Low-frequency Sound Absorption Research Articles

Ultrathin and pressure-resistant meta-coating with embedded tree-shaped acoustic black hole for broadbnd low-frequency underwater sound absorption

Acoustic coating is the main technical means to absorb incoming sound from detection sonars. Considering the advance of sonar technology to cope with low frequencies and the underwater working environment, acoustic coatings are required to provide effective broadband low-frequency sound absorption (SA) under hydrostatic pressure. However, current underwater acoustic coating materials have poor low-frequency broadband SA performance, especially under hydrostatic pressure. Herein, we propose an ultrathin meta-coating with tree-shaped acoustic black hole (ABH) units embedded in an elastomer matrix. Each tree-shaped ABH unit consists of multi-scale ABH plates as scatterers, a center-support column as “trunk” and a frame as “skeleton”. This design combines the rich dynamics of multi-scale ABH plates, the pressure-resistance characteristics of the frame and the column, and the damping of the matrix, to collectively lead to superior performance. Capitalizing on the rich local and coupled large-amplitude local resonances of the unit, and strong resistance to static-pressure-induced deformation, high and quasi-perfect SA is achieved in multiple and ultra-wide low-frequency bands at a deep subwavelength scale under different hydrostatic pressures. Notably, quasi-perfect SA (>0.92) is achieved at 500 Hz with a meta-coating whose thickness is 2 % of the sound wavelength. The quasi-perfect SA is also demonstrated experimentally within 1200–7500 Hz, which persists till 1.0 MPa. The SA is still above 0.6 from 3–10 kHz at 4.5 MPa. For SA model under hydrostatic pressures, hyperelastic constitutive model and pressurized dynamic mechanical parameters of the matrix were established. Numerical results are then verified by experimental tests under different hydrostatic pressures. With both high local-modal densities and hydrostatic-pressure resistance, this design represents a new type of acoustic metamaterials and promises broad underwater applications.

A compact acoustic metamaterial based on Helmholtz resonators with side slits for low-frequency sound absorption

The advancement of acoustic metamaterials enables the highly efficient absorption of low-frequency noise with a subwavelength structure thickness, but the complexity of these structures often hinders their large-scale practical applications. Here, we propose a straightforward and compact acoustic metamaterial structure composed of Helmholtz resonators with side slits (HRSS) for low-frequency noise absorption. The introduction of side slits not only simplifies the overall structure but also allows for easy adjustment of acoustic characteristics. By adjusting the depth of the resonator within the slit across 25 distinct units, an absorption coefficient above 0.8 is realized from 470 to 930 Hz. This work demonstrates the extensive low-frequency sound absorption capability of HRSS, providing valuable insights into the design of future practical acoustic materials.

Design and Optimization of One Special Microperforated Plate-Labyrinth Coupled Structure (MPPS-LS): From the View of Numerical Simulation

Controlling large-wavelength acoustic waves via small-size structures is an important direction for the field of noise control. Since porous acoustic material can only absorb low-frequency sound waves when their thickness is approximately one-fourth of the wavelength, such acoustic material is not effective when controlling the large-wavelength acoustic waves with small-size structures. Therefore, the resonant acoustic material has been usually chosen to absorb low-frequency sound waves instead. As a typical resonant acoustic material, microperforated plate structures (MPPS), need one large cavity with low space utilization to absorb the low-frequency acoustic waves. The microperforated plate-labyrinth coupled structure (MPPS-LS) has been proposed with the goal of improving low-frequency sound absorption ability, which is consisted of a MPPS and three separated second-order LS coupled together. The mathematical model has been established using the acousto-electric analogy method and transfer matrix method to examine the impact of the structural parameters of each part on the acoustic characteristics. According to this study, the coupled structure is compact in size and has a good low and medium-frequency acoustic absorption effect, combining low and medium-frequency acoustic absorption characteristics of both the MPPS and LS. The sound absorption of the structure after optimized design is testing using impedance tube.

