Acoustic Metamaterial Structure Research Articles

Bandgap and Vibration Suppression Mechanisms of Novel Star‐Shaped Hybrid Metamaterials

Herein, a novel star‐shaped hybrid acoustic metamaterial structure is proposed. Based on Bloch's theorem and the finite element method, the bandgap characteristics and the bandgap formation mechanism of different combinations of structures are first analyzed. Then, the propagation characteristics of elastic waves in Model C2 are studied with focus on equal frequency contours, phase velocity, and group velocity. And the attenuation of elastic waves in the finite size structure of Model C2 illustrates that the elastic waves are effectively suppressed in the bandgap frequency range, which further confirms the perfect correspondence between the bandgap and the vibration attenuation region. Finally, the influence of structural parameters on the bandgap is analyzed, as well as the change of the bandgap when the structure undergoes large deformation, which can realize the tunability and flexibility of the bandgap. This study provides important clues for the design of vibration isolators and metamaterials, and shows the good engineering application potential of the structure for vibration isolation.

Experimental investigation of mechanical, acoustic and hybrid metamaterial designs for enhanced and multi-band electric motor noise dissipation

Acoustic metastructures exhibit an unprecedented ability to arbitrarily manipulate the properties of sound waves, including amplitude and phase modulation which has attracted much attention in both the academic and industrial communities. Design of metastructures has been extensively investigated for the purpose of the independent manipulation of acoustic and elastic waves. It is up to now unclear however if the optimal dissipation approach involves locally resonant structural elements, or locally resonant acoustic cavities or a hybrid mix of both noise abatement mechanisms. To study this problem, we investigate numerically and experimentally the noise reduction performance of acoustic and mechanically locally resonant structures and how they perform when integrated into the same application scenario. Taking the noise emission of an electric motor as the excitation regime for our study, the acoustic and mechanical metamaterial structures with respective C-shape and T-shaped resonators are judiciously designed to maximize noise dissipation within two targeted frequency bands where the motor noise peaks. A hybrid structure with a mix of both resonators is also designed. The results demonstrate that all three metastructure designs improved the noise attenuation performance with an additional sound insertion loss of up to 16 dB when compared to a flat wall structure of equivalent mass. The findings in this work demonstrate that significant noise attenuation can be obtained by incorporating these acoustic and mechanical meta-atoms into a wall architecture which may have great potential for real world applications such as surrounding electric motors which tend to emit monotonic and irritating noise.

Enhanced vibration suppression using diatomic acoustic metamaterial with negative stiffness mechanism

This paper presents a diatomic-chain locally resonant acoustic metamaterial (LRAM) structure with negative-stiffness mechanism for enhanced suppression of vibration transmission. The bandgap properties of the diatomic configuration were studied and shown to enhance performance benefits by introducing two extra bandgaps that exploit Bragg scattering, located on both sides of the local resonant bandgap. The upper band-folding-induced bandgap exhibits better performance than the lower band-folding-induced bandgap. Converting a monoatomic configuration into a diatomic configuration is shown to be beneficial. A dispersion relation analysis is performed and new phenomena are revealed from the viewpoint of the vibration power flow and wave transmittance, demonstrating the potential application of negative stiffness for performance improvement. A geometrical nonlinear mechanism is studied, and the results demonstrate the possibility of providing a constant negative stiffness under specific material parameters. With the application of this negative-stiffness mechanism for the critical, effective stiffness value, the locally resonant bandgap of the metamaterial configuration shifted toward the lower frequency range, effective from zero frequency, thus achieving ultralow frequency vibration control. The proposed implementation of the negative-stiffness mechanism in a diatomic metamaterial structure is shown to yield desirable bandgap properties, providing potential benefits for vibration suppression.

Low Frequency Band Gap and Wave Propagation Properties of a Novel Fractal Hybrid Metamaterial

In order to achieve the attenuation of low‐frequency sound, in this paper, a hybrid metamaterial structure with a complete band gap below 500 Hz is proposed, which is subjected to second‐order fractal, series fusion, and whether the structure is framed or not to form 8 different structures. Based on the finite element method and Bloch's theorem, the bandgap and transport properties of all model structures are evaluated, and the bandgap and propagation properties of metamaterial structures with different fractal levels are elucidated in terms of band structure and group velocity. In addition, the third‐order fractal structure and the dependence of the band gap on the material are also studied. The results show that the structure has a superior band gap width, and the complete band gap of its tandem fusion structure accounts for up to 45.502% in the range of 500 Hz, and the second‐order fractal has a complete band gap of 45.588% in the range of 1000 Hz. Compared with other structures, it can produce lower and better band gap. The research in this paper provides a new strategy for realizing acoustic metamaterial structures with low frequency band gaps.

