Sound Absorption Mechanism Research Articles

Sound absorption mechanism of underwater anechoic coating with spherical cavities

Abstract An axisymmetric theoretical model is simplified to investigate sound absorption in the underwater anechoic coating with spherical cavities. An empirical equation is put forward to predict the frequency at the first absorption peak. The sound absorption mechanism of viscoelastic solid with cavity is clarified by the power analysis method. The quantitative analysis of viscoelastic power dissipation on sound energy provides a new idea for the structural topology optimization of anechoic coatings. These formulas are given to calculate the dissipation power of viscoelastic solid. The dependence of sound absorption performance of the anechoic coating upon radius and position of single or two spherical cavities of the unit cell is analyzed in detail. The study provides guidance for the design of anechoic coatings with cavities.

Mechanism analysis and optimal design of sound-absorbing metastructure constructed by slit-embedded Helmholtz resonators

<sec>Low-frequency noise has always been a thorny problem in the field of noise control. In recent years, the development of sound-absorbing metastructures has provided new ideas for controlling low-frequency noise. In this work, we propose a low-frequency sound-absorbing metastructure constructed by Helmholtz resonators with embedded slit. Analytical and numerical models are established to analyze the sound absorption performance and mechanism of the proposed sound-absorbing metastructure, and optimization design is conducted to achieve low-frequency wideband absorption performance. The analytical modeling method and the performance of the proposed sound-absorbing metastructure are also experimentally verified. The main conclusions are summarized as follows.</sec><sec>1) By using transfer matrix method and finite element method, analytical and numerical models for calculating sound absorption coefficient are established. It is shown that analytical predictions are in good agreement with numerical calculations. It is demonstrated that a typical design of a 30-mm-thick single-cell metastructure can achieve a sound absorption coefficient of 0.88 at 404 Hz. Typical designs of two-cell parallel structure and the four-cell parallel structure (both with a thickness of 50 mm) can achieve two and four nearly perfect low-frequency sound absorption peaks in a frequency band of 200–400 Hz, respectively.</sec><sec>2) The low-frequency sound absorption mechanisms of the proposed metastructures are explained from four aspects: simplified equivalent model parameters, normalized acoustic impedance, complex-plane zero/pole distribution, and sound pressure cloud image and particle velocity field distribution. It is demonstrated that the main sound absorption mechanism is related to the thermal viscous loss of sound waves, caused by the inner wall of embedded slit.</sec><sec>3) The design for broadband low-frequency absorption performance is optimized by using differential evolution optimization algorithm. An optimized parallel-multi-cell coupled metastructure with multiple perfect sound absorption peaks below 500 Hz is realized. For a thickness of 90 mm, the sound absorption coefficient curve of an optimized metastructure exhibits 8 almost perfect sound absorption peaks and an average sound absorption coefficient of 0.86 in a frequency range of 170－380 Hz.</sec><sec>4) Experimental samples are fabricated to test sound absorption. Experimental results are basically consistent with the analytical predictions. The results from analytical model, numerical calculations and experimental measurements are mutually verified.</sec><sec>In summary, the sound-absorbing metastructures with a thickness of sub-wavelength, proposed in this work, exhibit outstanding sound absorption performance at low frequencies. We demonstrate that they are suitable for low frequency broadband sound absorption below 500 Hz. Owing to their thin thickness and relatively simple construction, they have broad application prospects in practical noise control engineering.</sec>

An Investigation of Modular Composable Acoustic Metamaterials with Multiple Nonunique Chambers.

