. The influences of the interlayer and back cavity depth of two kinds of fibrous materials with the mass of 20, 30, 40 g were investigated experimentally in an impedance tube and applied to control the oscillating ethanol flame. The results showed that the best depth of interlayer and back cavity were 20 cm and 30 cm, respectively. The addition of two kinds of fiber sandwich structures could reduce the amplitude of the sound pressure oscillation in the combustion chamber by 89.2 % and 92.6 %, as well as 88.0 % and 91.2 % in the plenum chamber, meanwhile, restrained the flame heat release fluctuation by 73 %. It was proved that the porous sound absorber could act as a damp in the acoustic transmission path and suppress the sound source.
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Aiming at poor noise reduction performance of light and thin fiber aggregate materials, the polyurethane/ferroferric oxide/activated carbon fiber felt coated flexible composite materials were prepared by coating composite method. The effects of composite structure, ferroferric oxide particle content (10–25%), and coating thickness (0.25–1 mm) on the composites' sound absorption and sound insulation properties were systematically studied. The results show that with the increase of coating thickness and ferroferric oxide particle content, the sound absorption curve of the composite obviously moves to low frequency, and the resonance frequency decreases. When the particle content increased from 0% to 20%, the resonance frequency shifted 967 Hz. When the ferroferric oxide content is 25%, the maximum sound absorption coefficient and maximum transmission loss of the composite are 3.5 times and 25 times of that of the fiber felt, respectively. When the polyurethane/ferroferric oxide film thickness is only 0.1 mm, the maximum sound absorption coefficient and the average sound absorption coefficient of the composite are 3.5 times and 2.3 times of those of activated carbon fiber felt, respectively. Compared with single fiber felt, the composite material has better sound absorption, sound insulation, and mechanical properties while maintaining the thin and soft characteristics of the fiber felt.
Article
The structural characteristics of the polymer composite foams play an important role in their properties. Herein, the polypropylene/polylactic acid (PP/PLA) composite foams with the controllable hollow radially gradient porous structures are designed and prepared by the supercritical CO2-assisted foam extrusion coupled with postdrawing auxiliary technology. The above structures consist of the dense skin layer, hollow-core, closed-cell, and open-cell. Under the constant extrusion foaming temperature and pressure, the outer diameter of the as-prepared foams can be adjusted from about 9.8 mm to 5.7 mm, while the inner diameter changes from about 5.5 mm to 1.7 mm, by adding the PLA to regulate the viscoelasticity and thermodynamic differences of the polymeric melt. Besides, the formation mechanism of the hollow radially gradient porous structures is theoretically analyzed. The as-prepared PP/PLA composite foams exhibit outstanding oil/water separation performance, which show potential application in wastewater treatment.
Article
Noise pollution has become one of the four major pollutants in modern society, and the development of acoustical materials with superior noise reduction performance is urgent. In order to develop new and high-performance acoustic materials, braid-reinforced (BR) polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via the dry–wet spinning process and were used to prepare woven fabrics with different basic textures. The effects of pump flux on the structure and performance of BR PVDF hollow fiber membranes was studied. The results showed that pump flux had an impact on the structural parameters, including the diameter, inner coating layer thickness and porosity, and the fabricated BR PVDF hollow fibers with a porous-resonant composite sound absorption structure had good interface bonding performance. Furthermore, the sound absorption of the woven fabrics was measured by using the impedance tube method in frequency range of 100–6300 Hz. The results demonstrated that plain fabric had a smaller thickness of 2.17 mm and better acoustical properties with a maximum sound absorption coefficient of 0.71. These woven fabrics may potentially be used as ideal materials for controlling noise in fields such as building and transportation.
Article
The low-frequency sound-absorption performance of porous acoustic materials is poor, while the combination of porous materials and scatterer-based structures can improve the low-frequency absorption coefficient of porous materials. This paper investigates the use of a topology optimization with a genetic algorithm (TOGA) to automatically design the optimal structure for better broadband low-frequency sound-absorption performance. This study is designed to optimize the distribution of scatterers inside a 5-cm-thick melamine sponge, and the optimization results in an average absorption coefficient of 0.8 in the frequency range of 200–1600 Hz. The effects of different structures, different locations, different material parameters, different acoustic incident angles and simplified structures on the sound-absorption coefficients are investigated. The simulation calculation results and experimental results verify the correctness of the results. This composite acoustic material and the method provide more design ideas for the design of future acoustic materials.
Article
Achieving broadband sound absorption in low-frequency ranges using thin acoustic materials has been a long-standing and challenging problem in acoustics. When using the existing acoustic materials such as porous and fibrous materials, they are inevitably thick for low-frequency sound absorption. In this study, we propose a thin acoustic metasurface for broadband absorption of low-frequency sound by using hybrid resonances at multiple target frequencies. Supercells of the proposed metasurface are partitioned into multiple unit cells, and each of them is composed of two adjacent subwavelength Helmholtz resonators for perfect sound absorption based on hybrid resonance at each target frequency. When the hybrid resonance is derived, the unit cell exhibits perfect sound absorption at the target frequency with the lower Q-factor than the existing absorbing structures with same thicknesses. To take this advantage, we herein propose design procedures for the proposed supercells to achieve perfect sound absorption based on hybrid resonances at multiple target frequencies. The designed supercells are fabricated by 3D printing apparatus and their absorbing performance is experimentally evaluated in impedance tube. A thin (<λ/11) acoustic metasurface composed of the designed supercells achieves high (>90%) absorption band over broad frequency range, whose relative Q-factor is reduced to about one third compared to the one of the designed unit cell for a single target frequency. This work opens possibilities for practical applications of acoustic metasurfaces in noise mitigation of various mechanical systems (e.g., home appliances and power transformers) in that the target frequencies of supercells are eligible to be customized for each system by using the proposed design procedures.