Research on the low-frequency sound absorption characteristics of coiled Helmholtz cavity acoustic metamaterials

A coiled Helmholtz cavity acoustic metamaterial structure, which consists of three perforated plates, two channel layers and an air back cavity, is designed to address the problem of poor absorption of low-frequency band noise by conventional acoustic materials, and the results demonstrate that the absorption peak frequency of this structure is lower than that of a conventional Helmholtz cavity. The finite element method based on COMSOL multiphysics field coupling software is used to analyze the sound absorption characteristics of the metamaterial single-cell structure and to investigate the effect of structural parameter variations on its sound absorption performance. The problem of broadband sound absorption is solved by synergistically coupling two single-cell structures and then by designing the geometrical parameters of the coiled Helmholtz cavity structure, multiple basic units with absorption peaks at different frequencies are connected in parallel to design a multiunit coupled structure with low-frequency broadband sound absorption performance. In addition, the multiunit coupled structure is embedded in a porous material to design a composite space folding metamaterial structure, which can simultaneously absorb both low- and high-frequency noise and achieve a large broadband sound absorption performance above 200 Hz.

Acoustic metamaterial design framework using deep learning and generative modeling

This talk presents our findings and research performed on the application of deep learning and generative modeling in acoustic metamaterial design. Specifically, we will discuss our research findings published in recent papers on the application of deep learning algorithms, generative neural networks, reinforcement learning models, and global optimization for the inverse design of 2-D and 3-D acoustic metamaterial structures. The examples will be shown for the implementation of neural networks models for the inverse design of 3-D pentamode structures resulting in a low shear modulus, high bulk modulus, and an impedance matched with water. The generative 2-D GLO-Nets and the reinforcement learning models producing broadband low scattering effects for 2-D planar configurations of scatterers under plane wave incidence will be presented. The current challenges encountered during the application of deep learning methods in scaled metamaterial design will be discussed.

Ultra-broadband sound absorption performance of a multi-cavity composite structure filled with polyurethane

In order to improve the sound absorption performance of the resonator, porous materials are introduced, an acoustic metamaterial structure (P_TMCCR) composed of a tunable multi-cavity coupled resonator (TMCCR) and polyurethane-filled slits was established. Firstly, the TMCCR was designed by combining theoretical research with finite element simulation, which enables the continuous ultra-broadband sound absorption performance by adjusting the resonant frequencies of each component. Secondly, a simulation model of the P_TMCCR was established and its effectiveness verified through an impedance-tube test. Finally, sound absorption effects of polyurethane infill and the influences of various filling modes were investigated. The results demonstrate that the P_TMCCR (after filling) exhibited a significantly improved sound absorption bandwidth and coefficient, leading to more stable sound absorption effects. The double-layer P_TMCCR effectively absorbs sound within 375 to 4270 Hz and the average sound absorption coefficient exceeded 85% from 430 to 2370 Hz, filling the bottom with polyurethane and communicating with the incident surface can significantly improve sound absorption in the frequency band less than 250 Hz. The P_TMCCR is easy to prepare and its frequency band of sound absorption is adjustable on demand to realize continuous and stable sound absorption effects, suggesting potential for use in sound absorption and noise reduction applications.

A Tunable Open Planar Acoustic Notch Filter Utilizing a Pneumatically Modulated Helmholtz Resonator Array

Acoustic filters provide sound insulation unpleasant noises at a specific frequency only. Ordinary passive acoustic filters made up of acoustic metamaterial structures can work without a power supply. However, their working frequency is not adjustable. This study reports a tunable open planar acoustic notch filter that incorporates a pneumatically controlled Helmholtz resonator (HR) array. The HR array consisted of three layers: cavities, a membrane, and microchannels. The cavities function as the HR chambers, whose volume determines the resonant frequency. The acoustic wave is effectively attenuated at this frequency owing to the HR effect. The HR chamber volume can be varied by displacing the stretchable membrane with air pressure through the microchannels. This helps in adjusting the resonant frequency, thereby realizing a tunable acoustic notch filter. We conducted theoretical, and experimental examinations along with simulations of the frequency response for an HR array with three different cavity thicknesses. The prototype device realized a frequency tuning of approximately 4.1–4.9 kHz with 10 dB filtering. Furthermore, an improved notch frequency tunability was observed for the thinner cavities, along with a decrease in the notch performance. Thus, the proposed HR array filter can be useful for high-frequency noise that is generally uncomfortable for humans.