To make the sound absorber easy to fabricate and convenient for practical application, a modular composable acoustic metamaterial with multiple nonunique chambers (MCAM-MNCs) was proposed and investigated, which was divided into a front panel with the same perforated apertures and a rear chamber with a nonunique grouped cavity. Through the acoustic finite element simulation, the parametric studies of the diameter of aperture d, depth of chamber T0, and thickness of panel t0 were conducted, which could tune the sound absorption performances of MCAM-MNCs-1 and MCAM-MNCs-2 for the expected noise reduction effect. The effective sound absorption band of MCAM-MNCs-1 was 556 Hz (773-1329 Hz), 456 Hz (646-1102 Hz), and 387 Hz (564-951 Hz) for T = 30 mm, T = 40 mm, and T = 50 mm, respectively, and the corresponding average sound absorption coefficient was 0.8696, 0.8854, and 0.8916, accordingly, which exhibited excellent noise attenuation performance. The sound absorption mechanism of MCAM-MNCs was investigated by the distributions of the total sound energy density (TSED). The components used to assemble the MCAM-MNCs sample were fabricated by additive manufacturing, and its actual sound absorption coefficients were tested according to the transfer matrix method, which demonstrated its feasibility and promoted its actual application.

A multi-layered corrugated resonator acoustic metamaterial with excellent low-frequency broadband sound absorption performance

How to design acoustic metamaterials with excellent low frequency broadband sound absorption and mechanical properties is a concern and bottleneck in the field of structural vibration and noise control. Based on the principle of Helmholtz resonator and lightweight multi-functional sandwich structure, we propose a multi-layered corrugated resonator acoustic metamaterial (MLCRAM) and fabricate a sample using 3D printing technology. Through theoretical and simulated studies of the MLCRAM model, it is presumed that the MLCRAM has excellent sound absorption performance in the frequency range of 500 to 1000 Hz, which is mainly attributed to the parallel combination of resonant cavities. Then the experimental test of the sample is also carried out to verify the reliability of both theoretical and simulated methods. By analyzing the complex frequency plane, the influence of coupling effects between resonant cavities on the sound absorption properties of the MLCRAM. Subsequently, the overall sound absorption mechanism of the MLCRAM is further analyzed in terms of sound velocity, sound pressure amplitude, and power dissipation density. It has been demonstrated that MLCRAM exhibits excellent low-frequency broadband sound absorption performance, providing a reference for the design of novel acoustic metamaterials.

Hand-made sound absorber using familiar materials for nursery facilities

We have been proposing a hand-made sound absorber using familiar materials to develop a comfortable sound environment for children. By installing the hand-made absorber, anyone can easily add sound absorption to nursery rooms. In this study, as one of the familiar materials, we focused on bubble wrap, which is used as a packaging material. We measured the normal incidence absorption coefficient of bubble wrap arranged in layers and found that bubble wrap had reasonable sound absorption. As an example of a hand-made absorber, we created round-shaped fabric balls filling bubble wrap and measured the sound absorption performance in a reverberation room. As a result, the fabric ball absorber had sufficiently high sound absorption performance. Additionally, we clarified the sound absorption mechanism of bubble wrap by predicting based on simple assumptions. Therefore, it could be possible to design sound absorbers with higher sound absorption performance at low frequencies by stacking bubble wrap horizontally or vertically.

Principles of progressive slow-sound and critical coupling condition in broadband sonic black hole absorber

Recent advances in sonic black hole (SBH) provide new opportunities for controlling sound waves and designing wave manipulation devices. SBH is a device that consists of partitions with gradually decreasing inner radii inserted into an acoustic duct. Several studies have reported that SBH can achieve a broadband sound absorption coefficient close to 1, avoiding the issue of alternating high and low absorption coefficients observed in traditional sound absorbers. However, the fundamental mechanisms and principles behind this behavior are not yet fully understood. This study aims to investigate the detailed sound absorption mechanisms of SBH, including the progressive slow-sound effect and the critical coupling condition that leads to broadband sound absorption. To achieve this goal, an analytical model based on the effective medium approach is developed to investigate the layer-by-layer retardation in sound propagation. The sound absorption coefficient is then determined based on the surface impedance calculation. The effective medium analysis reveals that SBH enables a unique condition to progressively decelerate wave propagation across its layers. As a result, the critical coupling condition becomes more easily established with smoothly increasing SBH partitions and more discretised layers, as elucidated by the complex frequency analysis results. The physical insights gained from this study reveal the distinctive features of SBH compared to classical sound absorbers, paving the way for its engineering applications.

