Absorption In Frequency Range Research Articles

Biomass-derived carbon heterostructure composites modified with magnetic iron oxide for excellent EMW-absorbing materials

With the gradual spread of satellite navigation, broadcasting, and communication technologies, the problem of electromagnetic pollution in C- and Ku-band has increased. Electromagnetic wave (EMW) absorbing materials can effectively mitigate this problem. Here, we report the design of a biomass carbon heterostructure for the absorption of EMWs. This design takes advantage of the natural fiber mesh structure of the loofah and the voids, veins, and basal tissues surrounding the fibers. The material is then carbonized to create a radar antenna-like structure of fibers and nanosheets. This improves the material's conductive loss. In addition, modification with magnetic iron oxide in loofah sponges improves the impedance matching of the materials, enhances the magnetic loss, and increases the heterogeneous interface. We found that the maximum effective absorption frequency range of the prepared EMW-absorbing materials is 3.80 GHz, which is mainly concentrated in the C-band and Ku-band, and its maximum RLminvalue is −59.20 dB. The material's ability to absorb EMWs was also verified by simulating the radar cross section (RCS). In conclusion, this work develops a green and efficient composite material for absorbing EMWs and provides new design ideas for the development of EMW-absorbing materials.

The effects of phase interfaces in SmFeN/YSZ composite thermal barrier materials on electromagnetic wave-absorbing properties

Yttria-stabilized zirconia (YSZ) oxide ceramic powder is a thermal-barrier material used to fabricate thermal barrier coatings (TBC). In this study, to improve the electromagnetic wave (EMW) absorbing properties of TBCs, microwave-absorbing samarium iron nitrogen (SmFeN) particles were used to prepare SmFeN/yttria-stabilized zirconia (YSZ) composites. To study the effect of the SmFeN/YSZ interfaces on the EMW-absorbing properties of the SmFeN/YSZ composite, SmFeN/YSZ interfaces were built in different ways in composite materials having the same mass ratio of 1:1, for example, SmFeN and YSZ particles homogeneously mixed together, SmFeN/YSZ interfaces perpendicular and parallel to the EMW incident direction, and different interfaces between YSZ and SmFeN. Thus, the microwave absorbing performance of the samples, including the electromagnetic parameters, attenuation coefficient, eddy-loss current (C0), impedance matching, and minimum reflection loss (RLmin) values were examined and analyzed. The results demonstrate that interfaces perpendicular to the direction of the EMWs can increase the contact area and enhance the interfacial absorption effect. Further, when the wave-absorbing agent was placed in the middle of the composite, its wave-absorbing effect was the best among all the samples, yielding RLmin = −27.824 dB and effective absorption bandwidth (EAB) = 8.5453 GHz (effective absorption frequency range 7.9256–16.4709 GHz for a 7.2-mm-thick layer). Crucially, the frequency and bandwidth at which the best absorption occurred could be tuned by adjusting the thickness of the coating. This is because the interfaces in the samples resulted in polarization enhancement and anomalous dispersion, leading to large dielectric losses. Overall, the use of a coaxial measurement technique is highly efficient for EMW absorbing body design and process simulations.

Highly electrically conductive MOF/conducting polymer nanocomposites toward tunable electromagnetic wave absorption