Article
Different porous structures of biomass-based aerogels were regulated by adjusting the ratios of carboxymethyl chitosan (CCS) and montmorillonite (Mt). The hierarchically porous structures were obtained via a directional freeze method and the freeze-drying process. The diverse microstructures and flame retardant and sound absorption properties of as-prepared CCS/Mt. aerogels were studied. The results revealed that the incorporation of Mt. could dramatically improve the flame retardancy of the resultant aerogel, with the limiting oxygen index reaching up to 85%, attaining the nonflammable level. And the CCS/Mt. aerogels exhibit a super low density (33.85–38.27 mg/cm³) and high porosity (98.20%). The honeycomb-like structure of the aerogel could apparently improve the sound adsorption properties at low-frequency. Overall, this work provides a novel approach to develop bio-composites featured with excellent flame retardant and sound-adsorption properties that would find a broad range of potential applications as building material.
Article
Sheep wool and poultry feathers have unique properties that provide excellent noise and thermal insulation, and flame resistance when used as reinforcement for poly propylene. Discarded wool and poultry feathers are not only available in large quantities at low cost but have distinct characteristics that make them ideal to develop composites for various applications. In this study, we have studied the changes in tensile properties, thermal conductivity, sound absorption, and flame resistance of composites fabricated using wool and poultry feathers individually and as blends in various proportions. Reinforcing with sheep wool provided higher tensile and flexural strength compared to feathers. Compared to neat PP, higher tensile strength and modulus were obtained with 70% sheep wool as reinforcement but the flexural strength and modulus of the individual wool or feather and hybrid composites were considerably lower. When equal proportions of wool and feathers are used, the strength and modulus decreased compared to using the reinforcements individually. However, combining wool and feathers in 50/50 ratio and with 80% reinforcement, the composites had high sound absorption co-efficient of 0.55 with peak sound absorption found throughout the 1000 to 6000 Hz range depending on the proportion of wool and feathers. A flame resistance rating of V1 and thermal conductivity of 0.630 W/mK was obtained for the composites. Biocomposites with desirable properties could be obtained by blending sheep wool and poultry feathers for specific applications, particularly where acoustic, thermal and flame resistance is necessary.
Article
The radiated sound power in an enclosure with a vent hole is actively controlled by radiation mode. Active structural acoustic control (ASAC), which generates a sound field through the bending wave of a structure, is suitable for global noise reduction. However, ASAC applications have focused on reducing the transmission noise in completely enclosed structures. Herein, a new ASAC method for reducing the noise in an opened structure is proposed. This method uses radiation mode and optimal control theory simultaneously to achieve sufficient sound power reduction with only a few actuators. The optimal vibration velocity of a panel is controlled by three radiation modes. The proposed method showed a maximum sound power reduction of 14 dB in three actuators, while the existing optimal control algorithm showed only a reduction of 1 dB. They can be used for active noise attenuation of various products with vent holes by reducing the number of actuators required.
Article
Sound absorption efficiency of the fibrous composite material could be increased by an additional polymer thin layer deposited on the substrate. In order to control the thickness of the polymer surface layer, a new method based on thermographic measurement is presented. The proposed method is much easier to use and does not need any expensive tools in contrast to the Optical Coherence Tomography (OCT) image analysis. The research results show the prospects of this method in the development of sound-absorbing composites. The acoustic measurements confirmed that in the case of fibrous composite materials of viscose fibres/polylactide with an additional polylactide surface layer, the nature of low frequency sound absorption depends on the thickness of this layer.
Conference Paper
Sound absorption of structures developed from coir fibres is studied. The aim of the present work is to develop a hybrid structure from coir nonwoven matts and snippets (finely shredded denim fabric) for broad frequency sound absorption. This is achieved by first developing a multilayer density gradient structure using compressed coir matts. The sound absorption behaviour of this structure shows good absorption in the mid and high frequency range because the structure has increasing density, tortuosity and flow resistivity through the thickness that leads to better frictional loss of the sound wave. Hybrid structure is developed by layering the density gradient coir structure with a sound facing layer (made up of 1mm snippets sprayed on the surface of coir sheet). Addition of snippets forms small sized pores on the surface of the coir sheet thus increasing the surface area. The thin panel like structure of the sound facing layer improves low frequency sound absorption showing characteristic peaks but decreases absorption in high frequency. Snippets improve this vibrational loss and increases low frequency absorption. The hybrid structure is developed to cater to broad frequency absorption. Results show an improvement of 66.67% in NRC when 5% snippets sprayed on the coir sheet surface is used as a face layer along with the density gradient structure, thus showing improved absorption in hybrid structures. Further optimization of the parameters of the hybrid structure will ensure enhanced broad frequency sound absorption.
