Low-frequency Absorption Research Articles

Electromagnetic characteristics and 3D-printing realization of a lightweight hierarchical wave-absorbing metastructure for low-frequency broadband absorption

Electromagnetic absorbing metastructure with low frequency and broadband absorbing capacity is desirable for scientific community and real-life application. Here, we designed a lightweight hierarchical wave-absorbing metastructure by replacing the facets of the stepped structure with smaller-scale square grid. The electromagnetic characteristics of high-efficiency absorption in low and high frequency ranges were also studied. The specific dielectric constant and dielectric loss were found to be the key factors of low-frequency absorption, which was realized by regulating the proportion of Ag-Cu alloy-based composite material based on fused deposition modeling 3D printing technology. The hierarchical gradient design for the wave-absorbing metastructure is helpful to not only being lightweight but also enhancing the electromagnetic wave absorbing capacity. The existing heterogeneous interfaces of flaky alloy particles and spheric polymer particles also enhance the dielectric properties of composite material. The fabricated wave-absorbing metastructure sample with the bulk density of 0.5 g/cm3 shows high-efficiency absorption in 2.5–40 GHz, and the absorbing performance keeps stable even though the incident angle varied in the range of 50°. This paper provides a convenient realization method of absorbing metastructure with low frequency broadband absorption performance, which has outstanding practical application value.

Ventilation duct silencer design for broad low-frequency sound absorption

Low frequency sound absorption in ventilation duct is an important technical issue. In this study, silencer consisting of decoupled annular Helmholtz resonators (AHR) is designed to achieve low frequency sound absorption. The proposed silencer can achieve broadband sound absorption whilst avoiding hindering ventilation. Theoretical model that based on the temporal coupled mode theory (TCMT) is derived to reveal the working mechanism. To improve feasibility of the designed silencer and to reduce the required space for installation, arc-shaped Helmholtz resonator (ASHR) is then proposed as optimisation. Such structure allows the silencer to produce multiple peak absorption frequencies in a smaller dimension. The sound absorption performance of both the structures is investigated numerically and experimentally. For the AHR silencer, measured results indicate a >95% sound absorption coefficient from 690 Hz to 1350 Hz. The ASHR silencer can generate four effective peak absorption peaks over the range 570–1720 Hz. In addition, both structures are installed as the side-branches of the duct, so ventilation condition is completely not affected. The usage of porous materials prevents design process from repeated geometrical parameter tuning and enables continuous operational frequencies. The proposed designs are experimentally verified to possess good feasibility for practical application.

The acoustic performances of a subwavelength hierarchical honeycomb structure: Analytical, numerical, and experimental investigations.

This paper proposes a subwavelength hierarchical honeycomb structure (SHHS) with a compact lateral dimension and double-band perfect absorption in low frequencies. Unlike the conventional micro-perforated panel (MPP)-honeycomb sandwich absorber, this structure has an additional internal honeycomb with a perforated wall. Therefore, there are two resonant cavities in the SHHS to realize multiple absorption peaks. Analytical, numerical, and experimental investigations are performed to study the proposed system's acoustic performance in absorption. The SSHS is simplified into four parts and its analytical model is constructed by combining various analytical models by acoustic-electro analogy. The analytical model is presented to explore the physical properties of sound absorption and the influence of parameters, which has been validated by comparisons with the numerical model, and the experimental data is measured by an impedance tube. It is found that the main incident energy is lost by the inside hole, which is different from the conventional absorbers with surface MPP. Moreover, the side length of the internal honeycomb can adjust the resonant frequencies to achieve an absorber with the subwavelength. A SSHS is designed with a perfect absorption at 320 Hz whose thickness is 1/31 of the resonant frequency wavelength. The SHHS has excellent potential for noise control engineering applications.

