Acoustic Cloaking Research Articles

Why active Willis metamaterials? A controllability and observability perspective

Recently, active Willis metamaterials (AWM) have been the focus of extensive investigations because of their unique electro-elastic coupling characteristics. However, the treatments of this class of materials have been carried out exclusively, in all the available literature, by approaches that do not rely on solid control theory basis. In this paper, the emphasis is placed on revealing very important control features that are inherent to this class of materials because of their Willis coupling characteristics. These features lie in the enhanced controllability and observability properties of the AWM as compared to non-Willis active materials. Such control properties enable the AWM to possess broad sensing and actuation capabilities that can lend this material to be an effective means for monitoring and controlling the behavior of numerous critical applications, such as acoustic cloaking, particularly when integrated with appropriate robust control strategies. A simple example of a piezoelectric-based AWM is presented to demonstrate its effective control capabilities and distinguish this class of materials from conventional materials. In the selected example, the AWM is structured from two dissimilar masses connected by a piezoelectric spring. Lagrange dynamics formulation is utilized to generate the equations governing the Willis coupling, the piezoelectric coupling, and reveal the inherent control features. With this developed controlled-based structure of the AWM, it is shown that the AWM can simultaneously monitor and control both the strain and velocity whereas the conventional active material, which is formed from two similar masses connected by a piezoelectric spring, can only measure and control the strain alone. It is envisioned that the revealed control metrics for the simple one-dimensional AMW example can serve as means for investigating the potential of AMW's of higher dimensionality.

CVAE-Based Inverse Design of Two-Dimensional Honeycomb Pentamode Metastructure for Acoustic Cloaking

In this work, a method for inverse design of two-dimensional honeycomb pentamode metastructures (HPM) based on the Conditional Variational Auto-Encoder (CVAE) is proposed to achieve acoustic cloaking. The parameter distribution of the perfect acoustic cloak with two-dimensional cylindrical Kohn-Shen-Vogelius-Weinstein (KSVW) mapping is first derived. The CVAE model framework is then established along with its loss function in terms of the design parameters of the HPM. The inverse design performance of the deep generative model is evaluated using a large number of random test samples based on finite element simulations, showing that the equivalent mechanical parameters obtained from inverse design are highly consistent with the target parameters of the perfect acoustic cloak. For the HPM cloak design given by the trained deep generative model, the total scattering cross section (TSCS) is significantly reduced as compared to the case without a cloak, thereby demonstrating the effectiveness of the CVAE-based inverse design of acoustic cloak.

Amplitude‐Phase Dual‐Channel Encrypted Acoustic Meta‐Holograms

AbstractEncrypted acoustic meta‐holograms have potential applications in confidential acoustic information communication, noise control, acoustic cloaking, acoustic illusion, etc. Conventional encrypted optical and acoustic meta‐holograms only focus on the encrypted hologram in a single channel, viz. modulating spatial amplitude to project a holographic image. Here, the concept of an amplitude‐phase dual‐channel encrypted acoustic meta‐hologram (DrEAM) is proposed, capable of generating, encrypting, and decrypting both amplitude and phase holographic images. This unique concept of “phase holographic image” introduces a new degree of freedom for meta‐holograms, leading to dual‐channel encrypted acoustic holography. Acoustic metasurface with simultaneous amplitude and phase modulations is employed to realize this dual‐channel encryption. The findings may offer the possibility to increase the capacity of the encrypted acoustic meta‐holograms, which can lead to practical applications in, for example, multi‐channel acoustic field communications, complex noise control, and acoustic illusions.