Metal-organic frameworks (MOFs) have attracted significant interest as self-templates and precursors for the synthesis of carbon-based composites aimed at electromagnetic wave (EMW) absorption. However, the utilization of high-temperature treatments has introduced uncertainties regarding the compositions and microstructures of resulting derivatives. Additionally, complete carbonization has led to diminished yields of the produced carbon composites, significantly limiting their practical applications. Consequently, the exploration of pristine MOF-based EMW absorbers presents an intriguing yet challenging endeavor, primarily due to inherently low electrical conductivity. In this study, we showcase the utilization of structurally robust Zr-MOFs as scaffolds to build highly conductive Zr-MOF/PPy composites via an inner-outer dual-modification approach, which involves the production of conducting polypyrrole (PPy) both within the confined nanoporous channels and the external surface of Zr-MOFs via post-synthetic modification. The interconnection of confined PPy and surface-lined PPy together leads to a consecutive and extensive conducting network to the maximum extent. This therefore entails outstanding conductivity up to ∼14.3 S cm−1 in Zr-MOF/PPy composites, which is approximately 1–2 orders of magnitude higher than that for conductive MOF nanocomposites constructed from either inner or outer modification. Benefiting from the strong and tunable conduction loss, as well as the induced dielectric polarization originated from the porous structures and MOF-polymer interfaces, Zr-MOF/PPy exhibits excellent microwave attenuation capabilities and a tunable absorption frequency range. Specifically, with only 15 wt.% loading, the minimum reflection loss (RLmin) can reach up to –67.4 dB, accompanied by an effective absorption bandwidth (EAB) extending to 6.74 GHz. Furthermore, the microwave absorption characteristics can be tailored from the C-band to the Ku-band by adjusting the loading of PPy. This work provides valuable insights into the fabrication of conductive MOF composites by presenting a straightforward pathway to enhance and regulate electrical conduction in MOF-based nanocomposites, thus paving a way to facilely fabricate pristine MOF-based microwave absorbers.

Active low frequency band expansion mechanism of electromagnetic wave absorber composite structure

Bandwidth expansion of modern antennas and communication systems raises growing demand for electromagnetic wave absorber in compatibility equipment. Single absorption mechanism is hard to reach well-balanced characteristics including wide bandwidth, low frequency absorption and small thickness. Based on the composite structure concept, composite structure for electromagnetic wave absorber’s active low frequency band expansion is proposed, by combining circuit-analog absorber and active absorber to form a superimposed structure. The characteristic of composite structure is better than the single absorber structure, which improves enhancement mechanism brought by the composite structure. Sample fabrication and experiments indicate reflectivity curve envelope reaches −10 dB in 2.15–18 GHz frequency range (relative bandwidth 157.3%), and the tunable absorption frequency range in low-frequency domain is greater than 1.3 GHz. The structure has good adaptation on TE and TM polarizations, as well as good angle insensitivity. This composite structure has application potential in intelligent response system related with electromagnetic compatibility, energy absorption and object security management.

Structural, Magnetic, and Electrical Investigations of PVA/Ni Ferrite/(MoS2)x Nanocomposites.

Transition metal sulfides (TMDs) possess exceptional dielectric properties and a narrow band gap, rendering them highly efficient as electromagnetic absorbing materials. Among these TMDs, the two-dimensional MoS2 nanosheet has received significant attention in research. However, the quest for new absorbers no longer finds satisfaction in solitary absorption mechanisms. This article introduces a successful method for creating PVA/NiFe2O4/(MoS2)x nanocomposites via a straightforward sol-gel technique, wherein porous amorphous NiFe2O4 microspheres are integrated into MoS2 nanosheets. The investigation uncovers that the incorporation of MoS2 results in an enhanced complex permittivity, facilitating the attainment of a desirable permittivity level. The PVA/NiFe2O4/(MoS2)x nanocomposite absorber exhibits an incredibly low reflection loss (RL) of -16.75 dB at a mere thickness of 1 mm, achieved through the cooperative interaction of dielectric and magnetic loss, along with the advantages of the structure and composition. Consequently, the PVA/NiFe2O4/(MoS2)x nanocomposites effectively absorb electromagnetic waves. Therefore, it is posited that MoS2-based composites hold great promise as highly effective microwave absorbers, boasting strong absorption intensity and a wide absorption frequency range, given the exceptional performance of the as-fabricated PVA/NiFe2O4/(MoS2)x nanocomposites.

Design of broadband gradient resistive film metamaterial absorber based on genetic algorithm