Simulated and Experimental Research on Highly Efficient Sound Absorption of Low‐Frequency Broadband Acoustic Metamaterial Based on Coiled‐Up Space

Acoustic metamaterials are broadly employed in vibration and noise reduction fields for large defense industrial equipment such as engines and compressors thanks to their excellent subwavelength characteristics and extraordinary acoustic performance. However, the low‐frequency broadband sound absorption remains a thorny challenge in the scientific and engineering communities. This paper focuses on the low‐frequency broadband acoustic metamaterial composed of eight spiral absorbers coupled in two rows, researching its thermoviscous dissipation and sound absorption mechanism, and further analyzing its sound absorption characteristics. In the frequency range of 510–990 Hz, the average sound absorption coefficient of acoustic metamaterial with sound absorption area ratio of 2.72% reaches 0.80, and the cross‐sectional height is only 34 mm, which achieves the tradeoffs between structural thickness, frequency bandwidth and sound absorption performance. Furthermore, continuous and highly efficient broadband sound absorption within the target frequency band in practical applications can be achieved by optimizing adjustable parameters. In addition, finite element simulations are verified by experiments, which presents good agreement. Compared with traditional sound absorbing structure, the low‐frequency broadband acoustic metamaterial proposed in this paper owns the characteristics of small size and easy processing, which has broad application prospects in the field of low‐frequency noise control engineering.

Analysis of Influencing Factors for Stackable and Expandable Acoustic Metamaterial with Multiple Tortuous Channels.

To reduce the noise generated by large mechanical equipment, a stackable and expandable acoustic metamaterial with multiple tortuous channels (SEAM-MTCs) was developed in this study. The proposed SEAM-MTCs consisted of odd panels, even panels, chambers, and a final closing plate, and these component parts could be fabricated separately and then assembled. The influencing factors, including the number of layers N, the thickness of panel t0, the size of square aperture a, and the depth of chamber T0 were investigated using acoustic finite element simulation. The sound absorption mechanism was exhibited by the distributions of the total acoustic energy density at the resonance frequencies. The number of resonance frequencies increased from 13 to 31 with the number of layers N increasing from 2 to 6, and the average sound absorption coefficients in [200 Hz, 6000 Hz] was improved from 0.5169 to 0.6160. The experimental validation of actual sound absorption coefficients in [200 Hz, 1600 Hz] showed excellent consistency with simulation data, which proved the accuracy of the finite element simulation model and the reliability of the analysis of influencing factors. The proposed SEAM-MTCs has great potential in the field of equipment noise reduction.

Underwater low-frequency sound absorption performance and broadband absorption design of membrane-type acoustic metamaterials

The underwater low-frequency sound absorption performance of membrane-type acoustic metamaterials (MAM) is investigated in this paper. The MAM unit is composed of a tensioned membrane, an upper cavity frame, a lower cavity frame, a steel backing and a cylindrical mass block located at the geometric center of the membrane. The sound absorption mechanism of MAM and the influence of critical structural parameters are analyzed by numerical simulation. The absorption performance of MAM is further optimized by the parallel combination design of multiple MAM units and the acoustical siphon effect. A design of MAM with 19 units is proposed to improve the absorption bandwidth. The calculation result demonstrates that the structure can effectively absorb sound in the low-frequency range of 250–750 Hz, and the average absorption coefficient is above 0.6. Lastly, the corresponding sample is fabricated, and the underwater sound absorption performance is tested in the standing wave tube. The accuracy of numerical simulation is validated by experiment.

