Low-frequency Absorption Research Articles

Carbon based Composite Materials for Microwave Absorption in Low Frequency S and C Band: A Review

AbstractWith the rapid advancement of 5G technologies and electronic devices, there is a growing demand for microwave absorbing materials, especially those effective in low frequency microwave bands (2–8 GHz), making it an essential requirement. In recent years, many innovative microwave absorbers have been developed for low frequency absorption, with carbon/magnetic composite materials becoming particularly promising due to their various loss mechanisms and optimized impedance matching characteristics over a wide frequency range. However, there is currently a lack of a comprehensive review that summarizes the findings on low frequency absorbers. This review article thoroughly examines the current research on carbon/magnetic composite materials with efficient absorption performance in the low‐frequency S and C bands. It provides a detailed discussion of the microwave absorption performance of these composite materials. Furthermore, the composite design strategies, synthesis techniques, microstructure performance relationship, and microwave attenuation mechanisms are summarized. Lastly, the challenges and future outlook for low frequency microwave absorbing materials are addressed. This review article aspires to provide new insights into designing and synthesizing composite materials to accomplish effective low‐frequency microwave absorption, thereby promoting practical applications.

Tunable underwater sound absorption via piezoelectric materials with local resonators

In recent years, piezoelectric composite materials have been widely used in the design of underwater anechoic coatings due to their adaptability to tuning parameters. However, there are also some shortcomings, such as a single dissipation mechanism, narrow bandwidth, and poor low-frequency sound absorption. This work proposes an acoustic composite structure combining piezoelectric composite materials with local resonance units, which effectively enhances the sound absorption performance of the structure through the coupling effect of the piezoelectric energy consumption mechanism and local resonance mechanism. Compared to conventional acoustic structures, the proposed acoustic composite structure not only has a strong low-frequency sound absorption effect but also enriches the mid-high frequency sound absorption modes by connecting shunt damping circuits. On this basis, the effect of piezoelectric parameters and resonator morphological properties on structural sound absorption performance is further investigated, and the results show that the designed structure has the characteristic of sound absorption performance that is tunable. In addition, key factors affecting the sound absorption performance of the structure have been optimized to achieve better broadband sound absorption performance. This work may provide valuable ideas for the development of low-frequency broadband adjustable underwater sound-absorbing coatings.

Viscoelastic material enhancement of underwater sound absorption in higher-order resonators: from low-frequency to ultra-broadband

Underwater noise control is crucial for improving the marine acoustic environment. This study introduces a novel underwater acoustic absorber design that integrates viscoelastic materials with higher-order resonance structures to enhance the absorption efficiency of low-frequency sound waves. By embedding a thin layer of rubber within a conventional second-order Helmholtz resonator, the rubber's viscoelastic and damping properties significantly improve this structure's underwater low-frequency acoustic performance. Theoretical and simulation calculations have shown that the structure has two perfect absorption peaks in the low-frequency range. The simulation results show that in the frequency range of 415-1848 Hz (Simulation), only four units are needed to effectively absorb up to 90% of noise energy, and the total volume is only. Moreover, the absorber maintains excellent absorption performance over a wide range of incidence angles from 0 to 60 degrees. This work is helpful to design ultra-thin low-frequency underwater absorbers.

Experimental and numerical study on underwater noise control of multigrooved metasurface coating with antifouling and drag reduction potential

Acoustic metasurfaces have garnered increasing attention due to their efficacy in low-frequency sound absorption, while achieving broadband sound absorption performance remains challenging. In this study, a novel approach employing a multigrooved metasurface integrated onto a nanocomposite material is undertaken. Before modification, the nanocomposite material exhibits commendable underwater sound absorption capabilities above 4000 Hz, but demonstrates lower performance below this threshold. Integrating the multigrooved metasurface yields a notable enhancement in sound absorption performance below 4000 Hz, with the average absorption coefficient increasing from 0.29 to 0.63. Remarkably, this enhancement almost does not impact the performance above 4000 Hz. Experimental findings additionally reveal improved performance under variable hydrostatic pressures. Notably, the multigrooved surfaces show enhanced antifouling properties, and also exhibit potential for drag reduction compared to smooth surfaces. The proposed multigrooved metasurface in this study introduces a novel strategy towards the development of multifunctional underwater sound absorption coatings.