A resonator installed in a wooden puzzle board greatly enhances sound absorption capability at low frequency: A new approach

Loud noise has become a regular part of life due to growing urbanization and lifestyle changes, making noise pollution a severe health concern. However, eco-friendly materials for low-frequency sound absorption still need improvement. Wooden puzzle-assembled boards are being used to construct wooden buildings in South Korea, but they require better sound absorption performance. Therefore, we have developed a strategy to enhance the sound absorption capability of wooden puzzle-assembled boards by suggesting the installation of a single-frequency resonator on the board. The resonator contains nine pinholes with a 6 mm diameter perforation connected to a 30 mm diameter hole leading to the center hollowed cavity (CHC). We evaluated the sound absorption coefficient (SAC) of the control (i.e., cross-laminated timber (CLT)) and a resonator board. The SAC of the resonator with zero pinholes (i.e., CHC), one pinhole (PH-1), five pinholes (PH-5), and nine pinholes (PH-9) were estimated using a two-microphone transfer function method at low frequencies ranging from 100 to 1600 Hz. The average SAC of PH-9 showed significant improvement at the frequency of 450 Hz (0.637 ± 0.001; 818%) compared to the control samples. The noise reduction coefficient was improved by 239% (0.07 for the control and 0.23 for the PH-9). We compared the experimental results in this study with theoretical results and found that the orientation of the neck length significantly impacted the resonant frequency of the pinhole resonator. This finding provides valuable insights into the design and optimization of pinhole resonators for specific acoustic applications.

Low-frequency sound-absorbing metasurface constructed by a membrane-covered and coiled Helmholtz resonator

Achieving broadband absorption of sound waves below 500 Hz with materials of sub-wavelength thickness is significant but still a great challenge in academia and industries. Here, we present and theoretically analyze an airtight sound-absorbing metasurface constructed by a membrane-covered and coiled Helmholtz resonator. It is discovered that the metasurface possesses a near-perfect absorption with a working wavelength approximately 33.6 times greater than the total thickness, which stems from synthetic modulation on acoustic reactance brought by the membrane, air gap formed behind the membrane, and a coiled channel. Furthermore, on-demand broadband absorption below 500 Hz is achieved by parallel assemblies consisting of four subunits. An excellent agreement between measurements and predictions confirms the validity of the proposed structures. The airtight construction also broadens its application scenarios compared to the common perforated absorbers with open pores directly exposed to external environments. Our design provides a new structure paradigm for low-frequency sound absorption.

Study on the Low-Frequency and Broadband Sound Absorption Performance of an Underwater Anechoic Layer with Novel Design

To further improve the low-frequency broadband sound absorption capability of the underwater anechoic layer (UAL) on the surface of marine equipment, a novel sound absorption structure with cavities (NSSC) is designed by adding resonators and honeycombs to the traditional sound absorption structure with cavities (SSC). Based on the principle of shear dissipation, the original intention of the design is to allow more parts of the viscoelastic material to participate the dissipation of acoustic energy. The approximate multilayer sound absorption theoretical model based on the modified transfer matrix method is used to verify the accuracy of finite element calculations. In the frequency range of 1100 Hz–10,000 Hz, the sound absorption coefficient (α) of NSSC can reach 0.8. The effects of the presence and size of cylindrical oscillators and honeycomb structures on sound absorption are discussed in detail. The results show that expanding the effective sound absorption range of the damping area of the structure is the key to improve the wideband sound absorption effect. This design concept could guide the structural design of the UAL.

Broadband and low-frequency sound absorption by a slit-perforated multi-layered porous metamaterial

A novel slit-perforated multi-layered porous metamaterial (SMPM) is proposed in this paper, which exhibits a broadband and low-frequency sound absorption characteristic. The proposed SMPM is composed of periodic porous matrix layers and periodically distributed adjacent second-type porous layers containing slits. The theoretical model is established by applying the double porosity (DP) theory for the slit and the porous layer which form an equivalent inclusion layer, and then the homogenization method is used for the unit-cell of the SMPM composed of the equivalent inclusion layer embedded in the porous matrix layer. The theoretical results are validated by the numerical results obtained by the finite element (FE) software COMSOL Multiphysics, and a broadband and low-frequency sound absorption performance of the SMPM is achieved. Furthermore, the acoustic impedance of the proposed SMPM, the time-averaged power dissipation density and the acoustic energy dissipation ratio of the two constituent porous materials are investigated in detail. The effects of the slit-width and the material composition ratio on the sound absorption performance are also explored. The results demonstrate that the introduction of the periodic slits can promote the acoustic wave propagation in the SMPM to enhance the sound absorption especially at low frequencies, and the porous matrix layers and the second-type porous layers play different roles in the sound energy dissipation process. To be more specific, the porous material with a lower air-flow resistivity contributes more to the energy dissipation at low frequencies while that with a higher air-flow resistivity becomes more involved in the sound energy dissipation process at middle to high frequencies.