Article
The effect of aluminum foam on the pressure oscillation of premixed methane-air deflagrations was investigated using experiments performed in an empty duct and a duct with aluminum foam attached to the upper and lower walls. For a fair comparison, the passage areas of the two ducts are identical when considering a 10 mm thickness of the porous medium. The flame propagation, pressure oscillation, and interaction mechanism between the acoustic wave and flame front are discussed by comparing the flame structure and pressure waveform during premixed methane-air mixture deflagration. The results demonstrate that for the empty duct configuration, the flame propagates as a cellular structure and then evolves into an oscillating flame when approaching the closed end of the duct. The coupling effect of the flame and acoustic waves induces the flame to oscillate violently as the distance from the ignition source to the closed end of the duct (i.e., the ignition position) increases to a threshold value. The magnitude of pressure oscillation increases as the distance between the ignition source and the closed end of the duct increases and reaches a maximum when the ignition position is 700 mm from the closed end. Additionally, the pressure oscillation is consistent with the flame oscillation. This oscillation results in a large deflagration pressure and a high rate of pressure rise. By contrast, the aluminum foam-laden duct completely suppresses the oscillation of the premixed methane-air deflagrations for all ignition positions. The aluminum foam inhibits the coupling effect and consequently restrains the prominent deformation and oscillation of the flame front. Flame oscillation vanishes as the acoustic damping coefficient increases. In this case, there is no oscillation and the leading position of the left flame exhibits a linear relationship with time. Whether the foam material has an inhibitive effect on the deflagration pressure depends on the ignition position. The aluminum foam enhances the deflagration pressure when the ignition position is 100 mm from the closed end, with a 29% increase in the maximum overpressure. By contrast, the aluminum foam attenuates the deflagration pressure for other ignition positions, with a maximum drop ratio of 61.7% at the ignition position being 700 mm from the closed end.
Article
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A phase-gradient acoustic metasurface is designed based on wavefront manipulation for achieving broadband noise reduction in a flow duct. It is constructed by eight periodically arranged varying-depth units filled with porous materials, forming a linear phase shift of reflected wave at 3000Hz. The linear reflected phase-shifting can be roughly maintained in a wide frequency range. By employing the broadband phase-shift feature of the units, the metasurface exhibits an enhanced sound absorption performance in terms of the absorption coefficient and bandwidth when comparing to the individual units, which originates from forcing the higher-order modes of reflection waves to be evanescent. Then, its transmission attenuation characteristics in the presence of background flow are assessed by installing it on the side wall of a flow tube. The effects of flow speed (Mach number is up to 0.3) and sound source position (at the upstream and downstream sides of the metasurface) are experimentally and numerically investigated. Results show that the attenuation performance of the metasurface decreases with flow speed for the upstream sound source while increasing for the downstream sound source. For both upstream and downstream sound sources, a high transmission loss (≥20dB) is achieved by the metasurface in the frequency range of 1500–3000Hz at different flow speeds. Hence, the proposed metasurface is promising for achieving broadband noise attenuation in the presence of flow.
Article
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Compared with optical black, few attempts have focused on achieving broadband microwave blackbodies. In this study, all‐ceramic metamaterial microwave blackbodies are created by integrating a graded Gyroid shellular (GGS) metastructure design with additive manufacturing of polymer‐derived SiOC (PDCs‐SiOC) ceramics encapsulated by Si3N4 (SiOC@Si3N4). Hardly influenced by the destructive interference effect, as‐fabricated GGS‐structured SiOC@Si3N4 microwave blackbodies demonstrate a broadband microwave absorption (MA) above 83.6% (91.3% on average) across the entire X‐Ku band and encompassing higher frequencies above 18 GHz as well, together with the temperature insensitivity from room temperature to 500 °C. Based on the flexible electromagnetic tunability of PDCs‐SiOC, exceptional structural scalability is experimentally validated for metal‐doped modified CuSiOC and CoSiOC substrates with the same GGS metastructures, retaining high‐efficiency MA capability. Furthermore, attachment of perfectly reflecting metal backplanes further enhances the MA performance, with an ultrawide MA exceeding 67.9% (89.1% on average) achievable at 2.95–18 GHz for CoSiOC substrate. Meanwhile, the GGS‐structured SiOC@Si3N4 metamaterials possess additional multifunctional properties, such as good noise reduction performance as well as ultrahigh wear resistance. As a proof of concept, this study provides important guidance on achieving multifunctional coupling broadband MA characteristics by fully tapping the application potential of existing materials.
Article
Vehicle sound package has a substantial role in attenuation of vehicle noise for the purpose of improving interior comfort and reducing environmental noise pollution. In this paper, two approaches of absorptive design and multi-layer design are investigated to improve the performance of a sound package in automotive applications. For this purpose, the sound package of the engine compartment including hoodliner and dash insulator is studied. Furthermore, this paper proposes an experimentally validated prediction model through Statistical Energy Analysis (SEA) to evaluate the sound insulation of a vehicle body sound package. For this purpose, the research includes characterization techniques of sound package materials to determine intrinsic parameters of different layers required to develop a reliable SEA model. The inverse method and time-temperature superposition principle (TTSP) are utilized to characterize porous materials and heavy layer properties, respectively. The comparison of results that are experimentally obtained by acoustic cabin shows a fairly well agreement with the proposed SEA modeling procedure for the desired frequency range. By observing the results of the simulation for the hoodliner and dash insulator, it was revealed that the absorptive design combines the benefits of high sound absorption capability, wide absorption frequency band, and lightweight. Indeed, with an appropriate combination of layers, the multi-layer design can simultaneously provide a mixture of absorptive design attributes and sound insulation capabilities. The transmission loss obtained from the proposed design of the dash insulator revealed an increase by 42% without any reduction in the amount of sound absorption. Also, the sound absorption of the proposed hoodliner is enhanced more than twice in reference to the base design.