Ultra-thin composite underwater honeycomb-type acoustic metamaterial with broadband sound insulation and high hydrostatic pressure resistance

This research proposes a new type of composite underwater honeycomb-type acoustic metamaterial (AM) plate with the advantages of low-frequency broadband sound insulation and high hydrostatic pressure resistance. The proposed AM is composed of a thin plate (rubber-steel-rubber, RSR) clamped between two layers of honeycomb plate, and the thickness of the whole structure is 20.25 mm. The underwater sound insulation performance and mechanisms of the new type of AM were investigated numerically and experimentally. Wave propagation properties of the proposed AM were modeled using the finite element method (FEM). In addition, modal analysis of the eigenmode shapes were used to investigate the underwater sound transmission loss (STL) performance and local resonance bandgap formation. The results show that the honeycomb-type AM has good underwater sound insulation performance at low frequencies and the STL is more than 20 dB higher than that of homogeneous materials with the same area density. Furthermore, the theoretical STL of the proposed metamaterial was consistent with experimental values, thus verifying the STL performance and validating the theoretical modeling approach for the proposed AM. Moreover, the proposed AM structure can still achieve an average STL of 10 dB in the frequency range [2 kHz, 10 kHz] under the hydrostatic pressure of 3 MPa.

Precise and target-oriented control of the low-frequency Lamb wave bandgaps

Precise and target-oriented control of acoustic/elastic wave bandgaps in phononic crystals or acoustic metamaterials according to particular requirements is of both theoretical and practical importance. A very challenging and also interesting task is to tune the widths and the overall positions of the bandgaps of interest independently and arbitrarily. This work presents a simple and smart acoustic metamaterial structure, which can achieve the above mentioned goal by considering the symmetric Lamb waves. The proposed acoustic metamaterial structure consists of a homogenous piezoelectric plate which is connected to the LC circuits through an array of periodic surface electrodes. The control of the resonance bandgaps (RBGs), which are generated by the coupled resonance of the piezoelectric plate with the external LC circuits, is the focus of this study. To solve the problem at hand, we develop a two-dimensional (2D) spectral element method (SEM) considering the full electro-mechanical coupling, and the high tunability of the RBGs is demonstrated numerically in the low-frequency range. Especially, we find that the upper/lower boundary of a RGB can be intentionally fixed at any specified frequency while changing its’ bandwidth. To explore the corresponding physical phenomena and reveal the regulation mechanism, a simplified theory is developed based on the assumption of thin piezoelectric plates. Several simple analytical formulas are established, which are essential to the precise control of the RBGs for both the thin and thick piezoelectric plates. A numerical example is given to demonstrate the detailed procedure of precisely tuning a RBG in a targeted frequency range, which is also verified by the transmission spectrum for the corresponding finite piezoelectric acoustic metamaterial plate.

Structural designs, principles, and applications of thin-walled membrane and plate-type acoustic/elastic metamaterials

Many advanced physical properties can be realized by using well-designed acoustic metamaterial (AM) structures, which have significant application value in engineering. In particular, thin-walled membrane, plate, and shell-type structures with deep subwavelength thicknesses that can meet light weight requirements have attracted the attention of many researchers and engineers from various specialized fields. This Tutorial systematically introduced the structural design methods, acoustic/elastic wave attenuation and regulation principles, and engineering applications of thin-walled AMs for low-frequency sound insulation, sound absorption, and vibration reduction. In particular, the design methods and sound insulation/absorption properties of thin-walled AMs for realizing narrow-band and broadband sound attenuation were explored. Furthermore, the local resonance bandgap characteristics, quantitative extraction method for the bending wave bandgap, vibration suppression properties, and the design method for local resonance vibration dampers for elastic wave regulation by thin-walled elastic metamaterials were summarized successively. Moreover, other thin-walled AM applications, such as the wavefront steering performance of thin-walled acoustic/elastic metasurfaces, and the active thin-walled AMs, were introduced as well.

Study on broadband low-frequency sound insulation of multi-channel resonator acoustic metamaterials

A novel acoustic metamaterial structure, made up of both resonator and multiple channels, is proposed to insulate low-frequency broadband noises. Three bandgaps with a total width of 658 Hz can be obtained below 900 Hz when the structure appears as a single negative characteristic within specific frequency ranges. These bandgaps are caused by the standing-wave characteristics under the coupling action of the interior and exterior cavities and the multiple channels. By changing structural parameters and materials, the low-frequency broadband sound insulation performance of the structure can be improved. Experimental results further prove the sound insulation characteristics. The results obtained herein can be used as a guide toward achieving low-frequency broadband sound insulation using a lightweight structure.

