Tunable Acoustic Metamaterials Research Articles

Actively tunable sandwich acoustic metamaterials with magnetorheological elastomers

Actively tunable sandwich acoustic metamaterials with magnetorheological elastomers

A pneumatic soft acoustic metamaterial through modular design

Tunable acoustic metamaterials have excellent sound waves control and manipulation properties because of their deformations under different stimuli. Pneumatic actuation has recently attracted the attention due to its low-cost, fast in response and easy to integrate. However, due to the difficulty in fabricating soft enough scatterers and ensuring their airtightness, the experimental realization of pneumatic soft acoustic metamaterials remains a great challenge. In this paper, a pneumatic soft acoustic metamaterial is designed and its tunable band gap has been experimentally demonstrated. The designed pneumatic soft acoustic metamaterial comprises an array of soft inflatable rubber cavities in the background of air. And the scatterer in the soft acoustic metamaterials can deform by adjusting the air pressure, which can switch on or off the band gaps. The effects of scatterer shapes and orientations on the adjustable band gap are studied using numerical simulation methods. Furthermore, the modular design is introduced to ensure the flexibility of the designed soft acoustic metamaterials. And we fabricate modularized pneumatic soft acoustic metamaterials with square scatterers through the casting molding approach. The acoustic experiment results are agreement with the simulation results, which demonstrates that the band gap can be efficiently tuned when applying the air pressure. Additionally, the average transmission drops by about 20.2 dB in the maximum band gap of 3209.7–4639.7 Hz. This study provides a guide for designing and fabricating a pneumatic soft acoustic metamaterial, and confirms the feasibility and application potential of the design of acoustic devices by harnessing pneumatic actuation.

On-demand tunable metamaterials design for noise attenuation with machine learning

Metamaterials with structure-dominated properties provide a new way to design structures to obtain desired performance. To achieve a wide range of applications, on-demand tunable metamaterials would fulfill various and changing needs. The design of on-demand tunable metamaterials requires a higher-level understanding of the relationship between the properties of the metamaterials and the geometrical parameters, which in many cases are complicated and implicit. With the advancement of machine learning and evolutionary methods, it becomes possible to design on-demand tunable metamaterials. This paper designs on-demand tunable acoustic metamaterials for noise attenuation at varying frequencies by employing a genetic algorithm based neural network method. The C-shaped acoustic metamaterials with slidable shells are combined with the specifically designed tri-stable origami-inspired metamaterials to realize the on-demand tunable structure. Experiments were conducted and showed that the designed tunable metamaterials exhibited desired characteristics in different targeting frequency ranges. The present general methodology is expected to provide a route for on-demand tunable design while exploring more possibilities for the application of metamaterials.

A tunable slit-plate absorber with multiparameter cooperative control

Tunable acoustic metamaterials have a more flexible operating spectrum while still controlling acoustic waves via subwavelength thickness, but their operating range and acoustic absorption performance are limited by a single-parameter tuning mechanism. In this study, we first propose a slit-plate absorber with reconstructed porosity, which has a dynamic continuous tuning capability and is easier to achieve low-frequency resonance. Then, in order to improve its acoustic absorption performance, we study the tuning laws of numerous structural factors as well as their cooperative interaction. Local parameter changes to achieve impedance compensation and isotropic control to enhance frequency shift are presented to greatly improve the tunable range and sound absorption level. A tunable absorber with multi-parameter cooperative control is constructed using this method. The results reveal that continuous tuning of 210–920 Hz is possible at a total thickness of 25 mm, and the sound absorption coefficient in the tuning range is more than 0.8, and the thickness of the structure at 210 Hz is 1/65 of the absorption wavelength. Our work demonstrates a new method for improving the performance of adjustable acoustic metamaterials and promotes the use of acoustic metamaterials in low frequency changing noise environments.

Realization of the Quantum Spin Hall Effect Using Tunable Acoustic Metamaterials

We report pseudospin-dependent hybrid topological acoustic metamaterials constructed from tunable acoustic metamaterials and honeycomb-latticed sonic crystals. Using a theoretical model analogous to electrostatic interactions, we provide an efficient approach to evaluate intra- and interlattice couplings, whose competition is the major physical reason arousing the quantum spin Hall effect. The intralattice coupling is from the intrinsic design of the acoustic metamaterial, and the interlattice coupling is affected by multiple scattering among periodic scatterers. By independently manipulating the acoustic metamaterial and sonic crystal, we can successfully induce topological phase transitions at will. An alternative type of topological material is thus realized with great tunability. With identical building blocks for all hybrid topological acoustic metamaterials, a simple tuning of the corotation angle of each acoustic metamaterial can directly induce a topological phase transition, which makes the reconfiguration of the topological domains very convenient. A tunable multiport acoustic power divider is designed and demonstrated using numerical simulations and experiments. An alternative method to control the transmission rate and direction is thus reached and alternative routes for designing hybrid topological acoustic metamaterials with different functionalities and versatile applications are thus paved.