Analysis on sound absorption in Sonic Black Holes

Exploration of Acoustic black holes (ABHs) for sound wave manipulation is referred to as Sonic black holes (SBHs). Different from the ideal SBH structure with continuously varied wave propagation boundary formed by putting extensive number of rings inside, the wave trapping and energy dissipation mechanism of practically realized SBH structure with discrete varied wave propagation boundary formed by quite limited number of rings inside are still not clear. To fill the gap, finite element simulations are performed to assess the sound absorption performance of a practical discrete SBH structure and explore the sound absorption mechanism behind. The validity of the FEM results is demonstrated through comparisons with experimental data obtained through impedance tube tests. Analyses reveal that the physical mechanism underpinning the sound absorption of practically realized SBH device is attributed to the combined effects of the ABH-induced wave speed changes, energy trapping and the spatially graded local resonances of series cavities formed by the space between two adjacent rings. While shedding light on the underlying sound attenuation mechanism, studies provide guidelines for further investigations on SBH structures.

Sound absorption performance of micro-perforated plate sandwich structure based on triply periodic minimal surface

The sandwich structure based on Triply periodic minimal surface (TPMS) is a lightweight and high-strength multifunctional composite material that combines the versatility of heat exchange, impact resistance, and energy absorption, and has been widely used in various fields such as aviation and aerospace. However, its sound absorption performance has not meant fully studied. In this paper, a Micro-perforated plate Diamond sandwich structure (MPP-DSS) was designed based on TPMS method, which was composed of aluminum alloy solid panel, aluminum alloy macroscopically ordered porous Diamond structure and aluminum alloy micro-perforated plate. The acoustic absorption performance in low and medium frequency band was studied by impedance tube method. The results show that MPP-DSS has higher absorption coefficient and bandwidth than traditional perforated plate structure with the same structural parameters. Increasing the thickness of micro-perforated plate can improve the sound absorption capacity of MPP-DSS in the low frequency range, but the width of the sound absorption band will be narrowed accordingly. Different from the resonant sound absorption mechanism of the traditional perforated plate structure, the sound absorption mechanism of MPP-DSS is the combined effect of resonant sound absorption and friction loss sound absorption. This study broadens the versatility of the TPMS structure and can serve as a reference for the design of integrated load-bearing and sound-absorbing structures.

Acoustic metamaterials of modular nested Helmholtz resonators with multiple tunable absorption peaks

The noise control for machines and equipment in the workshop is important to reduce the noise occupational diseases for the workers, and a novel acoustic metamaterial of modular nested Helmholtz resonators was proposed and investigated in this study. Based on the acoustic finite element simulation, the distributions of sound pressure, acoustic velocity and temperature variation at these resonance frequencies were obtained, which showed the sound absorption mechanism intuitively based on the Helmholtz resonance principle. The 14 structural parameters for the modular nested Helmholtz resonators with 5 nests were studied one by one, and the corresponding impact indexes to the resonance frequencies and peak sound absorption coefficients were summarized. By testing sound absorption coefficients of the prepared samples, the feasibility of the proposed acoustic metamaterial, the accuracy of the utilized finite element simulation method, and the usefulness of the selected optimization algorithm were validated. 4 sound absorption peaks were obtained in the target frequency points with thickness about 1/35th of the maximal working wavelength of sound wave. The number of sound absorption peaks was tunable through adding or reducing the number of the nested resonators. The proposed adjustable modular nested Helmholtz resonators is favorable to suppress the unfixed multiple narrowband noises.

Theoretical study of sound absorption performance for Al2O3-polyurethane foam composite

To reveal the sound absorption property and mechanisms of Al2O3-polyurethane (Al2O3-PU) composite materials in theory, the impedance tube model is established by using the finite element method in this paper. Meantime, the theoretical modal and the calculation method adopt the Johnson-Champoux-Allard (JCA) modal and the standing wave ratio method, respectively. By comparing the experimental and simulation values of PU and Al2O3-PU composites from 0 to 1250 Hz, the results prove that the JCA predicted values can match the actual measured values. To further predict the sound property of Al2O3-PU composites in the medium and high frequency, the diameter of 29 mm and length of 600 mm for the impedance tube is installed. Finally, the sound pressure distribution diagram of the impedance tube is obtained.

