Low-frequency Absorption Research Articles

Coupled Helmholtz resonators for broadband Aeroacoustic noise mitigation

As a structurally simple acoustic element, Helmholtz resonators can exhibit strong resonance when acoustic waves enter the cavity, thus providing excellent sound absorption effects. Consequently, they are widely applied in automotive engine and exhaust systems. This paper systematically investigates the noise reduction performance of multiple coupled Helmholtz resonators under conditions with and without tangential flow. A finite element simulation model with multiple Helmholtz resonators is established by employing COMSOL Multiphysics software to solve the linearized Navier-Stokes equations in the frequency domain. The simulation results demonstrate that the structure, which couples multiple Helmholtz resonators, can effectively broaden the low-frequency sound absorption band under the influence of a flow field, enhancing the transmission loss across the entire low-frequency band. This structure holds significant potential for applications in automotive exhaust systems and aero-engine noise reduction.

A low-frequency anechoic coating under high hydrostatic pressure by pressure balancing strategy

It has always been a challenge to develop low-frequency anechoic coatings under high hydrostatic pressure. Based on active pressure balancing strategy (APBS), the airbag embedding in Helmholtz resonator model is proposed to achieve low-frequency and broadband sound absorption under high hydrostatic pressure. Compared with some existing pressure resistance schemes, the proposed anechoic coating has a thinner and lighter structure, lower sound absorption frequency band, and can withstand higher pressure. The proposed anechoic coating achieves sound absorption coefficient larger than 0.5 within the bandwidth of 24.5∼1000 Hz under pressure of 0.1 MPa, and within the bandwidth of 73.6∼1000 Hz under pressure of 5 MPa. The thickness of 10 mm is less than 1‰ of the incident wavelength at 100 Hz. The proposed model provides a new solution to the problem of improving low-frequency sound absorption performance of the anechoic coating under high hydrostatic pressure.

Continuous near-perfect sound absorption of a slit-resonator acoustic metastructure

A novel slit-resonator acoustic metastructure (SRAM) composed of Helmholtz resonators and porous materials is proposed to achieve a continuous perfect sound absorption at 200–3000 Hz. The Helmholtz resonator utilizes the resonance effect for low-frequency acoustic energy attenuation, and when its neck is small enough, it can be considered as an air slit. The air slit acts as a channel, from which most acoustic waves enter the metastructure and are absorbed by porous materials. Porous materials absorb high-frequency sound waves through thermoviscous dissipation. Unlike traditional filling forms, porous materials are filled around the air slits. To analyze the acoustic performance of this metamaterial, theoretical models and finite element models are developed and experimentally verified. The SRAM with melamine foam and rock wool can reach an absorption effect better than 0.5 at 331–3000 Hz and reaches a peak of 0.946 at 501 Hz with a thickness of 50 mm. Using the genetic algorithm, the parameters of SRAM are optimized for efficient sound absorption over a wider bandwidth. The optimized SRAM obtains an absorption coefficient of 0.8 in the range of 400–3000 Hz with a thickness of 50 mm. This study provides a new method of low-frequency ultra-broadband sound absorption.

Design of phononic crystal for enhancing low-frequency sound absorption in mufflers

To enhance the low-frequency sound absorption capabilities of expansion chamber mufflers, a novel Helmholtz-ring phononic crystal cell was developed. This innovative design integrates ring Helmholtz resonators as the phononic crystal scatterer, which is periodically arranged within the expansion chamber of a muffler to achieve enhanced sound attenuation at deep sub-wavelength scales. The transmission loss characteristics of the phononic crystal muffler were thoroughly examined and found to reveal a pronounced enhancement in sound absorption within the low-frequency bandgap. A critical aspect of this study was the analysis of the influence of defect states on transmission loss of the muffler. The introduction of defect states significantly expanded the sound attenuation bandwidth, effectively compensating for reduced sound absorption performance of the muffler outside the bandgap. The proposed phononic crystal muffler demonstrated a marked improvement in both transmission loss and aerodynamic performance compared to the traditional expansion chamber muffler. Notably, the sound attenuation was further augmented when in defective states. Corresponding experimental investigations were conducted and confirmed the effectiveness of the phononic crystal muffler within its designated bandgap range. This research presents a new way for the development of more efficient noise control solutions.

Multifunctional acoustic and mechanical metamaterials prepared from continuous CFRP composites.

The imperative advance towards achieving "carbon neutrality" necessitates the development of porous structures possessing dual acoustic and mechanical properties in order to mitigate energy consumption. Nevertheless, enhancing various functionalities often leads to an increase in the structural weight, which limits the feasibility of using such structures in weight-sensitive applications. In accordance with the outlined specifications, a novel structural design incorporating carbon fiber reinforced polymer (CFRP) composites alongside mechanical and acoustic metamaterials has been introduced for the first time. This innovative construction exhibits a lightweight composition with excellent mechanical and acoustic characteristics. Experimental findings demonstrate that with meticulous planning and fabrication, CFRP composite structures can achieve a balance of lightweight construction, high strength, exceptional energy absorption, and remarkable resilience. By introducing membrane and reasonable cavity design, the structure can produce low broadband noise reduction performance by a local resonance effect and impedance matching mechanism of metamaterials. The structural sound insulation capability breaks traditional mass law, resulting in an exceptionally broadband sound insulation peak (bandwidth of nearly 1000 Hz). Furthermore, the sound absorption characteristic of the structure surpasses that of the melamine sponge at frequencies below 300 Hz, demonstrating superior low-frequency sound absorption properties. The proposed structure provides new approaches for the design of multifunctional lightweight superstructures.