Folded planar resonator-based sonic Crystal scatterer: Part I. Shifting from split tube resonator design

A split tube resonator (STR) is one of the typical base structures in many previous acoustic metamaterial structures. This paper introduces a brand new approach to designing the tunable concentric wall resonating structure to look like a similar shape but with a different response and performance to STR. The central concept is a planar resonator folded outward and inward direction. We found that the proposing approach gives the possibility for a single geometrical dimension tunable sonic crystal element that could not occur in a single degree of freedom conventional STR. The test model made of stiff paper and the sound absorption performance test conducted experimentally with impedance tube refers to ASTM E-150-98 standard.

The role of material and geometric nonlinearities and damping effects in designing mechanically tunable acoustic metamaterials

Elastomeric material (as rubber) can be used to design the mechanically tunable acoustic metamaterial (AM) with reversible and repeatable deformation utilizing its geometric and material nonlinearities introduced by the large deformation. Meanwhile, an elastomeric material usually possesses the inherent damping effect, which will complicate the dynamic responses of AMs. In this paper, taking the AMs comprising different resonating elements (include soft elastomeric coating and hard core) embedded into an elastomeric matrix with a square array of circular holes of varying size as examples, we try to reveal the roles of material and geometric nonlinearities and damping effects (Rayleigh damping and linear viscoelastic damping) in the process of elastic wave manipulation by calculating the band structures and the transmittances of the finite-sized AM structures with and without damping. The numerical results indicate that the geometrical nonlinearity of the AM (mainly from matrix and coating) and the nonlinearity of coating material can simultaneously manipulate the locally resonant and Bragg scattering band gaps. Still, the nonlinearity of the matrix material mainly affects the Bragg scattering band gaps. The analysis of geometrical parameters indicates that the AM with holes of large size, thin coating, and a hard core of large radius benefits enhance its bandgap's tunability. The transmittances of the finite-sized AMs without damping drastically reduce in the frequency ranges of band gaps, which agree well with the numerical predictions of band gaps. The damping effect in the elastomeric matrix and coating materials can lead to the appearance of a new band region and the changes in the position and width of the band region. However, excessive damping in the coating and matrix materials suppresses the elastic wave propagation in the AM structures so that it is difficult to identify the band regions from the transmittance spectrums. The above researches demonstrate the roles of these influencing factors above in the process of elastic wave manipulation. They can help us design new AMs to meet the unique needs of noise and vibration control.

Optimization study of bandgaps properties for two-dimensional chiral phononic crystals base on lightweight design

The bandgap properties of phononic crystals are of great influence on the vibration and noise reduction performance for acoustic metamaterial structure. By analyzing the effects of material properties and geometric parameters on the complete bandgap width for phononic crystals, the maximizing the bandgap width problem is discussed. To achieve the goal of increasing the complete bandgap width, the bandgap optimization for chiral phononic crystals is investigated. The hexachiral phononic crystal for example, the material properties (density, wave velocity) and geometric parameters (scatterer diameter, ligaments or coating thickness) are defined as design variables, the bandgap optimization function which considering of mass factors is established. The multi-objective optimization problem is converted into a single objective optimization problem by normalizing the bandgap frequency coefficient. The bandgaps optimization is implemented, the first bandgap width is significantly increased and lower bound is decreased. Finally, the acoustic transmission analysis is conducted to verify the effectiveness of the proposed method. The results in present work provide more understanding the design and application of innovative acoustic metamaterial.

Sound transmission through a stiff double-panel structure periodically stabilized by negative stiffness module: Theoretical modeling

Analytic expressions are derived for models predicting the influence of periodically spaced structural links on sound transmission through a double-panel structure. The double panel has been configured to have two sets of structural links: a rib stiffener and negative stiffness component. The stiffener is identical and is spaced periodically at a distance. However, the negative element component is shifted by an amount q from the other set. The dynamic equation of the vibroacoustic of the system is formulated in terms of the space harmonics and by the principle of virtual work. The model is validated by comparing the model predictions with the existing result from the literature. Then, influences of the negative element, engineering safety, offset, and elevation angle are investigated. A new antiresonance with a huge sound transmission loss value can be engineered at the low-frequency region when these parameters are varied. In addition, the application of the stiff model is implemented for a periodic acoustic metamaterial structure. The negative stiffness inclusion can prevent wave propagation at low frequency. The periodic structure can be designed to obtain more and wider frequency bandgaps.