Явные решения в проблеме звукоизоляции с использованием многослойных структур при нормальном прохождении волн

The problem of sound isolation at ultra-low frequencies (30–200 Hz) by using multilayered structures is con-sidered, taking into account the normal wave propagation and an arbitrary number of layers. It is assumed that the multilayered structure consists of parallel acoustic layers, connected to each other along vertical boundary lines; acoustic half-spaces (air) are located to the left and right of the structure. For the problem, we define the acoustic parameters of the structure. The wave equations of the harmonic process are described for the left and right half-spaces of the structure, using the Helmholtz equation and a system of linear algebraic equations with given boundary conditions. Besides, explicit analytical solutions are obtained to find the transmission coefficient (transparency coefficient), with an arbitrary number of layers. Examples of solution for low-frequency sound absorption for building structures with different numbers of layers of alternating building materials and air spaces are considered. The effectiveness of using mixed structures, in which air layers are sandwiched between high impedance layers, for low-frequency sound absorption has been confirmed by assessing the level of noise reduction.

Rectangular extended neck Helmholtz resonant acoustic structure for low frequency broadband sound absorption

The Helmholtz resonant structure with rectangular extended neck is designed to solve low-frequency broadband sound absorption problem in this work. Theoretical and finite element absorption models are established and be used for low-frequency acoustic design. What makes it interesting is that all parameters of the rectangular extended neck Helmholtz resonator structure can be adjusted to shift the working frequency. Based on the regularity of the structural parameters, four coupling structures with different neck depths, neck opening areas, cavity cross-sectional areas, and cavity depths are designed respectively, each of which exhibited multiple sound absorption coefficient peaks to enhance the low-frequency absorption capacity of the structure. To further analyze the effectiveness of coupling structure, the broadband acoustic absorption mechanism of the coupled structure is analyzed based on particle vibration velocity distribution. It is found that cells with different acoustic impedance contributed differently to the sound absorption, and cells with longer necks provided better noise reduction for low-frequency. The experiment is verified in the impedance tube, result shows that the coupling structure with 9 cells and a cavity depth of only 4 cm achieved an average sound absorption coefficient of above 0.8 at 210–340 Hz, which verified the accuracy of the theoretical model. Overall, the Helmholtz resonant cavity acoustic structure with rectangular extension neck designed in this work has a simple structure with low-frequency broadband acoustic absorption performance. This provides a new approach for designing low-frequency broadband acoustic structure.

Review of Underwater Anechoic Coating Technology Under Hydrostatic Pressure

AbstractThe underwater anechoic coating technology, which considers pressure resistance and low-frequency broadband sound absorption, has become a research hotspot in underwater acoustics and has received wide attention to address the increasingly advanced low-frequency sonar detection technology and adapt to the working environment of underwater vehicles in deep submergence. One the one hand, controlling low-frequency sound waves in water is more challenging than in air. On the other hand, in addition to initiating structural deformation, hydrostatic pressure also changes material parameters, both of which have a major effect on the sound absorption performance of the anechoic coating. Therefore, resolving the pressure resistance and acoustic performance of underwater acoustic coatings is difficult. Particularly, a bottleneck problem that must be addressed in this field is the acoustic structure design with low-frequency broadband sound absorption under high hydrostatic pressure. Based on the influence of hydrostatic pressure on underwater anechoic coatings, the research status of underwater acoustic structures under hydrostatic pressure from the aspects of sound absorption mechanisms, analysis methods, and structural designs is reviewed in this paper. Finally, the challenges and research trends encountered by underwater anechoic coating technology under hydrostatic pressure are summarized, providing a reference for the design and research of low-frequency broadband anechoic coating.