In recent years, electromagnetic (EM) wave absorbing devices based on metamaterials have attracted widespread attention, due to their advantages such as broadband, easy preparation, and flexibility to tailor EM waves. Nevertheless, a review of the existing research reveals that the inherent sub-wavelength characteristics of metamaterials and metasurfaces impose certain constraints on their applications in low-frequency ranges. In order to achieve low detectability that takes into account both low-frequency and broadband absorbing performance, this work, presents a metamaterial absorber based on 5-layer gradient resistance film and dielectric composite structure, as shown in <xref ref-type="fig" rid="FigureAbstract">Fig. (a)</xref>. To begin with, we introduce the structural design of the initial element, and based on this, the transmission line theory and impedance matching principle are used to analyze the strong wave absorption conditions of the absorber element. In terms of the element structure optimization, the genetic algorithm is adopted to globally search for the optimal solution in the multi-variable domain, resulting in the rapid determination of metamaterial elements’ configurations and resistance parameters that meet the design goals. In the simulation, the wave absorption performance and mechanism of the designed absorbing element are also investigated in an in-depth manner. Simulation results show that the designed metamaterial absorber can achieve more than 90% EM wave absorption in a frequency range of 1.62–19.16 GHz (with a relative bandwidth of 168.8%) under normal incidence of linearly polarized plane waves, which effectively expands the absorption bandwidth to the L band and K band. In addition, the simulations for oblique incidence at different polarizations provide strong evidence for the device’s insensitivity to both polarization and angle. The radar cross section (RCS) curves obtained by the time domain (TD) simulation illustrate that the novel structure can achieve more than 10 dB RCS reduction in a frequency range of 1.7–20 GHz. In the device's performance verification process, a metamaterial absorber with 20 × 20 elements and dimensions of 1.566<inline-formula><tex-math id="M1">\begin{document}$ {\lambda }_{l} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231781_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231781_M1.png"/></alternatives></inline-formula>×1.566<inline-formula><tex-math id="M2">\begin{document}$ {\lambda }_{l} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231781_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231781_M2.png"/></alternatives></inline-formula>× 0.113<inline-formula><tex-math id="M3">\begin{document}$ {\lambda }_{l} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231781_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231781_M3.png"/></alternatives></inline-formula> is fabricated and tested by using the bow method reflectivity test system. The absorptivity curves under 5° oblique incidence of different polarizations, show that the proposed metamaterial absorber can realize more than 80% EM absorption in an entire frequency range from 2 to 18 GHz, the test results of different polarizations are basically consistent. The test results at oblique incidence (<i>θ</i> ≥ 30°) show that although the measured and simulated curves exhibit discrepancies in certain frequency bands due to human error or material dispersion characteristics, the overall experimental results are consistent with our expectations, which fully proves that the designed metamaterial absorber has potential application value in the field of low-frequency and broadband EM absorption.

In-depth investigations into symmetrical labyrinthine acoustic metamaterial with two micro-slit entries for low-frequency sound absorption.

Sound absorption below 1000 Hz has been extremely difficult through traditional barriers and absorbers, but it is required for noise control of appliances and machineries. Existing passive acoustic metamaterials attenuate low-frequency noise but with narrow bandwidths and bulky sizes. Hence, this paper proposes an acoustic metamaterial with enclosed symmetrical labyrinthine air channels and two micro-slits (configuration 1, identical slits; configuration 2, unequal length slits) at the end channels. Its theoretical model is established by acoustic impedance analysis using electro-acoustic analogy and validated numerically and experimentally. Sound absorption is found to happen as a result of impedance matching, Fabry-Perot-like labyrinthine resonances, and thermo-viscous losses in micro-slits. Parametric investigations reveal that increase in the number of channels, channel length, total height, and outer panel thickness shifts sound absorption peak to lower frequency but also decreases the magnitude and frequency range of absorption. Decreasing the channel width and slit width increases the sound absorption magnitude without changing absorption frequencies. Interestingly, unequal slit lengths perform better than equal slits by giving a lower frequency sound absorption with increased magnitude and frequency range, which is unlike that in existing labyrinthine metamaterials. Therefore, the proposed unequal slit metamaterial has enhanced low-frequency sound absorption and can be applied to appliances and machineries.

A compact low-frequency sound absorption metastructure realized by resonators with wavy bending necks

In this work, a compact low-frequency sound absorption metastructure composed of multiple resonators with embedded wavy bending necks is proposed. By arranging this metastructure in parallel and optimizing the parameters, it exhibits excellent broadband sound absorption capability in low-frequency range and has a much more compact volume. Compared with the traditional resonators, an individual resonator of this metastructure can move down the absorption frequency about 120 Hz while maintaining the same thickness. Furthermore, different resonator units are combined into a sound absorption array by employing appropriate design techniques. We first built a small metastructure composed of four units to demonstrate the correctness and accuracy of our design method. Both theoretical models and finite element simulation models are built and experimental results show good agreement between them. To achieve the same absorption value and frequency range, the thickest resonator in the traditional resonator array must be 30% thicker than the one in the wavy bending neck resonator array, which means the overall size of the structure is 30% larger. Following this design method, perfect sound absorption within the frequency range of 248 Hz–420 Hz is achieved with a compact volume of 53 mm in radius and 47 mm in height. The design strategy presents a new approach to achieve perfect broadband low-frequency sound absorption.