A Design Method for Sound Absorbing Structure at Low Frequency

Traditional acoustic absorbing materials are not effective for low-frequency engineering applications, but on the basis of the locally resonant principle, acoustic metamaterials can utilize the resonance of vibrators to dissipate acoustic energy and realize the subwavelength design of acoustic absorbers, therefore the acoustic metamaterials have great potential applications for noise reduction at low frequencies. This paper firstly employs the Bloch theory to investigate the effects of the parameters of the unit cell of the embedded membrane-and-mass metamaterials on the dispersion characteristics of the metamaterials, and the band gap is verified by the full wave finite element analysis. And then, a model of acoustic metamaterials is constructed by embeding an array of membrane-and-masses into a channel structure filled with acoustic materials. Next the transient frequency response analysis is performed to simulate the wave propagation in the model, the results show that the acoustic metamaterials can absorb the sound through the local resonance of the membrane-and-mass vibrators. Finally, an acoustic metamaterial maze structure is designed and analyzed, in the structure the membrane-and-mass array is embedded and the masses varies periodically. The research illustrates that the acoustic metamaterials with membrane-and-mass unit cells have excellent performances on the sound absorption at low frequency.

Design and characterization of tunable three-dimensional acoustic composite metamaterials

Purpose The purpose of this paper is to developing a kind of acoustic metamaterial with wide frequency band especially in low frequency region. At the same time, its the tunability of sound insulation frequency is achieved. Design/methodology/approach A three-dimensional (3D) acoustic metamaterial consisting of rigid frame, spherical attachment and thin film is proposed. The material parameters and the effect of the attachment hole on the forbidden band are investigated by finite element simulation. The sound insulation effect of the structure is validated by the combination of simulation and experiment. Findings The results show that the elastic modulus of the structural material determines the initial frequency of the forbidden band of the proposed 3D acoustic metamaterials. The lower the elastic modulus of the structural material, the lower the initial frequency of the forbidden band. The material parameters of the frame mainly affect the initial frequency of the first forbidden band, and the material parameters of the attachment will affect both the initial and termination frequency of the first forbidden band. Holes in the attachments reduce the band gap width. The characteristic curve moves down with the increase of subtracted mass. Research limitations/implications The findings may greatly benefit the application of the acoustic metamaterials in the fields of sound insulation and noise reduction. Originality/value This acoustic metamaterial structure has excellent sound insulation performance. At the same time, the single cell structure can be assembled into any shape. The structure can achieve sound selective filtering and combination control.

Low-frequency tunable locally resonant band gaps in acoustic metamaterials through large deformation

Acoustic metamaterials have subwavelength characteristics and hence they can be used for the low-frequency noise and vibration control. Usually, acoustic metamaterials have constant material properties and their dynamic responses are difficult to tune once manufactured, resulting in a fixed and narrow working band and limiting the number of possible applications. In this paper, by mean of elastomeric matrix with a square array of circular holes and resonating elements (including elastomeric coating and resonating mass), we design a new kind of tunable acoustic metamaterials with resonating elements embedded into the elastomeric matrix. Under equibiaxial compression, the deformation induced geometry and material nonlinearities of the elastomeric matrix and coating are intentionally exploited to tune the dynamic responses of acoustic metamaterials. We numerically investigate the stress–strain curves of the elastomeric matrix and coating, and the evolution of band gaps during deformation. Furthermore, the transmissions of the finite-sized acoustic metamaterial structures in the undeformed and deformed configurations are calculated to verify the tunability of the band structures. The numerical results indicate the locally resonant and Bragg scattering band gaps can be simultaneously controlled by deformation. The primitive low-frequency locally resonant band gap at 0.197∼0.226 is shifted and widened to 0.168∼0.364 in the deformed configuration and the width of band gap is increased 7.76 times, which provide a useful guideline for the low-frequency noise and vibration control.

Local acoustic field enhancement of single cell photoacoustic signal detection based on metamaterial structure

The photoacoustic signal from single cell plays an important role in medical diagnosis. Currently, the single-cell photoacoustic detection has limitations. To overcome the limits, an acoustic metamaterial model based on the acoustic waveguide theory for local enhancement of the ultrahigh frequency (UHF) acoustic signals was implemented in this study and it was applied to study the acoustic signal characteristics of individual cells. The local field enhancement effect at different frequencies and the specific mechanism of the model was evaluated in the simulation experiment. This proposing acoustic metamaterial structure of good practical value can be used in various applications for acoustic signal processing.