Design of an adjustable sound-absorbing structure with low frequency advantages

The thickness of microperforated panel absorbers with low-frequency sound absorption ability is often large and the absorption bandwidth is limited. To address this issue, a adjustable microperforated panel structure with low-frequency advantage (ASWLF) was designed. Compared to conventional structures, the introduction of inclined chambers allows the structure to absorb lower-frequency noise at the same thickness, enhancing spatial utilization efficiency. The addition of a rotating structure design broadens the frequency range over which the structure can operate effectively. The addition of a rotating structure design broadens the frequency range of the structure's action. The acoustic absorption coefficient of the model can be calculated using acoustic-electric analogy, which is used to predict the acoustic absorption coefficient of the ASWLF structure. The experimental results show that, with the same acoustic absorption performance, the thickness of the structure is reduced by 36.7%. The multi-stage combination allows the structure to achieve better acoustic absorption performance in a wider frequency band. By introducing the PSO optimization algorithm, the optimized ASWLF structure further improves the acoustic absorption frequency band by 32.9%. This study provides an effective method for solving noise environments under different conditions in industrial practical applications.

Sound absorber based on a sonic black hole and multi-layer micro-perforated panels

In order to achieve low-frequency and broadband sound absorption simultaneously, we propose a structure that combines a sonic black hole with multilayer micro-perforated panels. Firstly, we present finite element models for composite structures based on sonic black holes and micro-perforated panels and describe the sound absorption mechanism of the composite structure by comparing the sound absorption phenomena of micro-perforated panels with sonic black holes and micro-perforated panels with ordinary circular tubes. Secondly, the effects of the end coordinates of the sonic black hole, the number of panels and the parameters of the micro-perforated panels are discussed. Thirdly, the theoretical model of the proposed structure is developed using the transfer matrix method. Finally, the sound absorption test of the proposed structure is carried out using impedance tubes. The test results show that the sound absorption coefficient of the sample with a geometric length of 203 mm reaches 0.8 at 223 Hz and stabilizes above 0.9 at 398–1600 Hz. The sound absorber based on a sonic black hole and multi-layer micro-perforated panels has excellent sound absorption performance and has great research potential and application value.

Designing hybrid aerogel-3D printed absorbers for simultaneous low frequency and broadband noise control

Recent studies have demonstrated that granular aerogels with sub-50 μm particles surpass conventional acoustic materials like glass fibers and polyurethane foams in low-frequency sound absorption. However, incorporating such granular materials within practical structural solutions remains a challenge. In this study, we use additive manufacturing to overcome this challenge by designing a modular geometry that allows us to encapsulate such granular materials within a 3D printed scaffold. Using the 3D printed porous scaffold provides the added benefits of tunability and durability. Further, we introduce a design methodology and software tool to facilitate the efficient design of such hybrid sound absorbers. The proposed method uses the Johnson-Champoux-Allard model, Biot theory, and the transfer matrix method to model the acoustical behavior of hybrid absorber designs that use layered granular aerogels and 3D printed bulk absorbers. The tool can also be used to inversely characterize the acoustic bulk properties of such materials. We validate the design concept and tool by comparing the model predictions with experimental measurements. Finally, we outline strategies for designing hybrid absorbers that can provide application specific low-frequency and broadband absorption performance within specific frequency ranges, while considering engineering constraints on their total mass and depth.

Study on adjustable low-frequency sound absorbers based on digital shunt loudspeakers

Low-frequency noise is a critical frequency band that affects people’s physical and mental health. Traditional sound absorption technology is limited in effectiveness at low frequencies due to spatial constraints. Shunt loudspeakers, which consist of a loudspeaker connected to a shunt circuit, have emerged as a rapidly developing solution for low-frequency noise control. However, most shunt circuits are implemented using analog circuits, resulting in poor stability and accuracy due to the presence of parasitic resistance. As a result of this situation, this paper proposes an adjustable low-frequency sound absorber based on digital shunt loudspeakers, where the shunt circuit is realized through digital synthetic impedance based on FPGA technology. Additionally, the acoustic impedance can be easily adjusted by the digital shunt circuit without relying on cavity depth adjustments. Experimental results validate that the proposed sound absorber offers excellent performance in adjustable low-frequency sound absorption, thus confirming its theoretical basis.