Article
Thermal decomposition of inorganic salts, such as carbonates, oxalates, and nitrates, is a facile method for synthesizing porous oxides; however, it typically produces either micropores or mesopores with sizes below 50 nm. Macropores larger than 50 nm can capture fine particles from a liquid or gas flow and thus be employed not only for environmental purification but also for the preparation of functional composites. In this study, we investigate the role of water vapor in the formation of macroporous Mn3O4 by the thermal decomposition of MnCO3. It is found that water vapor accelerates the decomposition of MnCO3 and subsequent particle growth as well as the conversion of manganese oxides at lower temperatures than in air. As a result, maze-like open macropores are formed by the randomly growing primary particle walls. Single-particle compressive tests reveal that small microspheres with sizes of 3 µm are easily deformed to relieve compressive stress. The macropore formation through the thermal decomposition of MnCO3 in water vapor and microstructural tuning of the pore size, particle size ratio, and degree of curvature of interior walls can potentially expand the application range of porous oxide materials.
Article
Environmental noise characteristics are determined by factors besides its source. One such factor is reverberation time, which in city canyons tends to be high due to the reflective characteristics of materials commonly used in building facades. Incorporating sound absorbing materials into building facades can help improve urban environments. This research evaluates different facade materials (concrete mix, mortar mix, vinyl spackling, and epoxy resin) incorporated with rice husk nanoparticles (NPs). Rice husk, in addition to presenting good properties for acoustic absorption, is one of the main agricultural wastes worldwide. Additionally, the characteristic of rice husk nanoparticles is correlated with milling time (longer grinding times enhance production of rice husk NPs). Sound absorption coefficients levels increase for compounds with a greater amount of rice husk NPs.
Article
Ultrafine fibers have broad application prospects in the field of sound absorption. In this work, novel ultrafine sound absorbers of polypropylene/polyethylene glycol (PP/PEG) were successfully prepared by green and efficient melt differential electrospinning (MDES) process. The effects of fibers diameter, samples thickness and cavity depth on sound absorption performance were systematically analyzed by impedance tubes method. The samples have excellent sound-absorbing performance especially in low and medium frequencies (<1000 Hz). When the average fibers diameter reached 1.8 μm, samples thickness was 26 mm and the cavity depth was 40 mm, the optimal sound absorption coefficient (SAC) was 0.93 and the corresponding frequency was 235 Hz (It is difficult for the SAC of mainstream sound absorbers to reach 0.9 when frequency is below 400 Hz). This investigation provides critical insight on the design and fabrication of a highly efficient low and medium frequency sound absorbers.
Article
The silkworm derived carbon fibre aerogel (SA) could be a promising building material to address the ever-growing noise (EM and sound) issues and improve the energy efficiency of the architectures.
Article
Nanofibrous acoustic materials have gradually attracted more and more attentions in noise reduction applications. Significant progress has also been made concerning the fabrication of nanofibrous materials. The aim of this work is to provide a comprehensive review on recent advances on the acoustical properties of nanofibrous materials. In this review, studies on the sound absorption of nanofibrous membrane and 3D nanofibrous structure are summarized. Nanofibrous materials can effectively absorb the incident acoustic energy, due to its high surface area, unique porous structures, and suitable airflow resistivity. Laminated composites consist of different nanofibrous layers can efficiently increase sound absorption coefficients with reduced thickness and weight. Moreover, the low-weight aerogel embedded with nanofibers has also been considered as one of the most attractive materials for noise control and reduction. With the advantages of ultra-light weight and robust sound absorption, it has been proven to be promised potential alternative to commercial porous absorber.
Article
To maximize the potential of the lightweight and high structural performance of composite and to integrate noise reduction function at the same time, the paper reports a noise reduction composite based on honeycomb sandwich structure by filling the honeycomb with different plant and synthetic fibers and utilizing micro-perforated plate (MPP) as the face sheet. The sound-absorbing effect of these structural samples was characterized and compared. The porosity and density of the filler material and the fiber cloth configuration were calculated and measured. The results show that the filling in the honeycomb significantly increases the sound absorption coefficient and widens the sound absorption band. Meanwhile, the MPP parameter determines the peak frequency of the sound absorption curve. Especially in the range of 125Hz-4000Hz, cotton and polyester filling provide the highest average sound absorption coefficients of 0.532 and 0.483 among the five materials pre-studied, while the absorption coefficient of the unfilled sample is about 0.15. The results reveal that a filled honeycomb structure with an MPP face sheet can reduce noise while maintaining mechanical properties. The noise-reduction function-integrated structures can potentially influence construction, transportation, and infrastructure as lightweight and space-saving solutions.
Article
The present work aimed to investigate the effect of nanofiber enhancement on the sound absorption performance of fibrous materials through a brief review of the recent progress in the field and highlight the current challenges and issues for measuring and modeling the sound absorption coefficient (SAC) of such structures. Moreover, some composite samples consist of polyethylenterephthalat (PET) nonwoven fabric and polyacrylonitrile (PAN) nanofibers web were prepared to be studied as a case study. The nanofiber deposition amounts, nanofiber orientation, and nanofiber layering sequence were chosen as the main variables of samples. Various amounts of deposition were achieved through three different electrospinning (ES) time of 15, 60, and 180 min. The random and aligned orientation of the nanofiber web was obtained by using a rotating collector. The effect of the layering sequence of the nanofiber layer was studied in the form of quadruple-layered assemblies of samples. The results showed that the samples incorporated with the nanofiber layer with 60 min of ES and random orientation when it is used in the second position of the layering sequence have the best performance of sound absorption, mainly at lower critical frequency range. Furthermore, a discussion on some available empirical models and compare the results confirms that there is a need for developing a new approach for measuring and modeling the SAC and consequently designing such absorbers.