Ultra-thin low-frequency broadband absorber based on layered coiled channel structure

To improve and broaden the performance in absorbing sound waves of metamaterials at the lower and mid-frequency ranges, a layered coiled channel composite structure (LCCS) with deep subwavelengths is proposed in this paper, which extends the acoustic wave propagation paths longitudinally by connecting the Helmholtz resonator channels in each layer and achieves the multistep resonance of the coupling cavities, which results in the cancellation of acoustic energies, reduces the acoustic reflections and scattering, and enhances the absorption performance. A model to simulate theoretical sound absorption and finite elements simulation of the metamaterial were evolved to reveal its potential mechanismfor absorbing sound. Using the single-variable method to study the impact of altering structural parameters on the effectiveness of sound absorption, it shows good tunability in the target frequency range and obtains four fundamental unit LCCSs having distinct frequency absorption peaks, and designs a multi-unit coupling construction having low frequencies wideband sound absorption performance by connecting them in parallel to realize a large-broadband continuous and high-efficiency sound absorption within the scope of 370–1400 Hz, using a mean peak sound absorption value higher than 0.7. The structure was put together by 3D printing, and the impedance tube method test verified the precision of the simulation’s findings. This study offers a practical means to create lightweight, low-frequency broadband acoustic absorbing metamaterials.

Progress on the Sound Absorption of Viscoelastic Damping Porous Polymer Composites.

Porous polymer composites (PPC) have developed rapidly recently, which are widely used in various industrial fields. Viscoelastic damping is an important behavior of porous polymer composites, and it can determine the sound absorption for noise reduction applications. This review has mainly covered the viscoelastic damping and sound absorption of porous polymer composites. Different fabrication approaches of porous polymer composites are gathered. The mechanism of viscoelastic damping behavior is described, and also the sound absorption properties. Followed by the introduction of enhanced sound absorption of viscoelastic damping porous polymer composites, including the incorporation of fillers, microstructures modification, combination with nanofibrous materials, and multilayer configuration, etc. The incorporated fillers can effectively adjust the interfacial area in composites, and obtain desired bonding conditions. Microstructures modification is an effective tool to improve the morphologies of both polymer matrix and fillers, which can be achieved by chemical treatment and surface coating. The combination with lightweight nanofibrous layer can increase the low frequency absorption. The configuration of multilayer composites can improve both acoustical and mechanical properties for engineering applications. It is hoped that this comprehensive review is benefit for the promising development of porous polymer composites in related fields.

Analysis of Low-Frequency Sound Absorption Performance and Optimization of Structural Parameters for Acoustic Metamaterials for Spatial Double Helix Resonators

Low-frequency noise absorbers often require large structural dimensions, constraining their development in practical applications. In order to improve space utilization, an acoustic metamaterial with a spatial double helix, called a spatial double helix resonator (SDHR), is proposed in this paper. An analytical model of the spatial double-helix resonator is established and verified by numerical simulations and impedance tube experiments. By comparing the acoustic absorption coefficients of the spatial double-helix resonator, it is shown that the results of the analytical model, the numerical model, and the experiments are in good agreement, proving the accuracy of the theoretical model. The effects of different structural parameters on the peak sound absorption coefficient and resonance frequency are quantitatively revealed. The impedance variation law of the model is obtained, and the resistance and reactance distributions at the resonance frequency are analyzed. In the optimization model, the Back Propagation (BP) network is used to construct the mapping between the structural parameters and the resonance frequency and sound absorption coefficient, and this is used as the constraints of the equation, which is combined with Wild Horse Optimization (WHO) to establish the BP-WHO optimization model to minimize the volume of the spatial double helix resonator. The results show that, for a given noise frequency, the optimized structural parameters enhance the space utilization without affecting the performance of the space double helix resonator.

Excellent Low-Frequency Microwave Absorption and High Thermal Conductivity in Polydimethylsiloxane Composites Endowed by Hydrangea-Like CoNi@BN Heterostructure Fillers.