Meta-structure based on coupled resonators for low-frequency broadband sound absorption

This work investigates and explains the behavior of an acoustic meta-structure based on different coupled Helmholtz resonators (HR). The analytical formulation of the Maa model is used to represent both the acoustic impedance and the sound absorption mechanism of the absorber. The structures are numerically modeled using finite element method and validated experimentally in an impedance tube considering sound waves of normal incidence. The absorber samples were manufactured in ABS (acrylonitrile butadiene styrene) polymer material with the additive manufacturing technique by extrusion of material. The results of the impedance tube, numerical and theoretical tests showed agreement and the sound absorber showed a sub-wavelength scale with perfect sound absorption ( 0.97) and broadband of 292 Hz in the low frequency region (200 - 600 Hz). We deduced that the excellent experimental result obtained is due to the high resolution used in the sample manufacturing (± 0.1 mm) which minimized manufacture errors. Finally, the influences of the main parameters of the HR panel (width and depth of the micro-slit) on the sound absorption coefficient of the absorber were also analyzed.

Ultra-thin ventilated metasurface pipeline coating for broadband noise reduction✰

This paper proposes a method of designing a metasurface muffling coating for pipeline ventilation and noise reduction, which realises low-frequency broadband noise reduction by laying ultra-thin labyrinth-channel acoustic absorbing metasurface structures on the inner wall surface of the pipeline. With the advantage of relatively loose size constraints in the length direction of the fluid conveying pipeline, the thickness dimensions required for low-frequency broadband sound absorption are transferred to the length direction, which significantly increases the effective ventilation area, and the ventilatable area is as high as 55.5 % of the overall structural section area. Since sound waves can be gradually attenuated in the pipeline as the fluid medium flows along the length of the pipeline rather than needing to be attenuated at the same time as in a flat plate acoustic structure, the performance requirements of the acoustic structure are substantially reduced, enabling a significant increase in the operating bandwidth. We have tried the arrangement method of coating layers with gradient thickness, which shows that the combination of different thicknesses of metasurface coating layers can broaden the muffling frequency band and improve the transmission loss of the muffling structure. The design is equally suitable for the noise reduction of liquid-filled piping systems, and the peak transmission loss can be as high as 54.6 dB, which represents a pleasing combination of low-frequency muffling and lightweight design, as well as excellent ventilation capability, and embraces a promising future for application.

Hybrid ultrathin metasurface for broadband sound absorption

To this day, achieving broadband low-frequency sound absorption remains a challenge even with the possibilities promised by the advent of metamaterials and metasurfaces, especially when size and structural restrictions exist. Solving this engineering challenge relies on stringent impedance matching and coupling of the multiple independent local resonators in metasurface absorbers. In this Letter, we present an innovative design approach to broaden the sound absorption bandwidth at low-frequency regime. A hybrid metasurface design is proposed where four coupled planar coiled resonators are also coupled to a well-designed thin planar cavity. This hybrid metasurface creates a broad sound absorption band (130–200 Hz) that is twice as wide as that of the traditional single layer metasurface utilizing four coiled cavities at a deep subwavelength thickness (∼λ/51). This design strategy opens routes toward engineering a class of high-performance thin metasurfaces for ultra-broadband sound absorption, while keeping the planar size unchanged.

Theory and inverse design of microperforated panels comprising arbitrary axial pore profiles for broadband low-frequency sound absorption

Toward identifying efficient sound absorbers, we have formulated and analyzed the micro-perforated panels (MPPs) comprising cylindrical pores of arbitrary axial profiles for out-of-plane wave propagation. First, the forward problem was solved wherein an exact analytical expression for the absorption coefficient α was developed for these MPPs. The dependence of α was analyzed for various profiles comprising convex and concave-shaped sections including divergent and convergent linear and sinusoidal profiles, conic sections, Kilroy, and chirp shapes. The MPPs with pores having a diverging shape at the entrance were found to have higher sound absorption. The non-symmetric fluctuations in the profile led to fluctuations in the spectrum of α. Aiming to address the long-standing challenge of low-frequency sound absorption, we solved the inverse problem to identify the axial profile of the cylindrical pore for maximum sound absorption for frequencies up to 2500 Hz. Given the advances in additive manufacturing, the results of this comprehensive work help in designing MPPs comprising complex-shaped pores suitable for a particular spectral regime.