Rest length controlled tunable band structure in periodic tensegrity metastructure – a numerical study using a novel consistent stiffness and mass formulation

Elastic wave propagation in various periodic metastructures and its application to wave filtering, vibration isolation, acoustic cloaking, etc., have gained significant attention among researchers in recent years. Despite the presence of band gaps, traditional lattice structures lack dynamic tunability. The solution lies in using tensegrity as unit cells. This article presents analysis of tunable wave propagation through a metastructure formed by tessellation of a 2D tensegrity unit cell in one dimension. Equations of motion of the unit cell are derived with consideration of flexibility and distributed mass of the bars, which is often neglected in the literature. The formulation is extended for wave dispersion analysis using the Floquet–Bloch theory. Next, the order of the formulation is reduced for decoupled analysis of symmetric and antisymmetric modes. The dispersion relations for different modes have been validated with nonlinear as well as linearized frequency response analysis under impulse load. Next, the effect of rest length of strings and alternating arrangement of bars on band gaps is studied in detail.

A novel smart hybrid multimorph piezoelectric spherical shell cloak for broadband near-perfect underwater acoustic camouflage applications

Non-visual auditory camouflage plays a major role in the art of underwater deception. In this work, a hybrid active/semi-active omnidirectional cloaking shell structure composed of alternate complementary piezoelectric and smart viscoelastic (PZT/SVE) actuator layers is proposed that can effectively conceal a three dimensional underwater macroscopic object from broadband incident sound waves. The smart hybrid structure incorporates a finite sequence of fully active parallel-connected multimorph PZT constraining layers inter-stacked with semi-active SVE core layers both of which are collaboratively operative in the framework of a Particle Swarm Optimized (PSO) multiple-input multiple-output active damping control (MIMO-ADC) scheme. The elasto-acoustic modeling of the problem is conducted by coupling the spatial state space methodology based on the classical three-dimensional exact piezoelasticity theory with the wave equations for the inner and outer acoustic domains. The acoustic cloaking performance of proposed configuration is evaluated for four distinct classes of highly functional SVE interlayer materials with tunable (field-dependent) rheological properties, namely, magnetorheological elastomer (MRE), shape memory polymer (SMP), electrorheological fluid (ERF), and magnetorheological shear thickening polishing fluid (MRSTPF). Extensive numerical results reveal significant broadband reductions of the far-field backscattering amplitude in the (f∞θ=π,kexRex)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left|{f}_{\\infty }\\left(\ heta =\\pi ,{k}_{\ ext{ex}}{R}_{\ ext{ex }}\\right)\\right|)$$\\end{document} as well as the percentage error of external cloaked field (%Err)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\%\ ext{Err})$$\\end{document} by incorporating a sufficient number of smart multimorph PZT/SVE material layers. Furthermore, it is concluded that comparable low frequency acoustic cloaking effects is possible without expenditure of any external energy just by employing the entirely inactive MRSTPF-based cloak as an alternative to the semiactive or fully active multimorph PZT/SVE cloaks. The outcome of proposed study can advantageously serve as the first step towards practical development and experimental implementation of future high performance smart acoustic cloaking devices with expanded broadband near-perfect omnidirectional invisibility for three dimensional objects of diverse geometries.

Origami of multi-layered spaced sheets

Two-dimensional (2D) origami tessellations such as the Miura-ori are often generalized to build three-dimensional (3D) architected materials with sandwich or cellular structures. However, such 3D blocks are densely packed with continuity of the internal material, while for many engineering structures with multi-physical functionality, it is necessary to have thin sheets that are separately spaced and sparsely connected. This work presents a framework for the design and analysis of multi-layered spaced origami, which provides an origami solution for 3D structures where multiple flat sheets are intentionally spaced apart. We connect Miura-ori sheets with sparsely installed thin-sheet parallelogram-like linkages. To explore how this connectivity approach affects the behavior of the origami system, we model the rigid-folding kinematics using analytic trigonometry and rigid-body transformations, and we characterize the elastic-folding mechanics by generalizing a reduced order bar and hinge model for these 3D assemblies. The orientation of the linkages in the multi-layered spaced origami determines which of three folding paths the system will follow including a flat foldable type, a self-locking type, and a double-branch type. When the origami is flat foldable, a maximized packing ratio and a uniform in-plane shear stiffness can be achieved by strategically choosing the link orientation. We show possible applications by demonstrating how the multi-layered spaced origami can be used to build deployable acoustic cloaks and heat shields.