Article
Electrospun nanofiber materials, with the advantages of large specific surface area, small pore size, high porosity, good channel connectivity, and ease of functional modification, have been widely used in various fields including environmental governance, safety protection, and tissue engineering. With the development of functional fiber materials, the construction of three-dimensional (3D) fiber materials with stable structures has become a critical challenge to expanding application and improving the performance of electrospun fibers. In recent years, researchers have carried out a lot of studies on the 3D reconstruction of electrospun fiber membranes and direct electrospinning of fiber sponges. Specifically, a variety of 3D fibrous sponges were constructed by the 3D reconstruction of electrospun fiber membranes, including embedded hydrogels, 3D printing, gas-foaming, and freeze-drying methods. Meanwhile, the direct electrospinning methods of 3D fibrous sponges have also been successfully developed, which are mainly divided into layer-by-layer stacking, liquid-assisted collection, 3D template collection, particle leaching, and humidity field regulation. Moreover, the applications of these fibrous sponges in many fields have been explored, such as sound absorption, warmth retention, thermal insulation, air filtration, adsorption/separation, and tissue engineering. These research works provide new ideas and methods for the fabrication of 3D fiber materials. Herein, the electrospinning technology and principle were briefly introduced, the representative progress of 3D fiber sponges in recent years was summarized, and their future development prospected.Graphical Abstract
Article
For applications of porous materials where mean pressure drop is a concern, packaging the material into a corrugated structure is better compared to other geometries such as block or wedge shapes. The goal of this study is to integrate noise reduction functionality within that material, which requires an understanding of the sound propagation through corrugated porous structures, including flow effects. The corrugated porous structure involves porous partitions separating inlet and outlet fluid channels. The porous materials considered are periodic octet-truss and body-centered cubic unit cells, and sound propagation across these porous partitions is modeled using the Johnson-Champoux-Allard model. The predicted transmission loss (TL) is benchmarked using designed additively manufactured corrugated structures measured using a flow duct. The laminar flow regime is maintained across the porous structure to reduce flow-noise effects. It is shown that the TL for a given corrugated structure increases with a decrease in porosity, and the impact of flow becomes significant as the porosity decreases. The influence of flow on TL also depends on the unit cell configuration. Furthermore, the model provides insights on pressure and acoustic particle velocity distributions within the corrugated structure and reveals regions of the porous material that effectively participate in noise reduction.
Article
In the structural design of sound-absorbing materials, how to combine the merits of porous materials with acoustic functional fillers with special structures to improve the sound-absorbing performance of porous materials at low-medium frequencies is a challenging problem. Herein, a directionally antagonistic acoustic textile is proposed as a sound absorber fabricated via single-sided coating. It is found that the sound absorber presents a double gradient structure by controlling the distribution of filler on the porous material frame. Considering the incident plane of acoustic waves, two different paths are defined, namely A–B and B–A (A, coated side; B, uncoated side), under which the sound absorber shows remarkable anisotropic sound absorption. The peak frequency is from 5559 Hz of bare fabric to 3455 Hz of the A–B NW III (coated nonwoven when sound waves propagate along A–B) sound absorber, showing a significant tendency to move to the lower frequencies. The peak value of sound absorption coefficient of the A–B NW III is 0.94, indicating a high sound absorptivity. In addition, by adjusting the acoustic functional filler and weaving structure and thickness of the base fabric, the sound absorber exhibits the expected anisotropic sound absorption. The novel sound absorber can be fit for lightweight sound-absorbing applications because of the characteristics of light, soft, high efficiency and broadband sound absorption.
Article
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With the growing interest in foamed geopolymer manufacture, there is an increasing demand for tailoring these composites’ pore structure. The binary system of two surfactants offers a possibility for enhancing foam formation efficiency and stability in geopolymer paste, however, their effects are not well understood. The influence of binary system blends on pore formation, size, and distribution in fly ash-based geopolymer matrix is investigated and their synergistic effects are evaluated. The results show that combining a nonionic surfactant with anionic Sodium Dodecyl Sulfate (SDS) increases open porosity, whereas cationic Cetyltrimethylammonium Bromide (CTAB) promotes the development of closed pores, improving thermal insulating, hygrothermal and mechanical performance. Moreover, this paper proposes a mechanism to describe the creation of pores in the presence of mixed micelles, as well as the benefit of employing mixed surfactant systems in customizing porous composites.
Article
The harm of sound pollution to human health is nonnegligible. To construct a good sound absorbent fabric, we proposed a combination of flat-knitting and microfibers, which is a simple and effective way to prepare a hierarchical three-dimensional spacer fabric with macroscopic and microscopic pore structure. The effects of spacer layer structure, surface yarn thickness and sea islands composite filaments split durations on sound absorption property were investigated. The sound absorption property was evaluated by the sound absorption coefficient and noise reduction rate. The results show that the sound absorption property of the spacer fabric is improved by splitting, especially under the middle frequency range (1000–3000 Hz). What is more, the noise reduction rate of our layered hierarchical spacer fabric increased by 4.1% with the thickness decreased by 21.1% compared with a recently reported hierarchical porous sound absorbent fabric.