Lightweight cellular multifunctional metamaterials with superior low-frequency sound absorption, broadband energy harvesting and high load-bearing capacity

Multifunctional materials are highly desired for the design of compact engineering structures, such as aircraft where weight reduction, sound absorption, load carrying, and energy harvesting are key considerations. However, design challenge remains in the balance of multiple functionalities. Here, we combine the sandwich structure with the neck-embedded cavities to design a cellular metamaterial having sound-absorption, compression/impact resistance and energy harvesting functionalities. For sound absorption, an autoencoder-like neural network is constructed to generate an instant design, after which a probabilistic module is inserted to optimize it by searching solutions in a slightly expanded design space. This inverse design has been experimentally validated, showing broadband sound absorption from 400 to 650 Hz merely with nine ultra-thin resonators. Beyond serving as absorber, the resonant cavities, once installed with well-tuned piezoelectric membranes, can gather broadband acoustic energy at low frequencies. Additionally, the cellular metamaterial inherits the excellent mechanical properties of honeycomb cores, having a low density of 0.64 g/cm3 yet displaying a high yield strength (21.2 MPa) in out-of-plane compression test and a superior energy absorption capability (8.6 J/cm3) in low-velocity impact tests. This work presents an effective approach to design lightweight metamaterials of superior mechanical and acoustic functionalities highly sought-after in practical engineering.

Geometric design and performance analysis of a foldcore sandwich acoustic metastructure for tunable low-frequency sound absorption

Acoustic metamaterial structures have received extensive attention for sound and vibration engineering applications from the scientific community in recent years. However, the real-life application of conventional acoustic metamaterial structures is frequently limited by fixed frequency bands and increased structural thicknesses in low-frequency noise reduction. In this study, we introduce an origami-based acoustic metamaterial structure that consists of a Miura-ori foldcore, along with a perforated and an unperforated panel. The proposed Miura-ori foldcore sandwich acoustic metastructure (MOF-SAM) exhibits adjustable low-frequency sound absorption capacities due to the foldability of the origami foldcore. Moreover, we employ numerical methods to investigate the sound absorption properties of the MOF-SAM, quantified by the sound absorption coefficient. The results indicate that the structure has a single absorption peak which is superior to that of acoustic structures composed of conventional honeycomb cores. The dissipation of acoustic energy is due to the structural vibrations of the metastructure and the losses in the folding process of the origami foldcore. The numerical results of this study show that the proposed sound absorption mechanism enables tunable low-frequency sound absorption. The geometric design and periodicity of the origami unit fragments offer multiple distinct absorption peaks and thus tunable acoustic performance. These findings of this study are expected to inspire novel designs for next-generation acoustic devices.

Investigation on Low-Frequency and Broadband Sound Absorption of the Compact Anechoic Coating Considering Hydrostatic Pressure

The anechoic coating capable of absorbing sound energy in low frequencies within broadband is essential to conceal underwater vehicles. However, the geometric deformation and modification of mechanical parameters under hydrostatic pressure affect the prediction of absorption performance in deep water environments. An anechoic coating embedded with tandem resonant voids is proposed in this work to achieve quasi-perfect low-frequency and broadband absorption. The analytical method based on the effective medium approach and numerical simulation are performed to estimate the effects of hydrostatic pressure on sound absorption. When additionally considering the dynamic mechanical parameters of the compressed viscoelastic medium, the original absorption humps in low frequencies are inclined to higher band, accompanied by the expanded absorption bandwidth. Then, the tandem coating specimen is measured in a water-filled impedance tube. The experimental spectra are consistent with the analytical and numerical results under various hydrostatic pressures, demonstrating the efficient absorption (α > 0.7) in broadband low frequencies via ordinary pressure. At the same time, the absorption spectrum under higher hydrostatic pressures is also verified in the tube. Consequently, this work paves the way for a broadband low-frequency underwater absorber design and provides an efficient method to characterize the low-frequency and broadband absorption from the coupled resonant coatings in deep water environments.