Acoustic computational metamaterials

<p indent="0mm">Although digital signal processors have been widely used to perform complex computing tasks, they still suffer from some limitations, including the complexity, low speed, and high-power consumption caused by expensive analog-to-digital converters. For this reason, there has recently been a strong interest in wave-based analog computing, which avoids analog-to-digital conversion and can perform massively parallel operations, especially based on artificially designed metamaterials. A new analog computing scheme based on sound waves has been proposed. This computing system, called computational metamaterials, can be as fast as the wave speed and as small as the wavelength. It can perform complex mathematical operations on incoming wave packets, and can even perform calculations on integral and differential equations. These functions are expected to realize a new generation of ultra-fast, compact and efficient computing hardware based on sound wave propagation. The development of acoustic computing metamaterials has gone from the computation of a single mathematical operation, the tunable design and computing circuit and computational network that integrate multiple computing blocks, to the topological insulator computational metamaterial proposed for the robustness of computation. The computation of a single mathematical operation can be divided into space and time domain. The difference of main design principles is dimension of time and space. The spatial acoustic computational metamaterials mainly require the realization of spatial distribution of Green’s function of required calculation in materials. And the time-domain acoustic computational metamaterial needs to design the frequency spectrum to realize the transfer function. Both the space domain and the time domain acoustic computing metamaterials realize operations, such as integration, derivative, and differential equations. The proposed tunable acoustic computational metamaterials are aimed at the problem that the function of the metamaterials cannot be changed while the structure is fixed. By introducing tunable parameters into the metamaterial, it realizes the integration of integral, derivative and other functions on the same metamaterial. The proposal of acoustic metamaterial calculation circuit and calculation network has raised the complexity of metamaterial calculation by another dimension. By introducing the design of acoustic circuit components, such as acoustic switches, the sound waves are controlled to propagate in different circuits, and complex operations, such as series and parallel connection of multiple different metamaterial blocks are realized. Acoustic metamaterial computing network combines the design ideas of acoustic holography and convolutional neural network (CNN). Each pixel unit of the metamaterial is equivalent to a neuron in the CNN, and the propagation of sound waves between different metamaterial layers in the medium is equivalent to the signal propagation between neurons in different layers. Through training and learning, machine learning tasks, such as handwritten number recognition, have been completed. In view of the computational robustness, the proposed topological insulator computational metamaterials, by introducing the boundary mode of topological protection, ensure that the acoustic computational metamaterials can still guarantee the accuracy of the calculation even when the structure parameter is disturbed. In the future, the research of nonlinear computational metamaterials will receive more and more attention, and it will be possible to apply to the fields of nonlinear equation solving, nonlinear filtering, image processing methods and so on. In this review, we discussed the latest developments in the field of acoustic computational metamaterials and studied the latest superstructures used to perform analog computing. We further introduced the applications of acoustic computational metamaterials, including image processing, edge detection, equation solving, and machine learning. Finally, we look forward to key research issues and possible future directions.

Magnetoelastic instabilities in soft laminates with ferromagnetic hyperelastic phases

We investigate the microscopic and macroscopic instabilities developing in magnetoactive elastomer (MAE) composites undergoing large deformations in the presence of an external magnetic field. In particular, we consider the MAEs with bi-phasic layered microstructure, with phases exhibiting ferromagnetic behavior. We derive an explicit expression for the magnetic field-induced deformation of MAEs with hyperelastic phases. To perform the magnetoelastic instability analysis, we employ the small-amplitude perturbations superimposed on finite deformations in the presence of the magnetic field. We examine the interplay between the macroscopic and microscopic instabilities. We find that the layered MAEs can develop microscopic instability with antisymmetric buckling modes, in addition to the classical symmetric mode. Notably, the antisymmetric microscopic instability mode does not appear in a purely mechanical scenario (when a magnetic field is absent). Furthermore, our analysis reveals that the wavelength of buckling patterns is highly tunable by the applied magnetic field, and by the properties and volume fractions of the phases. Our findings provide the information for designing materials with reconfigurable microstructures. This material ability can be used to actively tune the behavior of materials by a remotely applied magnetic field. The results can be utilized in designing tunable acoustic metamaterials, soft actuators, sensors, and shape morphing devices.