Article
A high-temperature theoretical model is established to investigate the sound absorption performance of helically perforated porous metamaterials (HPPM) at high temperature. Finite element simulations are carried out to validate the theoretical model, and good agreements have been achieved. By perforating three-dimensional helical holes in homogeneous porous material, sound waves can enter the porous material matrix more fully through these extended macroscopic helical perforations, thereby obtaining good low-frequency sound absorption at high temperatures. The results show that with the increase of temperature, the impedance matching between the metamaterial and air becomes better, so as to improve the low-frequency sound absorption ability. Compared to homogeneous porous materials, the average absorption coefficient is greatly improved, especially at higher temperatures. The numerical results of sound pressure and energy dissipation at different temperatures show that high-temperature enhances the pressure diffusion effect at off-peak frequencies, resulting in more energy dissipation in high sound pressure regions. The helically perforated porous metamaterials with large helical diameter and low pitch exhibit better sound absorption performance in the low frequency range. The proposed metamaterial can be used for low-frequency sound absorption at high temperature.
Article
Environmental noise has been regarded as major noise pollution with severe hazards to human physical and mental health. The common commercial fiber sound-absorption materials have insufficient low frequencies wave absorbing and fire-resistant ability, limiting their wide application. To solve these problems, a novel strategy combining flexible nanofibers and rGO/MXene nanosheets was proposed to fabricate rGO/MXene/SiO2 nanofibers composite aerogel with hierarchically porous structure, which possessed an extremely low density of 9.8 mg/cm³ and superior low-frequency sound absorption ability (NRC value of 0.51). The obtained composite aerogel possessed a large deformation up to 80% (corresponding compressive stress of 17 kPa) and quickly recovered. In addition, the as-prepared aerogel could be easily produced on a large scale, providing a reference for the development of new generation of sound-absorbing products.
Article
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Several porous materials, especially natural fibres and polyurethane foams, are frequently used as sound absorbers in multiple noise reduction applications. Notwithstanding their excellent absorption performance, these materials usually lack the structural strength and fire resistance required for use in aggressive environments or situations requiring structural stability. This paper proposes the design of open-pore polymer and aluminum cellular materials with non-stochastic structures for sound absorption. These materials were fabricated using additive manufacturing (polymeric materials) and the replication method (aluminum materials), which involves infiltrating porous preforms formed by compacting spheres of a martyr material, such as NaCl, with liquid aluminum. The proposed materials can be employed as a resonator system when backed by an air cavity, with the change in cavity depth used to tune its sound absorption peak. Following the standard ASTM E1050, the sound absorption of these materials was investigated. In addition, the sound absorption performance of the materials was predicted using an Equivalent Circuit Method model. The experimental results are consistent with those predicted by the model, highlighting the potential of the microstructural and configurational design of these materials as sound absorbers. Graphical Abstract
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This paper reports the utilisation of fibres from the pineapple leaf (PALF) to be an alternative natural acoustic material. We fabricated samples from raw pineapple leaf fibres with different densities and thicknesses to ob- serve their effects on the sound absorption characteristic. Measurement was conducted for the normal incidence sound absorption coefficient in an impedance tube based on ISO 10534-2. It reveals that the pineapple leaf fibres can achieve sound absorption coefficient of 0.9 on average above 1 kHz by controlling the densities of the fibres and/or by introducing the air gap behind the samples. It is also demonstrated that the sound absorption per- formance is similar to that of the commercial rock wool fibres and synthetic polyurethane foam.
Article
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The sound absorption property of polyurethane (PU) foams loaded with natural tea-leaf fibers and luffa cylindrica (LC) has been studied. The results show a significant improvement in the sound absorption property parallel to an increase in the amount of tea-leaf fibers (TLF). Using luffa-cylindrica as a filler material improves sound absorption properties of soft foam at all frequency ranges. Moreover, an increase in the thickness of the sample resulted in an improvement of the sound absorption property. It is pleasing to see that adding tea-leaf fibers and luffa-cylindrica to the polyurethane foam demonstrate a significant contribution to sound absorption properties of the material and it encourages using environmental friendly products as sound absorption material in further studies.
Article
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A concept of hybrid local piezoelectric and electrical conductive functions for improving airborne sound absorption is proposed and demonstrated in composite foam made of porous polar polyvinylidene fluoride (PVDF) mixed with conductive single-walled carbon nanotube (SWCNT). According to our hybrid material function design, the local piezoelectric effect in the PVDF matrix with the polar structure and the electrical resistive loss of SWCNT enhanced sound energy conversion to electrical energy and subsequently to thermal energy, respectively, in addition to the other known sound absorption mechanisms in a porous material. It is found that the overall energy conversion and hence the sound absorption performance are maximized when the concentration of the SWCNT is around the conductivity percolation threshold. For the optimal composition of PVDF/5 wt. % SWCNT, a sound reduction coefficient of larger than 0.58 has been obtained, with a high sound absorption coefficient higher than 50% at 600 Hz, showing their great values for passive noise mitigation even at a low frequency.
Article
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Noise pollution is an important issue for automotive industries. In this article, the high molecular-weight copolymer polyol is blended in the polyol mixtures for fabricating flexible polyurethane foams to improve sound absorption efficiency. Changes of cavity size and material density of the foams are negligible by inclusion of copolymer polyol in the polyol mixture, but the closed pore ratio and specific airflow resistance increase for the copolymer polyol content higher than 20 wt% because of changes of phase separation behavior from drainage flow rate reduction that occurs with increased viscosity. Sound absorption efficiency increases with increasing copolymer polyol content up to 20 wt%, but it decreases beyond this point. The sound absorption property mainly results from the closed pore ratio, not from the cavity size. The compression strength increases with increasing copolymer polyol contents by increased amount of hard segments. Therefore, an optimum amount of high molecular-weight polyol is recommended for enhanced sound absorption property.