Результати експериментальних досліджень акустичних характеристик звукопоглинаючих конструкцій для ТРДД

The subject of the research in this article is an acoustic liner designed to reduce aircraft noise in a wide frequency range. The purpose of the study is to determine the acoustic characteristics of an acoustic liner. Tasks: development of a theoretical model of the acoustic liner in the form of tubes of different lengths, calculation of the natural frequencies of each tube of the developed liner, determination of the frequency range in which sound absorption will occur, production of a test sample, and experimental determination of the sound absorption coefficient. An experimental setup is used to determine the sound absorption coefficient of the developed liner, which determines the acoustic characteristics of the acoustic liners using the standing wave method. This study presents the results of: calculation of the natural frequencies of the proposed liner, calculation of the dependence between the amplitude transmission coefficient and the frequency of the exciting force, and measurement of the acoustic characteristics of the proposed acoustic liner. The scientific and practical novelty of the obtained results is as follows: to solve the problem of noise absorption in a wide frequency range, a theoretical model of the acoustic liner, which should consist of tubes of different lengths, was developed. Each tube has its own frequency of oscillation of the air inside the tube. As the path of sound travel inside the tube increases, the natural frequency of the tube decreases. When tubes of different lengths are placed on the test sample, it is possible to obtain an acoustic liner with a significant number of natural frequencies, that is, noise will be absorbed in a wide frequency range. According to the calculation results, the proposed acoustic liner, consisting of seven types of tubes of different lengths, has more than 20 natural frequencies in the frequency range from 500 Hz to 4500 Hz, that is, the frequency range of absorption is quite wide. In order to verify the theoretical calculations with the help of a 3D printer, experimental samples of the proposed acoustic liner, as well as a real acoustic liner used to reduce the noise of the D-18T engine, were made and their experimental studies were carried out. The research results show that the proposed acoustic liner reduces noise in a wide frequency range. In the frequency range from 1300 Hz to 3500 Hz, the sound absorption coefficient exceeds 0.4, and in the range from 1500 Hz to 2600 Hz, the sound absorption coefficient is above 0.5. In the specified frequency ranges, the proposed acoustic liner has significant advantages over the D-18T acoustic liner. By reducing the cross-section of the tubes, or increasing the number of tubes of the experimental sample, it is possible to significantly increase the frequency range of sound absorption.

Synthesis of NiCo2O4/CF composites with heterogeneous interfaces as excellent microwave absorbers

The absorbers with strong absorption capacity, wide effective absorption bandwidth (EAB), low filling, lightweight and thin thickness are the future development direction of electromagnetic wave (EMW) absorption field. NiCo2O4/CF composites were synthesized by simple hydrothermal method and calcination method in this study. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and vector network analyzer (VNA) were used to study the crystal structure, microstructure, surface chemical states and microwave absorption properties of the composites, respectively. Carbon fiber (CF) was used as nucleation carrier, and NiCo2O4 formed a dense conductive network on its surface in the shape of nanoneedles. In addition, there are abundant oxygen vacancies and heterogeneous interfaces on the surface of composite materials, which can cause dipole polarization and interface polarization and lead to more dissipation of electromagnetic wave energy. The minimum reflection loss (RLmin) of the NiCo2O4/CF composite is − 59.75 dB at the thickness of 2.01 mm. The maximum effective absorption bandwidth (RL < −10 dB) of the composite is 4.12 GHz (8.84–12.96 GHz) at the thickness of 2.18 mm. In the thickness range of 1.2–5 mm, the effective absorption frequency range of the composite material accounted for 80.78% of the measurement range (3.46–18 GHz), which covered the whole C-band, X-band and Ku-band, as well as a portion of the S-band. NiCo2O4/CF composites have the characteristics of strong absorption and wide absorption bandwidth due to the synergistic effect of dielectric loss and magnetic loss, good impedance matching characteristics and more Debye relaxation processes, which provide new inspiration for the structure and component design of lightweight microwave absorbing materials (MAMs).