Tunable pipe-type acoustic metamaterials based on piezoelectric composite side-branches

The ability to tune the performance of acoustic metamaterials without structural modifications or complex active control circuits is a remarkable challenge. In this work, we present a square piezoelectric side-branch (PSB) pipe-type structure that consists of the piezoelectric composite sheets (PCSs) on its sidewall and propose the corresponding equivalent electroacoustic circuit model. Compared with the traditional side-branch, theoretical and numerical results show that the thickness of the PSB can be up to λ/200, and the tunable phase shift of the transmitted wave in the PSB pipe can achieve nearly 2π by adjusting the capacitances that are connected to the PCSs. The experiment for a square pipe with one PCS is also given to prove the phase control ability of the PSB model. Furthermore, we show that by merely adjusting the external capacitances of the PCSs, switching the functions between the 45° extraordinary transmission and the acoustic focusing for the same incident wave can be exactly realized. Our work provides a promising potential in active controls and integration designs of acoustic metamaterials and devices.

Tunable underwater acoustic metamaterials via quasi-Helmholtz resonance: From low-frequency to ultra-broadband

We report a lightweight tunable acoustic metamaterial with deep subwavelength thickness (e.g., λ/300) and strong load-bearing capability for underwater low-frequency and ultra-broadband acoustic perfect absorption. The metamaterial is constructed by introducing a rubber coating and an embedded metallic neck into a metallic hexagonal honeycomb Helmholtz resonator. Physically, the quasi-Helmholtz resonance triggered by the rubber coating together with the anti-phase cancellation caused by the embedded neck leads to superior sound absorption. Theoretical predictions of the metamaterial performance agree well with finite element simulation results. With fixed external morphology (e.g., honeycomb-cored sandwich panel) and fixed overall thickness (e.g., 50 mm), key internal geometrical parameters of the proposed metamaterial can be tailored to achieve tunable perfect absorption from, e.g., 100 Hz to 300 Hz. Further, combining such tunable quasi-Helmholtz resonance leads to ultra-broadband quasi-perfect absorption from, e.g., 306 Hz to 921 Hz. This work contributes to designing underwater acoustic metamaterials and controlling underwater acoustic waves.

Ultralow-frequency tunable acoustic metamaterials through tuning gauge pressure and gas temperature

Acoustic metamaterials possessing subwavelength characteristics can be used for low-frequency noise and vibration control. Acoustic metamaterials with adjustable dynamic properties can further enhance their possible applications. This paper designs a kind of tunable acoustic metamaterials consisting of the frame structure, airbag, and balancing weight. And their dynamic characteristics can be manipulated by tuning gauge pressure and gas temperature in the airbag. The calculation results indicate that the tunable acoustic metamaterials can effectively suppress the wave propagation and vibration in the ultralow-frequency band gap (about 13Hz∼90Hz) during manipulation. The complete band gap is the overlapping frequency range of an out-of- and in-plane band gaps. Numerical analyses indicate that the formation of out-of-plane band gap is due to the coupling between the out-of-plane resonant mode of balancing weight and the anti-symmetric Lamb mode of the frame structure. In contrast, the formation of an in-plane band gap is due to the coupling between the in-plane resonant mode of balancing weight and the symmetric Lamb mode of the frame structure. Meanwhile, the manipulation mechanism of these band gaps stems from the fact that through tuning gauge pressure or gas temperature, the structural stiffness of the airbag (torsional stiffness, in-plane, and out-of-plane stiffness) and frame structure (in-plane and bending stiffness) can be significantly tuned so that the modes related to them can be manipulated. The tunable acoustic metamaterials can be used for active control of low-frequency vibration and noise. It is possible to provide guidelines for designing other acoustic metamaterials to suppress low-frequency vibration and noise.