Article
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The purpose of this work is to systematically study the effect of the throat and the pore sizes on the sound absorbing properties of open-cell foams. The three-dimensional idealized unit cell used in this work enables to mimic the acoustical macro-behavior of a large class of cellular solid foams. This study is carried out for a normal incidence and also for a diffuse field excitation, with a relatively large range of sample thicknesses. The transport and sound absorbing properties are numerically studied as a function of the throat size, the pore size, and the sample thickness. The resulting diagrams show the ranges of the specific throat sizes and pore sizes where the sound absorption grading is maximized due to the pore morphology as a function of the sample thickness, and how it correlates with the corresponding transport parameters. These charts demonstrate, together with typical examples, how the morphological characteristics of foam could be modified in order to increase the visco-thermal dissipation effects.
Article
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This paper proposes lightweight textile acoustic structure, wherein electrospun polyacrylonitrile-based nanofibers enhance sound absorption properties with no weight and thickness penalty. Polyacrylonitrile nanofibers with diameter of 110 ± 7 nm were electrospun on spacer-knitted fabrics by varying deposition amount and surface coating arrangement. Proposed novel approach eliminated additional processing steps such as handling and post-lamination and provided easy scalability of nanofibers at macro-scale. The results showed that the sound absorption of nano-enhanced specimens was improved drastically when deposited amount of nanofibers or its effective surface area increased. Sound propagation paths in different configurations were interpreted from sound absorption and air permeability measurements. The sound absorption coefficient values up to 0.7 are achieved in the low and medium frequency ranges with no weight and thickness penalty by tuning deposition amount and surface coating arrangement.
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Noise is an environmental pollutant with recognized impacts on the psychological and physiological health of humans. Many porous materials are often limited by low sound absorption over a broad frequency range, delicacy, excessive weight and thickness, poor moisture insulation, high temperature instability, and lack of readiness for high volume commercialization. Herein, an efficient and robust lamella-structure is reported as an acoustic absorber based on self-assembled interconnected graphene oxide (GO) sheets supported by a grill-shaped melamine skeleton. The fabricated lamella structure exhibits ≈60.3% enhancement over a broad absorption band between 128 and 4000 Hz (≈100% at lower frequencies) compared to the melamine foam. The enhanced acoustic absorption is identified to be structure dependent regardless of the density. The sound dissipation in the open-celled structure is due to the viscous and thermal losses, whereas it is predominantly tortuosity in wave propagation and enhanced surface area for the GO-based lamella. In addition to the enhanced acoustic absorption and mechanical robustness, the lamella provides superior structural functionality over many conventional sound absorbers including, moisture/mist insulation and fire retardancy. The fabrication of this new sound absorber is inexpensive, scalable and can be adapted for extensive applications in commercial, residential, and industrial building structures.
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The current study investigates the acoustic absorption property of nickel-based superalloy open-cell foams manufactured by a newly developed template replication process. Inconel 625 open cell foams with controllable porosities (92%–98%) and cell sizes (300 μm–900 μm) have been successfully produced and tested for their sound absorption performance. It is evident that foam samples with the smallest cell size among them exhibit the best acoustic absorption performance, with sound absorption coefficient > 0.9 at frequencies > 1500 Hz for 50 mm thick sample. In the numerical simulation, the classical Delany­Bazley model is employed to predict the acoustic absorption property across a broad frequency range, and it requires knowledge of foam's static air flow resistivity, which, as proposed in this work, can be analytically expressed as a function of foam's microstructure parameters. A good agreement between such microstructure-based numerical model and experimental results was obtained. The proposed model can be utilized as a material design tool to guide the production of foam with optimal microstructure for sound absorption through the controllable template replication process.
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Commercial 3D reticular nickel foam and its composite structure were investigated on the sound absorption at 200-2000 Hz. The absorption performance of foam plates 1-5 layers (1-layer thickness: 2.3 mm; porosity: 89%; average pore-diameter: 0.57 mm) was found to be poor, and could be improved by adding backed cavum or front perforated thin sheet. The absorption coefficient could reach about 0.4 at 1000-1600 Hz for the composite structure of 5-layer foam with a backed 5 mm-thick cavum, and even 0.68 at about 1000 Hz for that of 2-layer foam with the same cavum and a perforated plate closely in front of the foam.
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A semi-analytical model is developed to predict the influence of temperature on the sound absorbing performance of sintered metal fiber materials (SMFMs) by extending the Johnson-Champoux-Allard-Lafarge (JCAL) model. In this model, three micro-structure factors of the SMFMs – the Kozeny number and the thermal and viscous shape factors – are calculated by applying the multi-scale asymptotic method (MAM), and the temperature effect is taken into account by considering the variations of the thermo-physical parameters of saturated air with temperature. Key transport parameters (e.g., viscous and thermal permeability, tortuosity, viscous and thermal characteristic lengths) are then directly determined by fiber diameter and porosity of the material. High-efficiency of the semi-analytical model is demonstrated for estimating the influence of topological parameters upon sound absorption under different temperatures. The transport parameters obtained by the proposed model agree well with those calculated using fully numerical simulations. Also, the model predictions are in accordance with existing experimental measurements at different temperatures. The model reveals not only the underlying mechanisms of temperature effect on sound propagation in porous metals, but also provides a theoretical guideline for the sound absorption application of SMFMs in high temperature environments.