Co/C composites generated from biomass exhibit outstanding electromagnetic wave absorption

The development of high-performance electromagnetic wave-absorbent materials from inexpensive biomass feedstock is currently the subject of considerable interest. In this study, we developed Co/C composites with excellent electromagnetic wave absorption properties using a simple carbonization method with mangosteen shells as a precursor. The Co/C composite has a minimum reflection loss value (RLmin) of −61.88 dB and a maximum effective absorption bandwidth of 6.32 GHz. For thicknesses ranging from 1 to 5.5 mm, the effective absorption frequency range could cover the entire C, X, and Ku bands. The magnetic loss produced by Co and the dielectric loss, dipole polarization, interfacial polarization, and multiple reflections of biomass carbon synergies enhance the impedance matching capabilities and microwave absorption performance of the composites. The results demonstrate that the activated mangosteen shell magnetic composite has a wider effective absorption bandwidth and greater absorption intensity. This work provides a good potential method for preparation of magnetic particle/biomass-derived carbon microwave absorbing structural materials.

Fabrication of one-step shape memory gradient sound absorber with wrinkled inner wall and closed-pore structure

Plastic foam sound-absorbing material is of great significance to improve indoor sound environment. However, since the preparation of foamed material is fundamentally induced to form a homogeneous pore structure, the sound absorption frequency range is relatively narrow, limiting the sound absorption performance to a great extent. In order to improve the sound absorption performance in a wide frequency range, this work designed the combination of macro-porous gradient foam (MGF) and closed-pore resonator, introduced part of polyimide (PI) components based on the one-step free foaming method of polyurethane foam (PUF), and proposed a straightforward route to prepare basic polymer gradient foam. Through the arrangement and combination of different bubble holes, the sharp hole structure and the Helmholtz resonance structure were constructed by analogy. Importantly, the internal structure of the foam system was reorganized under the action of surface tension due to the inward collapse of the viscous bubbles during the process of bursting, resulting in the formation of wrinkled structures and small bubbles, which enhanced the resonant sound absorption at lower frequencies. In 200–6400 Hz, the MGF1 sample had the highest sound absorption capacity, with a cavity size range of 8.27–4.14 mm and the ability to absorb 90% of the acoustic wave above 1100 Hz. In addition, MGF samples also exhibited heat insulation behavior (0.047–0.061 W/(m·K)), shape memory, and energy absorption capacity. This gradient compound resonance structure satisfies the convenience of process manufacturing and high-efficiency sound absorption performance, which is essential for the wide application of plastic foam sound-absorbing materials.

Characterization and Optimization of the Angle-Dependent Acoustic Absorption of 2D Infinite Periodic Surfaces of Helmholtz Resonators

Acoustic metamaterials have emerged as alternative solutions to achieve useful physical effects that differ from the ones obtained with traditional materials. In terms of sound absorption, previous works have addressed their potential as compact surfaces with high performance. Nevertheless, studies on their angle-dependent behavior are scarce. In this work, an analytic model and a numerical model to estimate the performance of periodic surfaces with unit cells composed of 2D Helmholtz resonators are presented. By making use of these modeling tools, the absorption of surfaces with one and three different resonators is studied as a function of both incidence angle and frequency. Changes in the incidence angle can cause variation of the maximum absorption coefficient, the frequencies at which the maximum performance is observed, and the frequency range of significant absorption. Furthermore, the rate at which the performance changes as a function of the incidence angle is larger as the angle increases. Given the angle dependency of these absorbers, a strategy to optimize the dimensions of the surface elements to maximize the absorption performance for predefined ranges of incidence angles and frequencies is presented.