Induced effects of a rigid boundary on a pulsating spherical source

This work develops a comprehensive analytical formalism to study the boundary-induced effects on a pulsating spherical radiating source near a rigid planar boundary in an inviscid fluid. Using the modal multipole expansion method, the image method and the translational addition theorem for spherical wave functions, exact closed-form expressions are derived for the dimensionless axial acoustic radiation force function as well as the radiation, amplification and extinction energy efficiency factors. The source-boundary distance and the dimensionless size parameter are especially addressed in the numerical simulations for spherical sources with different vibration modes. It is shown that the axial radiation force is positive or negative, depending on the choice of parameters. Moreover, cases when the extinction acoustic power completely vanishes are predicted theoretically for dipole and quadrupole vibrations, which can be used to achieve acoustic invisibility in acoustic cloaking devices and acoustic stealth technologies. Combination of more than one vibration modes is also investigated to better simulate the acoustic manipulation in practical applications. This work can help us understand the underlying mechanism in manipulating the active spherical radiating sources in a bounded field.

Ultrathin acoustic metasurface carpet cloaking based on Helmholtz resonances

<sec>With the development of metamaterials, the acoustic cloaking has attracted extensive attention due to its novel physics and potential applications. In recent years, based on the phase compensation modulation from Generalized Snell’s law and coordinate transformation, the acoustic cloakings in underwater and air have been widely and deeply studied. However, there is still an urgent need to design acoustic cloaks that are thinner and less affected by the incident angle of acoustic waves. Further, the designed cloaks should have a wider operating band and be more suitable for irregular objects.</sec><sec>In this paper, an ultrathin curved acoustic metasurface carpet cloaking is studied by using of phase compensation modulation. The phase modulation is based on Helmholtz resonance (HR). The metasurface carpet is immersed in air, since the vibration mode of acoustic wave in the air is relatively single, thus the physical essence can be elucidated more clearly. The carpet cloak is composed of 52 Helmholtz resonant units, and the size of resonant unit is less than 0.2 of working wavelength.</sec><sec>The phase change of HR unit is solved analytically by using the Generalized Snell’s law, and confirmed by the Multiphysics COMSOL software. The parameter effects of HR unit on the phase change are studied, demonstrating that the phase change of HR unit is sensitive to the change of height and radius of HR unit, while the change of width of HR cavity neck can make the phase of HR unit change smoothly. Therefore, when building 52 HR units, the width of the HR cavity neck is designed, and the height and radius of HR unit stay fixed.</sec><sec>The simulating results demonstrate that the designed cloak works well in a frequency range from 5850 Hz to 7550 Hz. Also, we study the cloaking effect for oblique incidence, and the results show that the carpet cloak works well for incident angle less than 30°. To quantitatively analyze the bandwidth of the cloaking, we calculate the cosine similarity value. It elucidates that the value of the cloak is very close to that of the flat ground in a corresponding working frequency range. The cloak designed in this work is made of ultrathin Helmholtz Resonant structures. This cloak is simple and easy to realize and conducive to potential applications.</sec>

Sustainable Solutions in Sound Shielding: Harnessing Metamaterials for Acoustic Cloaking

The development of metamaterials promises to enable smallscale, worldwide industry-wide acoustic, electromagnetic, mechanical, and solar energy harvesting. Engineered structures surpass natural material limitations, offering capabilities unattainable in traditional counterparts. This paper explores metamaterials' manipulation of acoustic, electromagnetic, mechanical, and solar energy. Mechanical metamaterials convert strain into electrical energy, applicable from interstellar travel to terrestrial infrastructure. Precision-configured acoustic metamaterials efficiently harness dispersed acoustic energy, improving renewable energy methodologies. Integration into photovoltaic cells showcases metamaterials' solar potential, with innovative designs enhancing solar energy conversion efficiency. “Metamaterials” is a word used to describe artificial structures whose properties are based on the aggregate expression of individual components. Acoustic metamaterials are the term used for such constructions intended for the manipulation of acoustic waves. Controlled wave propagation is made possible by acoustic metamaterials, which is frequently not possible with bulk materials created chemically. This indicates that the wave propagation in acoustic metamaterials is directed and produces desired acoustic effects, independent of the mass-density properties of the material. The distinct properties of acoustic metamaterials have paved the way for the creation of practical solutions for a variety of uses, such as passive destructive interference, acoustic cloaking, sound focusing, low-frequency sound insulation, and biomedical acoustics. The kind of sound modification determines the general properties of an acoustic metamaterial. The properties of several of the most promising acoustic metamaterials from passive to active are introduced in this work. In order to achieve a sustainable future, it is necessary to combine environmentally friendly technologies with renewable energy sources for their final application. This is demonstrated by highlighting both the fundamental concepts and the physical models that were assessed.