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The present article aims to characterize and improve the sound absorption of wood-wool cement boards (WWCB) with varying strand widths, densities, thicknesses and applied with varying air cavity thicknesses by using impedance models. Different rigid-frame impedance models were analysed to predict the acoustic impedance of this material, and their suitability for the WWCB was evaluated. The Johnson-Champoux-Allard (JCA) model with its parameters, porosity, flow resistivity, tortuosity and characteristic lengths, was found to be the most appropriate to model the normal incidence sound absorption. The relations between the bulk density of the board and the impedance model parameters are established for material characterisation and optimization. Optimum density values were found per strand in terms of the sound absorption in the frequency range 200–2500 Hz. Moreover, the use of a density variation in the boards leads to improvement of the sound absorption. Regarding the application of the board, the use of an air cavity with a thickness of 100 mm leads to an optimized sound absorption for every strand width.
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Highly porous Si3N4 ceramics were fabricated via a volume-controlled mechanical foaming method. Slurries were fully filled in closed containers after foaming, thus porosity was controlled by adjusting the ratio of solid content and volume of foamed slurries. Pore size could also be controlled in this work by changing solid content of slurries. Porous Si3N4 ceramics with porosity varied from 70-90% were obtained after foaming and sintering. The as-obtained porosity of porous Si3N4 ceramics was in well consistence with evaluated porosity before experiments. These highly porous Si3N4 ceramics showed fine compressive strength ranging from 6.7 MPa to 59.9 MPa. Sound absorption characteristics of porous Si3N4 ceramics were investigated, both porosity and pore size had important effects on sound absorption coefficient.
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The rapid advances in technology, urbanisation and decrease in life span of electronic equipment have accelerated the generation of electronic waste significantly. Electronic waste is considered as serious social problem and environmental threat and hence sustainable methodology is critically required to recycle e-waste. In this paper, a novel approach to synthesise glass fiber-silicon carbide (SiC) composite by using waste printed circuit boards (PCBs) and compact discs (CDs) as resources is reported. The synthesis is based on carbothermal reduction using non-metallic PCB residue as glass fiber source, waste CD char as carbon source and silicon dioxide as silicon source. SEM, XRD, Raman and FTIR results confirm the formation of glass fiber-SiC composite and XRD signifies the major phases of β-SiC. SiC particles were mainly composed of sphere shaped nanoparticles and glass fiber sizes were in the range of 200–500 um length. This innovative approach of using electronic waste as resources could be an alternative for synthesis of glass fiber-SiC composite and also reduces the dependency on traditional raw materials.
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In the present work, acoustic wave absorber nanocomposites based on flexible polyurethane (PU) foam and sonicated multi walled carbon nanotube (CNT) has been fabricated by in-situ solution polymerization of toluene diisocyanate (TDI) and an ether based polyol in the presence of CNT particles. This study provides the influence of sonication time upon the acoustical and dynamic mechanical properties of the prepared composites. Flow resistivity measurement, scanning electron microscopy (SEM), dynamic mechanical thermal analysis (DMTA) and compressive mechanical measurement were performed on the prepared samples. The fabricated PU/CNT foamed nanocomposites were evaluated for their acoustic wave absorbency within a wide range of frequency (400-6300 Hz). Nanocomposites originated by highly sonicated CNT exhibited more acoustic absorbency. All CNT filled PU composites showed higher attenuation of the acoustic wave energy than the unfilled reference sample. Results revealed that sound absorbency of the foamed PU/CNT nanocomposites could be effectively enhanced by increasing the sonication time. Dynamic mechanical temperature sweep measurements showed decrease in mechanical loss tangent by enhancing the extent of CNT dispersion state. Results showed that improved sound absorbancy by CNT sonication time is mainly caused by increased CNT interacting surfaces rather than viscous response of the viscoelastic PU/CNT nanocomposites towards the applied dynamic field.
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Noises generated in automobile compartments can be controlled by utilizing sound absorption and insulation materials such as polyurethane foams. Polyurethane foams are synthesized by varying polymeric methylene diphenyl diisocyanate (MDI) content for exploring the effect of high functional isocyanate on cellular and acoustic properties. The use of polymeric MDI affects polyurethane matrix modulus and drainage flow rate of the foams, and it also has strong effects on cell structure and air flow resistance (AFR). The highest sound absorption coefficient is achieved at the optimum amount of the polymeric MDI. Therefore, the optimum amount of polymeric MDI content is recommended to achieve not only high sound absorption coefficient but also high transmission loss from the polyurethane foams.
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Herein, epoxy foams with tunable acoustic behavior were innovatively fabricated via non-traditional expandable microspheres. By adjusting the preparation parameters, changes in cellular structure from closed cells to partial open cells were observed, which exhibited tunability and resulted in diverse sound absorption behaviors. Short procuring time, high microsphere contents and foaming temperatures contribute to constructing more open cells and enhancing the sound absorption properties. High absorption coefficient up to 0.75 is achieved. Such novel light-weight epoxy foam can serve as a promising structural and sound-proof material, and satisfy the needs of many multifunctional systems including vehicles, buildings, aircrafts, etc.
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This paper presents a method for fabricating a tri-dimensional reticulated porous material (TRPM) made of copper fibers and an experimental study of the acoustic performance of the TRPM. Continuous copper fibers are produced through a multi-tooth tool, and sintered using a low-temperature solid-phase sintering (LSS) technique to fabricate the TRPM. The micro morphology of fiber surface and internal structure of the TRPM are investigated. The results show that a rock-like and fine-grained morphology on the fiber surface is achieved, and the complex micro-pores in the porous material give a large specific surface area. The sound absorption coefficient and transmission loss of the TRPM are also investigated under different bulk densities and porosity characteristics.