INFRARED SPECTROSCOPIC ANALYSIS OF COMPLEX OXIDE CATALYST 50%Cu3(PO4)2•50%Ni3(PO4)2

It is established that the frequencies and intensities of the absorption bands of the infrared spectra of the obtained air-dried sample 50%Cu3(PO4)2∙50%Ni3(PO4)2 and calcined at the optimum heat treatment temperature of 600оС are characterized by significant absorption in the valence (3460-3020 cm-1) and strain (2360-1400 сm-1) vibrations of hydroxyl OH-groups of water molecules. Valence vibrations of phosphate ion are most manifested in dehydration products in the form of clear bands in the absorption frequency range 1110-410 сm-1. For the investigated sample, three absorption bands were observed in the region: 3188,72 сm-1, 1621,84 сm-1 та 1434,78 сm-1. They confirm the existence of two types of crystalline hydrate water molecules, which are characterized by different hydrogen bonding energies. In the process of heat treatment of the synthesized sample in the temperature range of 120-700оС , the process of dehydration occurs. In this case, the intensity of the absorption bands in the region of valence and strain vibrations of water molecules gradually decreases in the obtained IR spectra. The phenomenon of complication of the spectrum of orthophosphate ion by additional absorption bands is obviously associated with the distortion of the tetrahedron structure in real crystals and the reduction of its symmetry. It can be a consequence of both low symmetry of the environment and perturbation of the orthophosphate anion РО43– by water molecules when intermolecular hydrogen bonds occur. It may also be due to possible additional coordination of orthophosphate anion with metal cations Сu2+ and Ni2+. Calcination of the sample at the final heat treatment temperature of 700оС leads to an anhydrous orthophosphate system. This will lead to a decrease in the value of surface acidity and catalytic inertness of the sample.&#x0D; Keywords: catalysts; heterogeneous catalysis; phosphates; complex oxides; oxidation; n-alkanes; hydrocarbons; ethane; ethylene; IR spectroscopy.

Experimental study of the elastic enhancement factor in a three-dimensional wave-chaotic microwave resonator exhibiting strongly overlapping resonances.

We study the elastic enhancement factor and the two-point correlation function of the scattering matrix obtained from measurements of reflection and transmission spectra of a three-dimensional (3D) wave-chaotic microwave cavity in regions of moderate and large absorption. They are used to identify the degree of chaoticity of the system in the presence of strongly overlapping resonances, where other measures such as short- and long-range level correlations cannot be applied. The average value of the experimentally determined elastic enhancement factor for two scattering channels agrees well with random-matrix theory predictions for quantum chaotic systems, thus corroborating that the 3D microwave cavity exhibits the features of a fully chaotic system with preserved time-reversal invariance. To confirm this finding we analyzed spectral properties in the frequency range of lowest achievable absorption using missing-level statistics.

Enhanced electromagnetic wave absorption performance of ZnO/N-doped carbon microspheres in 2–18 GHz

The properties of popular absorbing materials include low thickness, lightweight, a wide frequency range of absorption, and a significant absorption capacity. However, synthesizing a material that contains all these properties remain a considerable challenge. In this manuscript, Zn, N, O-doped microspheres precursors were innovatively synthesized using the one-pot method, which was efficient and simple. Zn, N, O-doped carbon composites (ZnO/N-CNs) were synthesized by heating precursors in an Ar atmosphere. The electromagnetic wave absorption (EMWA) performances for the composites were modified by altering the calcination temperature of the precursors. As a result, the performance of the ZnO/N-CNs calcined at 700 °C (ZnO/N-CNs-700) in electromagnetic wave absorption (EMWA) was outstanding. The effective absorption bandwidth (EAB) attained 5.68 GHz (12.32–18 GHz, 2.08 mm), while the minimum reflection loss (RLmin) achieved −44.6 dB (12.24 GHz, 2.5 mm). The synergistic interaction of the porous structure, ZnO/C heterostructure, ZnO particles, carbon, and nitrogen was responsible for the superior EMWA properties of ZnO/N-CNs composites. Therefore, the composites exhibited improved interface and dipole polarization loss, conduction loss, multiple reflection and scattering, and impedance matching. This technique made it possible to mass-produce brand-new N-doped carbon-based composites, which had effective EMWA performance. This article suggested a practical method for creating dielectric wave-absorbing composites with excellent all-around performances.