Hybrid acoustic materials through assembly of tubes and microchannels: design and experimental investigation

PurposeThe purpose of this paper is the design and experimental investigation of compact hybrid sound-absorbing materials presenting low-frequency and broadband sound absorption.Design/methodology/approachThe hybrid materials combine microchannels and helical tubes. Microchannels provide broadband sound absorption in the middle frequency range. Helical tubes provide low-frequency absorption. Optimal configurations of microchannels are used and analytical equations are developed to guide the design of the helical tubes. Nine hybrid materials with 30 mm thickness are produced via additive manufacturing. They are combinations of one-, two- and four-layer microchannels and helical tubes with 110, 151 and 250 mm length. The sound absorption coefficient of the hybrid materials is measured using an impedance tube.FindingsThe type of microchannels (i.e. one, two or four layers), the number of rotations and the number of tubes are key parameters affecting the acoustic performance. For instance, in the 500 Hz octave band (α500), sound absorption of a 30 mm thick hybrid material can reach 0.52 which is 5.7 times higher than the α500 of a typical periodic porous material with the same thickness. Moreover, the broadband sound absorption for mid-frequencies is reasonably high with and α1000 > 0.7. The ratio of first absorption peak wavelength to structure thickness λ/T can reach 17, which is characteristic of deep-subwavelength behaviour.Originality/valueThe concept and experimental validation of a compact hybrid material combining a periodic porous structure such as microchannels and long helical tubes are original. The ability to increase low-frequency sound absorption at constant depth is an asset for applications where volume and weight are constraints.

Bandwidth widening for sound absorption of a flexible microperforated panel backed by a multi-frequency panel-type resonator

A microperforated panel (MPP) often suffers from a limited absorption bandwidth and poor sound absorption in the low frequency range. The present study aimed to propose an MPP-panel-type resonator (PR) compound structure that can simultaneously widen the half-absorption bandwidth and improve the poor absorption at the low frequency range. A vibroacoustic model is developed and compared to finite element simulations to demonstrate its accuracy. The vibration characteristics of the compound structure and the surface impedance are analyzed to reveal the mechanism forming the absorption characteristics of an MPP-PR structure. It is found that backing an MPP with a panel-type resonator (PR) creates a low frequency absorption peak but followed by an absorption valley that decreases the absorption bandwidth. The analysis showed that the phase difference between velocity of air particles in perforation and the PR panel velocity dictates the absorption characteristics associated with the PR resonance. A multi-resonators panel-type resonator (MPR) is found that the absorption valley associated with the host panel resonance splits into multiple smaller amplitude absorption dips improving absorption performance. The proposed MPP-MPR compound structure demonstrated a relatively wider half-absorption bandwidth with improved low frequency absorption.

Deep subwavelength hybrid metamaterial for low-frequency underwater sound absorption by quasi-Helmholtz resonance

We proposed an acoustic metamaterial with deep subwavelength thickness for low-frequency underwater sound absorption. The proposed hybrid metamaterial has a perforated facesheet, a fluid-filled square honeycomb core with inside rubber coating, and a fixed backsheet. A theoretical model is established to predict the sound absorption performance of this perforated honeycomb hybrid metamaterial based on the sound absorption theory of the micro-perforated panel and electro-acoustic analogy. The theoretical model agrees well with our finite element simulation. Results suggest that perfect sound absorption (99.9%) of the metamaterial occurs at 375 Hz, at which the thickness of the metamaterial is only 1/80 of the underwater sound wavelength. According to the simulation, most of the sound energy is consumed by the rubber coating. It can be analyzed that the rubber coating replaces the fluid in the square honeycomb resonant cavity improving the acoustic capacitance and acoustic resistance and triggering a quasi-Helmholtz resonance. This acoustic metamaterial also exhibits a broadband underwater sound absorption performance by parallel design with different perforations, which has a promising potential in engineering applications.