Characteristics of Band Gap and Low-Frequency Wave Propagation of Mechanically Tunable Phononic Crystals With Scatterers in Periodic Porous Elastomeric Matrices

Abstract The characteristics of passive responses and fixed band gaps of phononic crystals (PnCs) limit their possible applications. For overcoming this shortcoming, a class of tunable PnCs comprised multiple scatterers and soft periodic porous elastomeric matrices are designed to manipulate the band structures and directionality of wave propagation through the applied deformation. During deformation, some tunable factors such as the coupling effect of scatterer and hole in the matrix, geometric and material nonlinearities, and the rearrangement of scatterer are activated by deformation to tune the dynamic responses of PnCs. The roles of these tunable factors in the manipulation of dynamic responses of PnCs are investigated in detail. The numerical results indicate that the tunability of the dynamic characteristic of PnCs is the result of the comprehensive function of these tunable factors mentioned earlier. The strong coupling effect between the hole in the matrix and the scatterer contributes to the formation of band gaps. The geometric nonlinearity of matrix and rearrangement of scatterer induced by deformation can simultaneously tune the band gaps and the directionality of wave propagation. However, the matrix’s material nonlinearity only adjusts the band gaps of PnCs and does not affect the directionality of wave propagation in them. The research extends our understanding of the formation mechanism of band gaps of PnCs and provides an excellent opportunity for the design of the optimized tunable PnCs and acoustic metamaterials (AMMs).

Manually tunable ventilated metamaterial absorbers

For most acoustic metamaterials, once they have been fabricated, their operating frequencies and functions cannot be adjusted, which is an intrinsic barrier for the development of realistic applications. The study to overcome this limit has become a significant issue in acoustic metamaterial engineering. Although with the advance of metamaterials in the past two decades, a series of methods such as electric or magnetic control have been proposed, most of them can only work in the condition of no fluid passage. Some metamaterials with large transmission losses have been proposed, but the sounds are essentially reflected rather than absorbed. Here, to overcome this intrinsic difficulty, we propose a ventilated sound absorber that can be manually tuned in a large range after being manufactured. During the tuning that is achieved through an intricately designed slider, high-performance absorption and ventilation are both ensured. The tunable ventilated sound absorber is demonstrated experimentally and the effective model of coupled lossy oscillators can be employed to understand its mechanism. The manually tunable ventilated metamaterial has potential application values in various complicated pipe systems that require frequency adjustment and it also establishes the foundation for future development of active tunable ventilated acoustic metamaterials.

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.

Superbroad-band actively tunable acoustic metamaterials driven from poly (ethylene terephthalate)/Carbon nanotube nanocomposite membranes

Actively tunable acoustic metamaterials have attracted ever increasing attention. However, their tunable frequency range is quite narrow (tens of Hz) even under ultrahigh applied voltage (about 1,000 V). Here, we report a superbroad-band actively tunable acoustic metamaterials with the bandwidth over 400 Hz under a low voltage. In the actively tunable acoustic metamaterials, the acoustic membrane is a laminated nanocomposite consisting of a poly (ethylene terephthalate) (PET) and super-aligned carbon nanotube (CNT) drawn from CNT forest array. The laminated nanocomposite membrane exhibits adjustable acoustic properties, whose modulus can be adjusted by applying external electric field. The maximum frequency bandwidth of PET/CNT nanocomposite membrane reaches 419 Hz when applying an external DC voltage of 60 V. Our actively tunable acoustic metamaterials with superbroad-band and lightweight show very promising foreground in noise reduction applications.

Minimizing Indoor Sound Energy with Tunable Metamaterial Surfaces

The advent of broadband and tunable acoustic metamaterials, amenable to be fabricated into flat panel geometry, offers broad, emerging possibilities for room acoustics. In particular, the fine tuning of surface acoustic impedance becomes possible to optimize specific room acoustic goals. In this initial simulation study we examine the possibility of minimizing the averaged level of energy density (${L}_{e}$), both globally and locally in an enclosed room. The relationship curve between surface impedance and global ${L}_{e}$ shows an optimal value that can reduce ${L}_{e}$ by about $15$--$20\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$ over a wide audible frequency regime. Moreover, partial coverage of the room surface by the optimally tuned acoustic metamaterial already demonstrates considerable effectiveness. We compare and verify our numerical model with the statistical-diffusive acoustics model and find good agreement in the high-frequency regime. In the low-frequency regime, the energy inhomogeneity caused by wave interference can be utilized to create a quiet zone for specific targeted frequency by optimally tuning the surface impedance values at discretized locations on the four walls.