Low-Frequency Sound Absorption Potential of Subwavelength Absorbers Based on Coupled Micro-Slit Panels

Due to space limitations during installation, reducing low-frequency noise has always been a challenging area. Sub-wavelength structures are typically favored in such scenarios for noise reduction. This paper explores the potential of micro-slit panels (MSP) for low-frequency sound absorption. To further optimize the panel thickness, coupled MSPs (CMSP) with a distance between two MSPs of less than 1 mm are proposed. Firstly, the low-frequency absorption performances of a single MSP based on two optimized schemes – the cavity-depth optimal scheme (COS) and the panel thickness optimal scheme (TOS) – are examined and compared with those of existing ultrathin metamaterials. The results demonstrate that MSP has significant potential for low frequency sound absorption, and COS allows for a smaller overall structural thickness but a larger panel thickness than TOS. Secondly, to reduce the panel thickness, the CMSP is developed and the theoretical model of its acoustic impedance is established and validated by experiments. Then, based on the theoretical model, the low-frequency absorption potential of CMSP is optimized using COS. The results show that both the overall thickness and the panel thickness of the CMSP absorber are reduced while maintaining better performance. Furthermore, the proposed absorber achieves a subwavelength scale since its total thickness can be as small as 0.138λ.

A multifunctional metastructure with energy dissipation and low-frequency sound-absorption optimized for decoupling by genetic algorithm

A multifunctional metastructure has been proposed with the aim of having both energy-absorbing and sound-absorbing capabilities through the use of a rubber-filled re-entrant honeycomb frame. The acoustic and mechanical properties of the metastructure are decoupled utilizing a genetic algorithm so that the properties can be designed separately. The re-entrant honeycomb frame has the role of energy absorption, while the rubber filling allows the structure to form a Helmholtz-type absorbing material. The form of deformation of the structure under quasi-static compression is investigated by means of experiments and simulations, which are used to determine the variation of the cavity volume with compression. With the optimization by genetic algorithm, the first-order and second-order acoustic structures with excellent acoustic performance before and after compression have been obtained. At the same time, a continuous broadband sound- and energy-absorbing metastructure has been produced by combining these two structures. This work provides a remarkably new strategy for the integrated design of sound- and energy-absorbing structures and has a significant prospect for engineering applications such as aerospace engineering.

Granular activated carbon sound absorption predictions made using measured material parameters

Granular activated carbon (GAC) is hierarchical porosity material that yields better in low frequency sound absorption performance compared with many traditional porous materials due to the sorption effect created by nanometer-scale pores within the particles. In 2017, a triple porosity model accounting for the sorption effect in the micropores was proposed by Venegas et al. In the proposed model, some of input parameters (i.e., mesopore size and micropore effective diffusion coefficient) were fitted by matching the model to measured the surface impedance of a GAC stack; however, some fitted values are questionable from an inorganic material perspective. In the present work, the GAC material parameters, such as micro- and meso-pore sizes, were estimated from a standard isotherm measurement. The measurement results showed a standard Langmuir type isotherm behavior at different temperatures: i.e., 273.15 and 293.15 K. Thus, if the isotherms can be described with the Langmuir model, then the Langmuir constant and heat of adsorption can be estimated based on the isotherms. Finally, these parameters were used as the input to the GAC model and the absorption coefficient was calculated. The calculated result was also compared with the absorption coefficient measured following the E1050 standard.