The advancement of thin, lightweight, and high-power electronic devices has increasingly exacerbated issues related to electromagnetic interference and heat accumulation. To address these challenges, a spray-drying-sintering process is employed to assemble chain-like CoNi and flake boron nitride (BN) into hydrangea-like CoNi@BN heterostructure fillers. These fillers are then composited with polydimethylsiloxane (PDMS) to develop CoNi@BN/PDMS composites, which integrate low-frequency microwave absorption and thermal conductivity. When the volume fraction of CoNi@BN is 44 vol% and the mass ratio of CoNi to BN is 3:1, the CoNi@BN/PDMS composites exhibit optimal performance in both low-frequency microwave absorption and thermal conductivity. These composites achieve a minimum reflection loss of -49.9dB and a low-frequency effective absorption bandwidth of 2.40GHz (3.92-6.32GHz) at a thickness of 4.4mm, fully covering the n79 band (4.4-5.0GHz) for 5G communications. Meanwhile, the in-plane thermal conductivity (λ∥) of the CoNi@BN/PDMS composites is 7.31W m-1 K-1, which is ≈11.4 times of the λ∥ (0.64W m-1 K-1) for pure PDMS, and 32% higher than that of the (CoNi/BN)/PDMS composites (5.52W m-1 K-1) with the same volume fraction of CoNi and BN obtained through direct mixing.

LaFe-MOFs derivatives with different compositions for boosting low-frequency and broadband electromagnetic wave absorption

To achieve low-frequency and broadband electromagnetic wave absorption (EMWA), building excellent metal-organic frameworks (MOFs)-derived EMWA materials is critical but remains challenging. In this research, La2O3/La2O2CN2/Fe/N-doped carbon (LFC) composites with different compositions were synthesized by adjusting the La3+ content to tune the electromagnetic parameters. As the La3+ content increased, the EMWA performance showed a trend of first increasing and then decreasing. As a result, LFC-2 achieved a minimum reflection loss (RLmin) value of −60.82 dB at a thickness of 2.60 mm when the molar ratio of La3+ to Fe3+ was 1: 1, along with an effective absorption bandwidth value of 5.76 GHz (9.84–15.60 GHz) at 2.29 mm. Moreover, the EMWA performance of LFC-3 at low-frequency (4.08 GHz) was enhanced when the La³⁺ to Fe³⁺ molar ratio was 2: 1, with an RLmin value of −47.47 dB. Comprehensive characterizations suggested that the formation of the La2O2CN2 phase played an indispensable role in optimizing impedance matching and enhancing magnetic loss and dielectric loss. In addition, La3+ had a high coordination number, which effectively regulated the electromagnetic parameters. Concurrently, radar cross-section simulation results confirmed the outstanding EMWA capability of the LFC coating. This work proposed a strategy for LaFe-MOFs derivatives, shedding light on the foundation for designing more efficient low-frequency and broadband absorbent materials.

Vegetal wools for building application: investigation of optimization approaches for the enhancement of low-frequency sound absorption

Bio-based building materials, such as vegetal wools for thermal and acoustic applications, are increasingly used in construction. Indeed, as they store atmospheric carbon dioxide, they have a lower carbon footprint than conventional materials. Despite high-level multi-functional properties, they suffer from poor low-frequency absorption for thin panels (less than 5 cm thick). Therefore, it seems relevant to investigate the optimization possibilities available, including meta-material approaches, and to adapt them to the specificities of vegetal wools, such as a low air resistivity for hemp and flax wools which are our main concern. To meet this challenge, a state-of-the-art is being carried out to identify the optimization methods best suited to the specific characteristics of vegetal wools. This preliminary work is based on the implementation of an initial optimization concept using a multilayer poroelastic material with a perforated resistive layer. The material structure is optimized using analytical modeling to target low frequencies. Experimental verifications are then carried out to determine the sound absorption coefficient of vegetal wools with optimized configurations.

Broadband acoustic metamaterial absorber

Constrained by the causality nature, it is a challenge to achieve low-frequency broadband efficient absorption via passive materials. By coupling local resonances as many as possible, broadband metamaterial absorbers can be realized. In this scenario, the coupling effect is crucial for improving the absorption efficiency. Here, we demonstrate a kind of acoustic metamaterial absorber capable of high-efficiency broadband absorption by optimizing the non-local coupling effect. Cascade neck-embedded Helmholtz resonator is designed as the subunit of the metamaterial absorber. By coupling 36 subunits, a metamaterial absorber is constructed with the aid of optimization algorithm. The simulation results suggest that strong non-locality is induced in the near field, which effectively suppresses the impedance oscillations and absorption dips at the antiresonance frequencies. Eventually, the presented metamaterial absorber realizes impedance matching to the air from 320 Hz to 6400 Hz, leading to efficient broadband absorption performance. The results and methodologies in this work reveal the significance of non-local coupling effect in coupled-resonant metamaterials.