Absorption characteristics analysis of membrane sound absorber with magnet

This paper presents a membrane sound absorber (MSA) with tunable properties for low-frequency sound absorption. A simple method to predict the resonance frequency is proposed; the resonate frequencies predicted show good agreements with the experimental ones. The analysis indicate that the iron-platelet-magnet resonance mechanism introduced by the tuned magnetic field is the main factor leads to the appearance and shift of absorption peaks in low-frequency region, which are almost independent of the back cavity. This demonstrates that the structure has strong potential to achieve an extremely thin and low-frequency tunable-sound-absorption design which can be easily adapted to the noise source variation.

Design of Miniaturized Frequency-Selective Rasorber With Embedded Dual-Bow Resonators

This letter proposed a miniaturized frequency-selective rasorber (FSR). This FSR consists of a conducting sheet layer with embedded dual-bow resonators and a bandpass frequency-selective surface layer with a cross-ring slot. The structure exhibits absorption–transmission–absorption (A-T-A) characteristics that shows excellent transmission performance in 2.4 GHz band. The unit size of structure is 0.113 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{L}$</tex-math></inline-formula> and the thickness is 0.071 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{L}$</tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{L}$</tex-math></inline-formula> is the lowest frequency of the absorption band satisfying 80% absorption rate. The simulation results show that the center frequency of the transmission band is located at 2.42 GHz with 0.24 dB insertion loss. Under the condition of 80% absorption rate, the absorption bandwidth is 58.82% for the low-frequency absorption band, and 58.6% for the high-frequency absorption band. The concept of slope in mathematics is introduced into FSR for the first time to define steep drop characteristics. Finally, in order to verify the correctness of the analysis results, a prototype of FSR with 20×20 units is processed and measured, and the measured results are in good agreement with the simulation results.

Hollow structured (Ni/C)/ZnFe2O4 composite with enhanced low-frequency microwave absorption performance

To enhance the low-frequency (2–10 GHz) microwave absorption performance of hollow magnetic metal–carbon composites, hollow (Ni/C)/ZnFe2O4 composite was prepared by introducing submicron ZnFe2O4 into hollow Ni/C microspheres. With the increased addition of ZnFe2O4, the low-frequency microwave absorption performance of the composites was gradually enhanced. When the addition of ZnFe2O4 was 12 wt%, the minimum reflection loss (RL) of −38.64 dB could be obtained at 5.33 GHz and the effective absorption bandwidth was 3.45 GHz. As the ZnFe2O4 addition increased to 18 wt%, the minimum RL reached −55.39 dB at 4.26 GHz and the effective absorption bandwidth was 3.82 GHz. The enhanced low-frequency microwave absorption performance can be attributed to the introduction of ZnFe2O4 improved attenuation coefficient and optimized impedance matching of the (Ni/C)/ZnFe2O4 composite. This work provides an effective idea to improve the low-frequency microwave absorption performance of hollow magnetic metal/carbon composites, and the prepared (Ni/C)/ZnFe2O4 composite can be used for microwave absorption in the low-frequency band.

Acoustic transmission characteristics based on coiled-up space metamaterials

Coiled-up metamaterials with solid plate (SC) and with micro-perforated plate (MPC) are proposed for broadband low-frequency acoustic insulation and absorption. Firstly, the sound transmission theory of the double-layer plate and the impendence of the micro-perforated plate coupled with the coiled-up channel are introduced to obtain the dependence of structural parameters on the acoustic characteristics. Then, the acoustic performances of SC and MPC are investigated numerically and verified by experiments, and the effects of geometry parameters are analysed to predict the acoustic performance of SC and MPC. The minimum sound transmission loss (STL) of SC with a thickness of less than 2 cm is higher than 20 dB and the average STL reaches 45 dB at [0, 1600] Hz. The multi-order resonances of coiled-up space induce STL valleys but provide 10 dB improvement at valleys compared with double-layer plates. The MPC achieves narrow absorption at 422 Hz and 1232 Hz, which are induced by the resonances of coiled-up channels and 99.99 % of total energy dissipation is viscous dissipation mainly occurring in MPP. Coiled-up space with embedded porous materials (PTM) achieves above 0.9 absorption coefficient at [700, 1600] Hz. The optimized MPTMs, which consists of three parallel MPCs guided by the genetic algorithm (GA) and a single PTM unit, can reach a normal incident average absorption coefficient of 0.80 at [342–2000] Hz and a random incident absorption coefficient above 0.5 at [352–1862] Hz.