Sound absorption mechanism of sonic black hole with labyrinthine units and micro perforated panel

Acoustic black hole (ABH) is a technique capable of manipulating the propagation of flexural wave, and the sonic black hole (SBH) is a kind of ABH which is used to manipulate sound wave in a fluid medium. In this paper, we propose an SBH structure with labyrinthine units and combine with micro-perforated panel (MPP) to form a composite sound absorption structure. The sound absorption mechanism of the absorption structure is deeply investigated using numerical and simulation methods. The simulation reveals the sound absorption mechanism by acoustic streaming effects of composite sound absorption structure. We analyze the flow characteristics of the acoustic medium under acoustic excitation, and the effect of the flow field on the distribution of the acoustic field, and the energy dissipation distribution. Our theoretical results show that the sound absorption is attributed to the effects of sound energy focusing of ABH, the local resonance of MPP, and the acoustic energy localization and dissipation effect of labyrinthine units caused by large flow velocity gradients. Finally, the proposed composite sound absorption structure has good sound absorption performance, which is also confirmed by impedance tube experiments. It can provide a new way of thinking for the design and optimization of the SBH structure.

Low-frequency sound-absorbing metamaterial based on a series-parallel arrangement structure

Low-frequency noise has a long wavelength, decays very slowly, and is extremely penetrating. Traditional marine acoustic insulation materials have difficulty achieving effective control of this low-frequency noise. In this paper, a series-parallel arrangement structure of a low-frequency metamaterial is designed, which mainly comprises convoluted channels and Helmholtz cavities in series and a sandwich arrangement of multiple cells (as opposed to the traditional parallel arrangement). We use a numerical method to establish an acoustic-solid coupling model for the metamaterial, consider the influence of thermal and viscous losses on its sound absorption performance, and investigate the sound absorption characteristics and mechanisms of the single-cell and multicell structures. The metamaterial designed in this paper shows an average absorption coefficient of 0.97 in the range of 130–145 Hz. An experimental model was prepared by 3D printing, and the intended sound absorption effect was experimentally verified.

Sound Absorption Performance and Mechanism of Aluminum Foams with Double Main Pore‐Porous Cell Wall Structure

To enhance the sound absorption performance of open‐cell aluminum foam, the double main pores‐porous cell walls (DMP‐PCW) aluminum foams via infiltration casting of preforms mixed with two sizes of NaCl particles are prepared. The pore structure, sound absorption performance, and mechanism of DMP‐PCW aluminum foam are investigated. The pore structure consists of double‐sized main pores similar to the NaCl particles and the cell wall pores formed by the connections between NaCl particles. It is found that the static flow resistivity of DMP‐PCW aluminum foam reaches an optimum value of 28105 Pa.s m−2 when the volume proportion of small main pores increases, the size of cell wall pores decreases, and the number of cell wall pores per unit main pore surface area (NPPA) increases. At 800–6300 Hz, the average absorption coefficient is 0.89. In addition, the Wilson model predicts the sound absorption properties of DMP‐PCW aluminum foam. The predicted values agree well with the measured values. The finite‐element acoustic simulations and dynamic viscous‐thermal permeability calculations reveal that the improved sound absorption performance of DMP‐PCW aluminum foam is correlated to the enhanced sound transmission caused by increased NPPA and increased viscous‐thermal loss due to the double main pore structure.

A modified sonic black hole structure for improving and broadening sound absorption

In order to improve and broaden the absorption performance of Sonic Black Hole (SBH), we propose a modified SBH structure with micro-perforated panels which is embedded the multiple resonator cavities, and investigate the sound absorption effect of the structure. Characteristics of acoustic streaming effects are embodied in sound pressure and flow velocity distribution. Further, proposed SBH structure can achieve high sound absorption under 6.0 kHz. Based on the symmetry of the structure, the Two-dimensional axisymmetric Finite Element Method (FEM) model is proposed to study the sound absorption mechanism, which shows that the change of flow velocity gradient of acoustic medium leads to the loss of acoustic energy inside the structure. By discussion on acoustic streaming effects of different frequencies, the resonator cavity and micro-perforated panels contributes more to the sound absorption performance at low frequencies, and SBH produces a sound absorption advantage at high frequencies.The theoretical calculation and simulation result match and prove the correctness of the sound streaming effect. Sound absorption tests by acoustic impedance tube confirm that the proposed structure can maintain a sound absorption coefficient of over 0.5 in the range of 0.05–6.0 kHz. The combination of micro-perforated panels and SBH structure is widely applicable to muffler design and has good application prospects.