Self-Templating Engineering of Hollow N-Doped Carbon Microspheres Anchored with Ternary FeCoNi Alloys for Low-Frequency Microwave Absorption.

Rational design and precision fabrication of magnetic-dielectric composites have significant application potential for microwave absorption in the low-frequency range of 2-8GHz. However, the composition and structure engineering of these composites in regulating their magnetic-dielectric balance to achieve high-performance low-frequency microwave absorption remains challenging. Herein, a self-templating engineering strategy is proposed to fabricate hollow N-doped carbon microspheres anchored with ternary FeCoNi alloys. The high-temperature pyrolysis of FeCoNi alloy precursors creates core-shell FeCoNi alloy-graphitic carbon nano-units that are confined in carbon shells. Moreover, the anchored FeCoNi alloys play a critical role in maintaining hollow structural stability. In conjunction with the additional contribution of multiple heterogeneous interfaces, graphitization, and N doping to the regulation of electromagnetic parameters, hollow FeCoNi@NCMs exhibit a minimum reflection loss (RLmin) of -53.5dB and an effective absorption bandwidth (EAB) of 2.48GHz in the low-frequency range of 2-8GHz. Furthermore, a filler loading of 20wt% can also be used to achieve a broader EAB of 5.34GHz with a matching thickness of 1.7mm. In brief, this work opens up new avenues for the self-templating engineering of magnetic-dielectric composites for low-frequency microwave absorption.

Reverberation room design optimisation for low-frequency diffuse sound absorption testing

The reproducibility of sound absorption testing with the reverberation room method is a long-standing concern. Absorptive samples induce directionality in the nearfield, while the farfield depends on the room geometry below the Schroeder frequency. Nevertheless, when properly accounting for nearfield effects, the theoretical diffuse absorption coefficient of a sample still represents its average performance across an ensemble of different rooms, even at very low frequencies. Recent research found that particular reverberation room designs allow for an accurate measurement of the diffuse sound absorption coefficient of highly absorptive samples at low frequencies. Pinpointing such designs hence opens up a possibility to sustainably improve the low-frequency reproducibility of sound absorption testing in reverberation rooms. The present paper introduces a numerical optimisation framework that serves this purpose. Specific room shapes are parametrised and the geometrical room parameters are optimised so as to minimise the difference between the measured and the diffuse absorption coefficient under appropriate constraints. The sound absorption testing of a sample in a particular reverberation room is numerically simulated using a method that is both accurate and computationally efficient at low frequencies. The diffuse absorption is computed with a hybrid deterministic-statistical energy analysis approach that accounts for the detailed absorber properties, geometry, and boundary conditions, as well as the nearfield effects. The methodology is applied to both cuboidal and hexahedral room shapes. Certain optimised designs are found not only to provide an excellent match for the absorber that was used during the optimisation, but they also maintain their performance across a range of absorptive samples. Additionally, potential geometrical deviations are found to be well tolerated by these reverberation room designs.

Construction of Chiral-Magnetic-Dielectric Trinity Structures with Different Magnetic Systems for Efficient Low-Frequency Microwave Absorption.

The fabrication of carbon nanocoil (CNC)-based chiral-dielectric-magnetic trinity composites holds great significance in low-frequency microwave absorption fields. However, it is not clear that how the different magnetic systems affect the magnetic and frequency responses of the composites. Herein, four types of magnetic metals, FeCo, CoNi, FeNi, and FeCoNi, are selected to be combined with the chiral templates respectively, resulting in four types of chiral-dielectric-magnetic composites with similar morphology. The CNC templates endow all the composites with excellent dielectric loss. Further permeability analysis and the micro-magnetic simulation confirm that the frequency response region can be well adjusted by changing the magnetic systems with specific magnetic resonance modes and magnetic domain motion. Due to the synergistic effect between magnetism, chirality, and dielectricity, the FeNi-based composites exhibit the best low-frequency microwave absorption performance. The minimum RL of -60.7 dB is achieved at 6.7 GHz with an ultra-low filling ratio of 10%, and the EAB value in low-frequency region is extended to 3.7 GHz. This study provides further guidelines for the design of the chiral-dielectric-magnetic trinity composites in low-frequency microwave absorption.