Two-dimensional honeycomb lattice structure for underwater acoustic cloaking using pentamode materials

This research article presents an innovative and novel approach to achieve underwater acoustic cloaking using a two-dimensional honeycomb lattice structure with pentamode materials in the kHz frequency range. Underwater acoustic cloaking holds substantial importance in various applications, such as marine engineering, imaging, and military operations, making the development of an efficient underwater acoustic shell imperative. The proposed cloak consists of a pentamode titanium material honeycomb lattice embedded in an air background, submerged in water. To attain effective camouflage and regulate the phase and energy flow, impedance matching was applied to the overall geometry of the structure. By fine-tuning the structural parameters of the cloaking shell, derived from the effective mass velocity and density for recovering reflected waves, impedance matching with water was ensured. Through simulation calculations and optimization design, the average total scattering cross-section of the acoustic cloak is determined to be 0.1. The results demonstrate that the pentamode material-based cloaking approach is not only suitable and efficient in achieving the cloaking phenomenon but also enhances operator flexibility. The operating frequency bandwidth of the acoustic cloaking system is approximately 8 kHz for lattice constant a = 5 mm. These findings pave the way for further advancements in underwater acoustic cloaking technologies.

Polydimethylsiloxane Polymerized Emulsions for Acoustic Materials Prepared Using Reactive Triblock Copolymer Surfactants.

Porous polymers have interesting acoustic properties including wave dampening and acoustic impedance matching and may be used in numerous acoustic applications, e.g., waveguiding or acoustic cloaking. These materials can be prepared by the inclusion of gas-filled voids, or pores, within an elastic polymer network; therefore, porous polymers that have controlled porosity values and a wide range of possible mechanical properties are needed, as these are key factors that impact the sound-dampening properties. Here, the synthesis of acoustic materials with varying porosities and mechanical properties that could be controlled independent of the pore morphology using emulsion-templated polymerizations is described. Polydimethylsiloxane-based ABA triblock copolymer surfactants were prepared using reversible addition-fragmentation chain transfer polymerizations to control the emulsion template and act as an additional cross-linker in the polymerization. Acoustic materials prepared with reactive surfactants possessed a storage modulus of ∼300 kPa at a total porosity of 71% compared to materials prepared using analogous nonreactive surfactants that possessed storage modulus values of ∼150 kPa at similar porosities. These materials display very low longitudinal sound speeds of ∼35 m/s at ultrasonic frequencies, making them excellent candidates in the preparation of acoustic devices such as metasurfaces or lenses.

Acoustic cloaking design based on penetration manipulation with combination acoustic metamaterials

The acoustic wave transmission manipulation ability is the most important performance for the acoustic metamaterials. To manipulate the acoustic transmission, the combination acoustic metamaterials structures are involved, and the two-directional acoustic penetration cloaking scheme are constructed. The combination acoustic metamaterials include chiral metamaterials and self-collimation metamaterials, the acoustic wave transmission path are manipulated to bypass from the acoustic scattering target in the penetration cloaking, and the target scattering stealth problem are effectively solved. In the end, the acoustic transmission manipulation performances are verified by numerical analysis with a finite-width acoustic metamaterial structure plates. The results provide technical support for design and application for acoustic transmission manipulation with acoustic metamaterials.