Coiled-up structure with porous material lining for enhanced sound absorption

This paper presents an acoustic metamaterial, coiled-up structure with porous lining (CSPL), which is constructed by adding porous material lining to a coiled-up air channel for the pursuit of broadband and low-frequency sound absorption at subwavelength scales. The porous material is fixed on both sides of the channel wall to enhance the losses of the system, and an air slit is left in the middle of the channel to regulate the impedance matching between the metamaterial and the air. A theoretical model based on double porosity theory and impedance transfer method is established to study the sound absorption performance of the metamaterial, and a finite element (FE) simulation model is constructed to reveal the underlying sound absorption mechanism. To validate these two models, the sound absorption performance of the designed sample is measured in a standing wave tube. The results show that the porous material lining not only enhances the sound energy dissipation, but also reduces the propagation speed of sound wave in the coiled channel, resulting in good low-frequency sound absorption performance of the CSPL. By adjusting the position and length of the porous lining, good sound absorption tunability can be achieved in the considered frequeny range. Moreover, the effect of acoustic properties of the porous material on the sound absorption of the CSPL is revealed and discussed in detail. Finally, several coupled CSPLs are designed to achieve broadband sound absorption in different frequency ranges. This work provides a new approach to design metamaterials for broadband low-frequency sound absorption.

An efficient underwater absorber using pentamode metamaterials for broadband frequency

To further design and develop broadband and efficient underwater sound absorption structure, we propose a hybrid acoustic structure composed of a rubber matrix layer, cylindrical cavity, pentamode metamaterial (PM) layer, impedance matching layer, and steel backing plate. Thanks to the special properties of the PMs, the hybrid acoustic structure obtains excellent broadband sound absorption performance (SAP) in the frequency range of 500–10000 Hz than that of the traditional cavity acoustic structure. To this end, we study the design principle of PM structure and its metal–water characteristics and analyze the sound absorption mechanism at the working frequency. In addition, the research shows that the material and geometric parameters of the structure have a significant impact on the SAP. By the optimization of structural material parameters, the average absorption coefficient at 500–10000 Hz can further increase to above 0.781. Finally, we also demonstrate that the structure still has good SAP under a wide range of oblique incidence angles. The research in this paper opens up bright perspectives for the design of underwater anechoic coatings for multifunctional applications.

Interpenetrating a Hollow Microlattice Metamaterial Enables Efficient Sound-Absorptive and Deformation-Recoverable Capabilities.

Owing to the pervasive noise and crash hazards, tough microlattices with sound absorption capabilities are sought-after. However, typical truss microlattices are unable to fulfill this requirement. To overcome this, herein, we report a new design strategy for truss microlattices via introducing the concept of interpenetrating hollow struts, which thereby constitutes a novel interpenetrating hollow microlattice metamaterial (IHMM). The design is based on interweaved unit cells of a hollow octet-truss (HOT) and a hollow rhombic dodecahedron-like (HRDL) truss. Experimentally demonstrated, the IHMM displays a synergistic gain in both sound absorption and mechanical properties that substantially surpass that of its constituent lattices. High sound absorption coefficients (α > 0.99) and broad half-absorption (3.2 kHz) are observed, with the average α being 110.6 and 85.3% higher than those of the HOT and HRDL, respectively. The sound absorption mechanism is attributed to the presence of cascaded Helmholtz resonance, which is then fully elucidated by impedance and damping analyses. The IHMM also outperforms its constituents in specific strength. A simultaneous high strength (4 MPa) and recoverability (80% strain) and pseudo-reusability are also observed. The mechanisms behind the mechanical reinforcement and exceptional robustness are thoroughly revealed. Overall, this work offers insights into developing multifunctional engineering materials.