Acoustic resonant metasurfaces with roughened necks for effective low-frequency sound absorption

The rise in environmental low-frequency noise from human-induced activities due to global urbanization has had a noticeable effect on the health and life quality of urban residents. Despite the implementation of conventional noise mitigation methods, they are only effective within a narrowband frequency range and their efficiency deteriorates when dealing with low-frequency noise. To address the existing problem in noise attenuation, an acoustic resonant metasurface that incorporates sliced Helmholtz-resonator-like substructures with embedded roughened necks is designed to achieve low-frequency sound absorption within a subwavelength scale. The theoretical model of this metasurface is first established, and then parametric studies are conducted to analyze the impact of various geometrical parameters on the sound absorption effect of the proposed metasurface. Subsequently, the sound absorption performance of the proposed design is investigated through numerical and experimental studies using two samples at both single and broadband frequencies. Excellent absorption performances by the samples at a low-frequency spectrum have been achieved. To understand the underlying absorptive mechanism of the proposed metasurface models, theoretical investigations are conducted for acoustic impedance and complex frequency, while numerical studies are also carried out to examine the distribution of acoustic pressure, acoustic velocity and thermoviscous power dissipation density. The research findings presented in this study may facilitate the development of an effective acoustic barrier for low-frequency noise (≤ 200 Hz).

Sound absorption performance of honeycomb metamaterials Inspired by Mortise-and-Tenon structures

To address the limitations of traditional honeycomb sandwich structures in attenuating mid to low-frequency sounds, particularly in configurations with minimal thickness and weight, this study introduces an innovative honeycomb acoustic metamaterial incorporating the traditional Chinese mortise-and-tenon joint. We systematically investigate the acoustic absorption characteristics of the modified honeycomb structure through theoretical analysis, empirical validation, and numerical simulations. Our experimental setup maintained consistent geometric parameters across all trials and demonstrated that the resonance frequency of the modified honeycomb structure decreased by 10 % relative to its conventional counterpart. We conducted detailed analyses on the influence of micropore positioning, tenon geometric dimensions, and micropore diameters on the acoustic performance. Notably, elongating the tenon from 2 mm to 6 mm resulted in a 15 % reduction in resonance frequency, whereas increasing the micropore-to-tenon distance from 0 mm to 4 mm led to a 30 % increase. The integration of the mortise-and-tenon joint significantly enhances the mid to low-frequency sound absorption performance of the honeycomb panels. This improvement is achieved while preserving the structural benefits of low panel thickness and shallow cavity depth, alongside simplified processing of micropores. Our findings elucidate a promising approach to augmenting the acoustic properties of lightweight structural materials, thereby extending their application potential in noise control engineering. This study not only contributes a novel perspective to the design and optimization of acoustic metamaterials but also highlights the potential for integrating traditional architectural techniques with modern material science to enhance noise control solutions.

Rational construction of dicarbonous CB/CNT@PI composites toward low filler loading and low-frequency electromagnetic wave absorption.

With the application of low frequency radar and the demand for stealth of high temperature resistant components, it is increasingly urgent to develop absorbing materials with both low frequency and high temperature resistant properties. Here, we successfully prepared various carbon/polyimide composites as low-frequency electromagnetic wave absorbing materials by simple blending method. The well-designed mesh lap structure introduces a large amount of free space, further optimizing the impedance matching of the material. At the same time, the multiple loss mechanism formed by the combination of carbon black dominated polarization and carbon nanotube dominated conductive loss enhances the loss of incident electromagnetic wave. The results showed that only 10 wt% filler loading of the CB/CNT@PI is achieved in the low frequency range (1-4 GHz) with a minimum RL strength of - 18.3 dB, which has obvious advantages compared with other works in recent years. This study provides a way for the design and preparation of resin-based absorbing materials.