A low-frequency wideband ventilation muffler based on an embedded rough-necked Helmholtz resonator

Aiming at the unsatisfactory low-frequency sound absorption effect of Helmholtz resonator, a novel broadband low-frequency ventilation absorber with rough neck is proposed. The roughness is introduced into the neck of Helmholtz resonator to change the shape of the neck and achieve the structure of rough neck Helmholtz resonator. The proposed absorber can effectively provide the acoustic impedance required for low-frequency sound absorption without changing the overall size, thereby reducing the resonant frequency. The finite element method is used to simulate the structure, and the impedance tube sound absorption test is carried out to verify it. The experimental and simulation results show high consistency with each other. The results also indicate that the rough neck Helmholtz resonator absorber with roughness introduced in the neck achieves an absorption peak at 58 Hz, with an absorption coefficient of about 0.63. Comparing with the absorber without roughness introduced, the resonant peak frequency becomes low, from 70 Hz to 58 Hz, reducing 17.1%. Therefore, adjusting the neck roughness can serve as a method of tuning the acoustic performance, and the absorption peak frequency can be adjusted by appropriately increasing the neck roughness so as to move it in the low frequency direction. Based on the verification that the roughness of the neck can effectively reduce the absorption peak frequency of Helmholtz resonator, a broadband low-frequency ventilation absorber with a rough neck, which is composed of eight absorption units, is designed. Through simulation calculation and experimental exploration, the absorption coefficient can achieve more than 0.8 in a target working frequency band of 500－1100 Hz. On this basis, the acoustic impedance of the structure can be adjusted by introducing roughness into the neck of Helmholtz resonator, so as to obtain the optimized broadband low-frequency ventilation absorber with a rough neck, which achieves a broadband sound absorption coefficient higher than 0.8 in a frequency range of 400–1200 Hz. The optimized structure also has 8 consecutive absorption peaks with amplitudes above 0.95. The proposed low-frequency broadband ventilation absorber provides a reference for designing and optimizing efficient low-frequency subwavelength acoustic absorbers. It has a wide range of applications in pipeline noise control.

Tunable composite lattice structure for low-frequency and ultra-broadband underwater sound absorption.

The underwater sound absorption technique in low-frequency and broadband has far-reaching prospects since it is essential for noise reduction of deep-sea operation requirements and evading advanced underwater target detection. Here, we propose an underwater sound-absorbing composite lattice with low-frequency and ultra-broadband characteristics. The composite lattice is constructed by regular spatially stacking cells with different sizes of metallic core spheres. All the core spheres are coated with silicon rubbers, and cells are embedded in the rubber matrix. In the composite lattice stereostructure, the lattice cells convert incident longitudinal waves into transverse waves through multiple local resonance coupling and multiple scattering. The energy is localized and dissipated in the composite lattice. We analyze the relationship among the corresponding absorption spectrums, the displacement clouds, and the resonance modes of lattice cells. Then, we construct a composite lattice and realize low-frequency broadband absorption from 693 to 1106 Hz with absorptance above 0.8. Further, our investigation demonstrates that the absorption bandwidth can be extended to ultra-broadband from 1077 to 10 000 Hz, where the thickness of the composite lattice is λ/17.05. The proposed composite lattice provides a practical approach to designing ultrathin low-frequency and ultra-broadband acoustic absorption coating for underwater noise suppression.

A lightweight metastructure for simultaneous low-frequency broadband sound absorption and vibration isolation.