Tunable Elastic wave Bandgaps and Waveguides by Acoustic Metamaterials with Magnetorheological Elastomer

There is proposed a tunable acoustic metamaterials cell composed of a micro-electromagnetic coil as the mass element and a magnetorheological elastomer (MRE) as cladding layer. The cell resonance frequency is changed by changing the magnetic field generated by the micro-electromagnetic coil to control the shear modulus of the MRE cladding layer. The tunable bandgaps, transmission loss, and waveguides of the tunable acoustic metamaterials are investigated using the finite element method. The results show that the bandgap frequency and width, the maximum attenuation frequency and transmission loss increase with the increasing of applied magnetic field strength. Meanwhile, without changing the structure of the acoustic metamaterials, the localization, guidance, demultiplex and multiplex of elastic waves can be realized by changing the cell resonance frequency. These results will greatly contribute to the application of tunable acoustic metamaterials in vibration control, new waveguides, and filter components.

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.

Acoustic tunable metamaterials based on anisotropic unit cells

We propose a tunable acoustic metamaterial based on anisotropic elements. By changing the incident angle or rotating the unit cell, the refractive index can be tuned continuously. Such an acoustic metamaterial element can be used to design and fabricate tunable gradient-index lenses, such as fundamental functional lenses, convergent lens, divergent lens, and deflection lens. These functions can be switched in real time without changing the physical structure. A prototype of the acoustic lens is fabricated by using three-dimensional printing technology, and the regulatory mechanism is implemented to testify its versatility. Our finding opens an avenue for designing tunable acoustic metamaterials and is expected to have wide applications in acoustic devices.

Thermal tuning of negative effective mass density in a two-dimensional acoustic metamaterial with hexagonal lattice

The unique characteristics like periodic and locally resonant behaviors of acoustic metamaterials result in an anomalous response to the wave propagation and exhibit complete elastic wave bandgaps. However, these unusual properties arise from the geometry of the artificially designed metastructure and can be significantly affected under the exposure of the thermal environment. In this paper, we present a thermally tunable two-dimensional acoustic metamaterial with a hexagonal lattice. The numerical results demonstrated that by adjusting the externally applied temperature to the structure and imposing several different load conditions like thermal deformation and thermal stress, a tunable effective mass density of the metamaterial could be obtained. The proposed concept further extends the ongoing research in the field of tunable acoustic metamaterials.

A kind of tunable acoustic metamaterial for low frequency absorption

Low frequency noise is always an important factor affecting people’s quality of life. At present, the most widely used sound absorbing materials include polyurethane foam, trimeric amine, mineral cotton, textiles, cotton and special sound insulation materials. However, the sizes of these materials are generally large, and the sound absorption efficiencies are often low, especially in a low frequency range (below 2000 Hz). Acoustic metamaterial is a kind of artificial composite material, which is constructed by microunits whose dimensions are much smaller than the working wavelength. The results show that if the strong coupling condition between the resonant scatter and the waveguide is satisfied, the sound energy flowing through the metamaterial will be completely offset by the internal loss of the resonant scatter. Therefore, it is believed that this kind of acoustic metamaterial can solve the absorption problem of low-frequency sound waves. In order to solve this problem, researchers have conducted a lot of exploratory researches. However, most of the structural units that are constructed with acoustic metamaterials are passive, that is, once the material is processed and shaped, its properties are fixed and cannot be changed. This defect greatly limits the development of acoustical metamaterials, so it is urgent to study acoustical metamaterials whose material properties and the working frequency bands are flexibly adjustable. Although tunable acoustic metamaterials have been studied, few people have extended this research to the field of low-frequency tunable sound absorption. In our previous work, we systematically studied the acoustic properties of two kinds of acoustic artificial " meta-atoms”, namely, open hollow sphere model with negative equivalent elastic modulus and hollow tube model with negative equivalent mass density. The research shows that these two kinds of " meta-atoms” both have obvious sound absorption effect. According to our previous studies, in this paper we couple these two kinds of " meta-atoms” into a whole, and design a new nested model of open loop. The model has the advantages of simple structure and easy preparation. Through theoretical analysis, numerical simulation and experimental testing, it is found that the strong coupling resonance effects between these " meta-atoms” can be excited by the low frequency incident acoustic wave in the nested structure, thus achieving nearly perfect sound energy absorption. In addition, the relative impedance of the metamaterial can be changed by simply rotating the inner splitting ring around the axis, therefore the position of the absorption peak can be freely controlled in a wide frequency band. Because of its deep sub-wavelength size, the metamaterial is very useful for miniaturizing and integrating the low-frequency acoustic absorption devices. What is more, this model also lays a foundation for designing the broadband absorbers.