Evaluation of the attenuation mechanism of the noise reduction performance of polyurethane porous elastic road surface

The polyurethane porous elastic road surface (PERS) demonstrates superior noise reduction performance, primarily due to its porous structure that facilitates effective sound absorption and its high elasticity that contributes to substantial vibration attenuation. However, it is susceptible to void clogging and long-term aging during service, leading to a gradual decline in noise reduction performance. To investigate the attenuation characteristics of noise reduction performance in PERS mixtures under void clogging and long-term aging conditions, this study investigated PERS mixtures subjected to various clogging cycles and aging times. Concurrently, the tire vertical drop test and standing wave tube absorption coefficient test were employed to analyze the variation rule in the noise sound pressure level (SPL) and sound absorption coefficient frequency curves under the above conditions. The results show that the noise reduction performance of the PERS mixture exhibits a variation rule of first decreasing and then increasing with the increase of the clogging cycles, and the required clogging amount to reach the void saturation state is 30–40 g. Compared to before clogging, the PERS mixture after clogging has an enhanced ability to absorb low-frequency noise and a weakened ability to absorb high-frequency noise; the sizes of the cloggings have a higher degree of attenuation in low-frequency sound absorption than in high-frequency. As long-term aging progresses, the noise reduction performance of the PERS mixture first increases and then decreases. Notably, long-term aging predominantly impacts the high-frequency noise reduction capabilities of the PERS mixture. The impact of void clogging on the noise reduction performance of PERS mixtures is found to be more pronounced than that of long-term aging.

Study on low-mid-frequency broadband sound absorption properties of bending acoustic metamaterials with embedded porous materials

With the rapid development of the traffic industry, noise issues are becoming increasingly serious, and the traditional noise control technologies have the problems of poor low-frequency noise absorption and narrow bandwidth. This study proposes a variable-section bending acoustic metamaterial with an embedded porous material (VS_BAMP). A theoretical model of the VS_BAMP unit is developed based on the Johnson-Champoux-Allard (JCA) model and the impedance transfer method. The sound absorption unit with a thickness of 48 mm exhibits a quasi-perfect (α = 0.98) at 736 Hz, and an efficient sound absorption (α > 0.8) in the range of 574 Hz–966 Hz. Based on the complex frequency plane method, this work designs sound absorption units that exhibit perfect sound absorption at discrete frequencies. By connecting two different absorption units (PVS_BAMP) in parallel, efficient sound absorption from 424 Hz to 1500 Hz is achieved. Finally, the accuracy of the theoretical model is verified by experiments and simulations, confirming the effective sound absorption of PVS_BAMP structure in the middle and low frequency bands. The prepared PVS_BAMP is highly adjustable, has a wide bandwidth, and can be prepared through a simple manufacturing process. Our results can provide a theoretical basis for the design of compact low-mid-frequency broadband noise reduction structures for practical application.

Modeling and optimizing a broadband sound absorber consisting of a granular activated carbon stack backed by a poro-elastic layer

High surface area particles, like granular activated carbon (GAC), have shown advantages in absorbing low-frequency sound, both by themselves and when combined with other treatments. In this article, a novel sound absorption treatment is described: i.e., a GAC stack supported by a soft, porous layer. At first, it was experimentally observed from impedance tube measurements that the low-frequency absorption performance of GAC stacks can be significantly enhanced by inserting a melamine foam between the stack and a rigid backing. To enable an analytical model of this type of absorber, the interfaces between GAC stacks and other materials were first characterized. Further, both 1-D layered transfer-matrix and 2-D axi-symmetric finite-difference approaches were implemented to predict the measured absorption spectra. Because of the constraint of the stack at the impedance tube wall, the 2-D model leads to more accurate absorption spectrum predictions. However, the difference between the two model predictions becomes negligible for large-area treatments. Therefore, amore efficient 1-D theory was incorporated in the multi-objective optimization to design broadband GAC-foam absorbers. Two optimization objectives were defined to balance the overall absorption performance: i.e., one associated with absorption at low frequencies, and the other with absorption at higher frequencies. It was found that a minimum thickness broadband GAC-foam absorber favors a thin, soft foam. In conclusion, this article presented a framework for the development of general broadband sound absorbers, and by combining both granular and porous materials, it may be possible to design practical sound-absorbing metamaterials that perform well at low frequencies.