An acoustic cloaking design considering flow effects by using topology optimization

A topology optimization method is utilized to design an acoustic cloak in the presence of background mean flows, highlighting both the scientific significance and practical importance of acoustic invisibility in fluids. This study focuses on understanding the inherent physical mechanism and considering realistic constraints for actual manufacturing, ultimately inspiring the development of an optimized cloak that is available for practical fabrication in the presence of flows. Here, we use the density-based topology optimization to assign specified materials and impose practical constraints on the material allocation to achieve the desired cloaking performance. The optimization problem is then efficiently solved using the gradient-based globally convergent method of moving asymptotes, leveraging the derivative information from the finite element simulation studies of the linearized acoustic potential equation. To validate the proposed method, several numerical simulations are conducted, exploring different frequencies, incident flow speeds, and incident angles of flow. The results confirm the effectiveness and efficiency of the optimization method, expanding the range of applications of the acoustic cloak and contributing to a deepened physical understanding of how it manipulates sound waves in flows.

Acoustic cloaking via a hybrid passive–active technique

Both passive and active methods have been employed extensively in recent times to create the effect of acoustic cloaking, with obstacles being made near-invisible over specific frequency ranges. Passive methods typically rely on metamaterials with complex microstructures, whereas active methods require an external power supply. Furthermore, a large number of active sources and sensors are typically required for active control. In this work, a hybrid method that combines passive and active approaches is developed. The passive acoustic cloak is designed theoretically via layered shells with alternating, functionally graded properties. The layer properties are informed by coupling transformation acoustics with homogenization theory and require unphysical values on the inner cloak wall. As such, some scattering occurs from the obstacle to be cloaked as well as due to the discontinuity in material properties across the shell boundaries. Using distributed monopole control sources, active control is then employed to nullify the non-zero scattered field outside the passive cloak domain. Effective cloaking is achieved using the hybrid passive–active control system.

A robust design strategy for active control of scattered sound based on virtual sensing.

Combining virtual sensing (VS) with scattered sound control enables active acoustic cloaking when there are limitations in sensor configurations. The remote microphone method and additional filter method (AFM) are two common VS methods, and both can be divided into the training and control stages; the consistency of the environments in these two stages is essential for the control system. This paper investigates the effects of uncertainties in the incidence angle of the detection wave on these two VS-based scattered sound control methods. Following the analysis, we propose a robust design strategy based on the optimal layout of physical sensors, and the placement scheme is chosen by minimax optimization. The feasibility of the proposed strategy is verified by numerical simulations of a finite-length cylindrical scattering model. The results demonstrate that the proposed strategy can effectively reduce the degradation of system performance over the examined range of variations in the detection waves. In particular, the AFM-based method, combined with optimal placement, shows a remarkable improvement in robustness. It improves the worst noise reduction by approximately 14.5 and 10.8 dB on average, respectively, compared with the uniform placement and the direct control method based on the Wiener solution.

Manipulation of acoustic wave scattering from cylindrical shells using a coating of FG magneto-electro-elastic composite materials: Active acoustic cloaking

Acoustically cloaking an object from view of a system that uses acoustic waves for detection is an attractive and practical capability. The aim of the current investigation is to hide a steel cylinder containing axial flow acoustically, which is achieved through the application of a coating of FG electro-magnetic material. This coating is employed to minimize the sound pressure that is scattered from the structure, and it comprises piezoelectric (BaTiO3) and magnetostrictive (CoFe2O4) make up the coating. Accordingly, the method presented is based on optimal control and incorporates an improved particle swarming optimization algorithm. The decision variables are the magnetic and electrical potentials on the inner and outer surfaces of the coating, whereas the cost function is the scattered acoustic pressure. In other words, what are the values of electrical and magnetic potentials on the inner and outer surfaces of the coating to minimize the scattered pressure? Subsequently, by defining the electrical and magnetic boundary conditions in a certain way, it is feasible to achieve the complete cancelation of scattered sound pressure. The governing equation of each layer is extracted using an orthotropic laminate composite model that is based on the 3-D theory of elasticity. A state-vector method is then applied to obtain the propagator matrix that links the state variables at each layer's surfaces. The results show that the proposed acoustic cloaking technique has an excellent capability for hiding the shell immersed in the fluid.