Contribution of Side Surfaces to the Effectiveness of Automotive Sound-Absorbing Materials

When developing noise-reducing structures for passenger, engine and luggage compartments of motor vehicles, an objective assessment of their sound-absorbing properties is important. The method of measuring the reverberation time of a diffuse sound has found the widest distribution. This method provides sufficient reliability and repeatability of measurement results. However, often the values of reverberation sound absorption coefficient are exceeded relative to the actual values. The article investigates the influence of side surfaces of sound-absorbing materials on equivalent sound absorption. The frequency range of effective absorption by side surfaces is indicated. An updated formula for approximate calculations is given.

Facilitating enhanced microwave absorption properties of barium hexaferrite/polyaniline composites based on tunable interfacial polarization by rare earth doping

Barium hexaferrite (BaM)/polyaniline (PANI) is a typical microwave absorbing composite that effectively integrates magnetic and electric losses. To enhance its microwave absorption properties, a facile method based on rare earth element doping was explored; the effects of rare earth elements on the microstructures and properties of both BaM nanopowders and the subsequent BaM/PANI composites were investigated. The use of rare earth elements, including La, Nd, and Ce, in the BaM particles alleviated the constraints of the electric polarization in PANI near the surfaces of the BaM particles; this resulted in more electric polarization at the interfaces of the Re-BaM and PANI phases. This provided a versatile way to enhance the absorption efficiency, expand the absorption frequency range, and reduce the thickness. The La-BaM/PANI composite demonstrated a maximum reflection loss of approximately − 63.6 dB at 7.76 GHz with an optimum thickness of 3.28 mm. The effective absorption range of the Nd-BaM/PANI composite was 4.72 GHz. Ce-BaM/PANI composites released a relatively broad effective frequency range of approximately 3.88 GHz and a small thickness of 1.58 mm.

Multiple resonances in lossy acoustic black holes - theory and experiment

Acoustic properties of the metamaterial graded absorber, also known as ``acoustic black hole'', are studied in the linear regime. The absorber consists of thin metallic circular plates, each with a central perforation, separated by annular air cavities. Radius of the perforation in each plate is gradually decreasing with the distance from the plate to the front surface, forming a central channel with a staircase radius profile. A semi-analytical equivalent fluid model accounting for the variations of both the effective density and compressibility of air inside this channel is developed, which incorporates the staircase variations of the perforation radius with distance and assumes motionless plates. The viscous and thermal losses inside the side cavities and the central channel are accounted for using a well-established Johnson-Champoux-Allard-Lafarge model. It is demonstrated that high absorption coefficient values are achieved in a wide range of frequencies starting from a few hundred Hz or less. At low frequencies, the resonances along the length of the structure, i.e. global resonances, are responsible for sound attenuation. At higher frequencies, the resonances of the lateral cavities, i.e. local resonances, play a major role. The upper boundary of the frequency range of high sound absorption is determined by the resonance frequency of the front annular plate. The model is validated against impedance tube measurements on five samples of different geometry and FEM models. FEM model predicts that the elasticity of the plates affects the absorption coefficient behaviour in the frequency range of plate resonances and at frequencies well below the plate resonances.

Enhancing sound absorption for an acoustic metastructure with extended tubes at ultra-low frequency

A micro-perforated plate (MPP) as a typical acoustic absorber has ideal sound absorption performance at mid-frequency. A novel metastructure with a MPP and multiple sub-cavities (MPP-MSC) is proposed to broaden the sound absorption frequency range based on Helmholtz resonance, and its sound absorption performance is studied theoretically, numerically, and experimentally. Then, the genetic algorithm is adopted to extend the effective sound absorption band by 120 Hz and reduce the structure thickness to less than 61 mm, which is about 40% thinner than the existing structures. Additionally, a perforated plate with extended tubes (PPET) and porous sound absorptive material (PSAM) are introduced into the metastructure to enhance sound absorption performance in the ultra-low and mid-frequency range. Therefore, the composite sound absorber (PP-MPP-MSC) composed of PPET, PSAM, and MPP with superior sound absorption performance is proposed. Specifically, the effective acoustic absorption frequency of the structure can be broadened to 200–1800 Hz with a maximum thickness of 61 mm. It is worth noting that the design and fabrication of PP-MPP-MSC are relatively accessible, and effective sound absorption performance can be achieved in a certain frequency range, which presents the guiding role for current noise reduction applications.