We theoretically, numerically, and experimentally study a lightweight metastructure that can simultaneously reduce vibration and noise in a broad low-frequency range. We introduce spiral slits and micro-perforations in the panel and core plate of a face-centered cubic sandwich structure, respectively. A bottom-up acoustic impedance theory is developed to describe the impedance of a single unit cell. Broadband low-frequency sound absorption is achieved for a 3 × 3 supercell via reinforcement learning optimization. The resonant coupling of the upper spiral panel and the lower panel of the unit can form a wide hybridized bandgap for flexural waves, which is further validated for vibration isolation with a one-dimensional supercell. The proposed multifunctional metastructure provides a new route to design lightweight load-bearing structures with noise and vibration reduction performance for potential applications such as aerospace engineering and transportation vehicles, among others.

Establishing a unified paradigm of microwave absorption inspired by the merging of traditional microwave absorbing materials and metamaterials.

The merging of traditional microwave absorbing materials with metamaterials holds significant potential for enhancing microwave absorber performance. To unlock this potential, a unified paradigm is urgently required. We have successfully established such a paradigm, focusing on regulating effective electromagnetic parameters and interfacial forms across microscopic, mesoscopic, and macroscopic scales. Building upon this foundation, we introduce an active co-design methodology for jointly optimizing full-scale structures and the concept of "full-scale microwave absorbers" (FSMAs). Under this guidance, performance improvements can be achieved efficiently, leading to crucial breakthroughs. For demonstration, we present a case study designing ultra-thin miniaturized FSMAs capable of ultra-broadband and low-frequency absorption. Simulation results show absorptivity exceeding 90% in the 2-28 GHz range, with absorptivity surpassing 85% and 74% in the 1.5-2 GHz and 1-1.5 GHz ranges, respectively. Additionally, the total thickness and macro period are only 5 mm, roughly equivalent to 0.033 wavelengths of the lowest operating frequency. Most importantly, we have broken the Rozanov limit, with experimental results further validating this design. This work significantly enhances our understanding of microwave absorption and offers a shortcut for pursuing improved performances and breakthroughs.

Hierarchically porous networks structure based on flexible SiO2 nanofibrous aerogel with excellent low frequency noise absorption

Environmental noise has been regarded as major noise pollution with severe hazards to human physical and mental health. The common commercial fiber sound-absorption materials have insufficient low frequencies wave absorbing and fire-resistant ability, limiting their wide application. To solve these problems, a novel strategy combining flexible nanofibers and rGO/MXene nanosheets was proposed to fabricate rGO/MXene/SiO2 nanofibers composite aerogel with hierarchically porous structure, which possessed an extremely low density of 9.8 mg/cm3 and superior low-frequency sound absorption ability (NRC value of 0.51). The obtained composite aerogel possessed a large deformation up to 80% (corresponding compressive stress of 17 kPa) and quickly recovered. In addition, the as-prepared aerogel could be easily produced on a large scale, providing a reference for the development of new generation of sound-absorbing products.

A dual ultra-broadband switchable high-performance terahertz absorber based on hybrid graphene and vanadium dioxide.

A tunable dual broadband switchable terahertz absorber based on vanadium dioxide and graphene is proposed. The tunability of graphene and the phase transition properties of vanadium dioxide are used to switch broadband absorption between low-frequency and high-frequency, as well as the absorption rate tuning function. The simulation results indicate that when vanadium dioxide is in the insulating phase and the graphene Fermi energy is 0.7 eV, the absorber achieves low-frequency broadband absorption within the range of 2.6-4.2 THz with an absorptance greater than 90%; when vanadium dioxide is in the metallic phase and the graphene Fermi energy is 0 eV, the absorber achieves high-frequency broadband absorption within the range of 4.9-10 THz with an absorptance greater than 90%. Furthermore, the absorptance can be tuned by adjusting the conductivity of vanadium dioxide or the Fermi energy of graphene. Due to the central symmetry of the proposed structure, the absorber is completely insensitive to polarization. For TE and TM polarized waves, both low and high-frequency broadband absorption are maintained over a range of incident angles from 0° to 50°. The simple structure, tunable absorption rate, insensitivity to polarization angle and incident angle properties are advantages of our proposed absorber. It has broad application prospects in adjustable filters and electromagnetic shielding.