A wideband acoustic cloak based on radar cross section reduction and sound absorption

In this paper, by analogy with the theory of radar cross section (RCS) reduction in electromagnetics, we propose a design method for an acoustic invisibility cloak, which can achieve wideband acoustic stealth performance. The cloak consists of two sets of subwavelength metasurface units with phase differences within a range, which can achieve good RCS reduction in a wide band. At the same time, it overcomes the shortcomings of narrowband and high directional dependence of previous metasurface stealth cloaks based on phase gradient distribution. Using a simple resonant cavity metasurface structure, the invisibility cloak based on wideband scattering reduction and sound absorption is realized. Because of its simple structure and wide working frequency band, the RCS stealth cloak has wide engineering application potential.

Post-research reflexivity in qualitative research: Through cloaks and cross-threading

This interdisciplinary Note is a creative form of writing that engages in post-research reflexivity through a process that it terms ‘cross-threading’. Using the trope of a cloak, which it links back to the author’s childhood imaginings of having an invisibility cloak, it cross-threads through the medium of this cloak a series of thoughts and feelings about a recently concluded research project (led by the author) exploring some of the ways that victims-/survivors of conflict-related sexual violence demonstrate resilience. Drawing on empirical data from Bosnia-Herzegovina, it illustrates the utility of the cloak as a thinking practice in relation to some of the stories that interviewees told, and it highlights the relevance of the cloak as a way of thinking about resilience. It also discusses the cloak as an identity, and in so doing it draws attention to an important aspect of the research process that is rarely talked about – the feelings, emotions and anxieties that researchers might experience when a major study or project ends. This Note concludes by underlining the potential benefits to researchers of having an acoustic cloak.

Acoustic metafluid for independent manipulation of the mass density and bulk modulus

Tuning the mass density and bulk modulus independently is the key to manipulate the propagation of sound wave. Acoustic metamaterials provide a feasible method to realize various acoustic parameters. However, the relevant studies are mainly concentrated in air, and the huge impedance difference makes it difficult to directly extend these airborne structures to underwater application. Here, we propose a metafluid to realize independent manipulation of the mass density and bulk modulus underwater. The metafluid is composed of hollow regular polygons immersed in the water. By adjusting the side number of the hollow regular polygons and choosing proper materials, the effective mass density and bulk modulus of the metafluid could be modulated independently. Based on the flexible adjustment method, metafluids with same impedance but different sound velocities are designed and used to realize an underwater impedance-matched gradient index lens. In addition, by combining the proposed metafluid with other artificial structures, acoustic parameters with great anisotropy can be achieved, which is exemplified by the design and demonstration of an impedance-matched underwater acoustic carpet cloak. This work can expand the practicability of underwater metamaterials and pave the way for future potential engineering applications in the practical underwater devices.

Sound attenuation enhancement of acoustic meta-atoms via coupling.

Arrangements of acoustic meta-atoms, better known as acoustic metamaterials, are commonly applied in acoustic cloaking, for the attenuation of acoustic fields or for acoustic focusing. A precise design of single meta-atoms is required for these purposes. Understanding the details of their interaction allows improvement of the collective performance of the meta-atoms as a system, for example, in sound attenuation. Destructive interference of their scattered fields, for example, can be mitigated by adjusting the coupling or tuning of individual meta-atoms. Comprehensive numerical studies of various configurations of a resonator pair show that the coupling can lead to degenerate modes at periodic distances between the resonators. We show how the resonators' separation and relative orientation influence the coupling and thereby tunes the sound attenuation. The simulation results are supported by experiments using a two-dimensional parallel-plate waveguide. It is shown that coupling parameters like distance, orientation, detuning, and radiation loss provide additional degrees of freedom for efficient acoustic meta-atom tuning to achieve unprecedented interactions with excellent sound attenuation properties.