Improved sound absorption by size gradient granular materials due to Brazil-nut effect

Granular material is attracting increasing research momentum due to its prevalence and rich unique properties. Nevertheless, compared with its numerous functions in other areas, the application of granular material in acoustic waves has received less attention. In this paper, we propose that by exploiting Brazil-nut effect induced particle size segregation in granular material, a significant improvement in the sound absorption can be achieved. Firstly, sound absorptions by particles with step-increasing sizes have been analyzed. It is found that size-dependent mechanisms of sound energy dissipation may occur. Secondly, sound absorptions by size-mixed particles with vibration treatment have been studied. As a result of the Brazil-nut effect induced size segregation, the size-mixed particles with initially limited sound absorptivity exhibited remarkably improved sound absorbing capability, with both broadened absorbing band and raised low-frequency absorption. This work demonstrates that simple processing of the granular material may become a promising way to create premium sound absorbers.

Facile synthesis of novel nitrogen-doped diamond with excellent microwave absorption and thermal conductive performance

Herein, nitrogen doped polycrystalline diamond was prepared using the microwave plasma chemical vapor deposition (MPCVD) method with H2–CH4–N2 gas sources to achieve excellent electromagnetic (EM) wave absorption and thermal management properties. The effect of the nitrogen content (0 to 50 ppm) on its performance was studied. The 25-ppm nitrogen doped diamond demonstrated excellent EM wave absorption performance, achieving a minimum reflection loss (RLmin) of −44.8 dB at 6.7 GHz and a maximum effective absorption band (EAB) of 5.4 GHz (3.7–9.1 GHz). The superior absorption performance could be attributed to synergistic attenuation mechanisms including dipole and interface polarization, conduction loss, eddy current loss, and magnetic polarization, intensified by nitrogen vacancy centers. This multifunctional material, combining high thermal conductivity with effective low-frequency EM wave absorption, showed promise for applications in 5G communications and electronic devices.

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.

Excellent broadband sound absorption in composites manufactured by embedding piezoelectric polymer aerogels in porous ceramics

Challenges remain in the design and manufacture of acoustic devices with excellent broadband sound absorption performance. Herein, an efficient acoustic absorber with hierarchical pore structure and additional acoustic-electrical energy conversion mechanism is reported in which combined zirconia porous ceramics and P(VDF-TrFE) piezoelectric aerogels. Reticular cross-scale pore structure enables sound waves to propagate and dissipate effectively. The vibrations generated by piezoelectric aerogels under acoustic excitation further consume sound waves through converting acoustic energy into electrical energy based on the local piezoelectric and triboelectric effect, which improves the medium and low-frequency sound absorption capability. The average sound absorption coefficient of the prepared acoustic composites reaches 0.87 while the noise reduction coefficient is 0.54. Furthermore, the composites also possess low density, high compressive strength, good hydrophobicity and thermal insulation properties for applications. Therefore, this innovative strategy offers new design ideas for the next generation of high-performance acoustic materials with promising applications in transportation and industrial construction.

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.

Electromagnetic Absorber Covering the P-band and THz band Based on Ferrite Materials with Multilayer Microstructural Units

In this work a novel absorber that can operate simultaneously at the previous (P) and terahertz (THz) frequencies is proposed. This innovation features a single configuration composed of NiCuZnBi ferrite with periodic microstructure on the surface. In P band, the absorber achieves an absorption efficiency of more than 90 % (with a reflection loss of less than −10 dB) across the frequency range of 429.5 to 1000 MHz. This extremely low frequency absorption is solely attributed to the inherent magnetic loss of NiCuZnBi ferrite and is independent of the surface structure of the absorber. In the THz band, with this same absorber, a single layer of NiCuZnBi microstructure unit can achieve broad absorption range from 351.8 to 1200 GHz due to local energy loss caused by diffraction effects. More importantly, we have revealed the principle that increasing the number of NiCuZnBi microstructure unit layers is beneficial for broadening the THz absorption bandwidth. By designing stacked multi-layer structural units, we increased the absorption bandwidth of the device from 848.2 GHz for single-layer structures to 932.8 GHz for two-layer structures and 969.1 GHz for three-layer structures, respectively. The cross-band device design method proposed here makes it possible to develop electromagnetic absorbers that cover the P-band, THz band, and even more other frequency bands, thereby meeting the requirements of future applications such as multi-spectrum electromagnetic stealth and 6G communication.