Active Acoustic Metamaterials Research Articles

Imaging with reconfigurable active acoustic metamaterials

We present a design procedure to obtain active metamaterial structures that cover a large range of linear and non-linear acoustic responses, and whose flexibility makes it suitable to numerous applications. The presentation will briefly outline the types of material properties enabled, and then focus on the potential of this approach to various imaging applications. The design method is illustrated experimentally using a very thin metamaterial slab whose functionality is set electronically. This ensures that multiple imaging techniques can be demonstrated using the same physical slab by simply swapping the electronic modules that determine the metamaterial behavior. Using this method, we implement a thin acoustic lens whose focal length can be controlled at will. In addition, we show how time reversal imaging techniques can be implemented in real-time and in a continuous manner. Finally, the slab is configured to act as an acoustic multiplexer that takes sound incident from multiple directions and funnels it in exactly one direction.

Finite element analysis of the macro fiber composite actuator: macroscopic elastic and piezoelectric properties and active control thereof by means of negative capacitance shunt circuit

The finite element method (FEM) model of a piezoelectric macro fiber composite (MFC) is presented. Using a specially developed numerical model, the complete set of macroscopic values of elastic compliance and piezoelectric tensors is computed. These values are useful in numerical FEM simulations of more complex systems such as noise and vibration suppression devices or active acoustic metamaterials, where the MFC actuator can be approximated by a plate-like uniform piezoelectric material. Using this approach, a great reduction of the FEM model complexity can be achieved. The computed numerical macroscopic values of the MFC actuator are compared with MFC manufacturerʼs data and with data obtained using different computational methods. A demonstration of active tuning of effective elastic constants of the piezoelectric MFC actuator by means of a shunt electric circuit is presented. The effective material constants are computed using the FEM model developed. The effect of the shunt circuit capacitance on the effective anisotropic Youngʼs moduli is analyzed in detail. A method for finding the proper shunt circuit adjustment that yields the maximum values of the MFC actuator Youngʼs modulus is shown. Possible applications to noise and vibration suppression are discussed.

Active acoustic metamaterials with tunable effective mass density by gradient magnetic fields

Magnetically controlled acoustic metamaterials are designed and experimentally studied. Magneto-acoustic metamaterials are fabricated by covering an aluminum circular ring with magnetorheological elastomer. The resonant frequency of the structured elastomer is actively tunable by external gradient magnetic field, allowing for values of effective mass density of metamaterials to be adjusted in the low-frequency region. A prestressed plate theory is proposed to explain the shifting of the resonant frequency induced by the magnetic field and coincides very well with the experimental results. It is found that the tunability of magneto-acoustic metamaterials is attributed to the competition between the magnetic-field-induced prestress and the structural flexural rigidity. The proposed magneto-acoustic metamaterials realize the dynamic tuning of effective mass density with non-contact and fast-response gradient magnetic fields, providing a degree of freedom for full control of sound.

Non-reciprocal and highly nonlinear active acoustic metamaterials

Unidirectional devices that pass acoustic energy in only one direction have numerous applications and, consequently, have recently received significant attention. However, for most practical applications that require unidirectionality at audio and low frequencies, subwavelength implementations capable of the necessary time-reversal symmetry breaking remain elusive. Here we describe a design approach based on metamaterial techniques that provides highly subwavelength and strongly non-reciprocal devices. We demonstrate this approach by designing and experimentally characterizing a non-reciprocal active acoustic metamaterial unit cell composed of a single piezoelectric membrane augmented by a nonlinear electronic circuit, and sandwiched between Helmholtz cavities tuned to different frequencies. The design is thinner than a tenth of a wavelength, yet it has an isolation factor of >10 dB. The design method generates relatively broadband unidirectional devices and is a good candidate for numerous acoustic applications.

Tunable active acoustic metamaterials

We describe and demonstrate an architecture for active acoustic metamaterials whose effective material parameters can be tuned independently over a wide range of values, including negative material parameters. The approach is demonstrated experimentally through the design and measurement of two types of unit cells that generate metamaterials in which either the effective density or bulk modulus can be tuned. We show that these unit cells achieve negative refraction, negative effective mass density, significantly nonunity bulk modulus, and are suitable for the design of negative refraction media and media with tunable gain and absorption. In this architecture, a transducer senses the pressure wave incident on the metamaterial and an electronic circuit manipulates the electric signal produced and drives a second transducer that creates the acoustic response consistent with the desired effective material parameters. The electronics between the two transducers allows easy dynamic tuning of these parameters. We show that this approach can be applied to both broadband and narrow band designs.

An effective gauge potential for nonreciprocal acoustics

It is shown that by using temporal modulation of phase velocity inside active acoustic metamaterials, an effective gauge potential can be produced to allow nonreciprocal acoustic wave propagation. Such effective gauge potential gives rise to the counterintuitive unidirectional Doppler shift. The proposed scheme provides a new class of time-varying metamaterials for nonreciprocal acoustics, which opens more possibilities for the one-way acoustic devices.

Active Acoustic Metamaterial With Simultaneously Programmable Density and Bulk Modulus

Acoustic metamaterials are those structurally engineered materials that are composed of periodic cells designed in such a fashion to yield specific material properties (density and bulk modulus) that would affect the wave propagation pattern within in a specific way. All the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a new class of composite one-dimensional active acoustic metamaterials (CAAMM) with effective densities and bulk moduli that are programmed to vary according to any prescribed patterns along its volume. A cylindrical water-filled cylinder coupled to two piezoelectric elements form a composite cell to act as a base unit for a periodic metamaterial structure. Two different configurations are considered. In the first configuration, a piezoelectric panel is flash-mounted to the face of the cylinder, while the other is side-mounted to the cylinder wall, introducing a variable stiffness along the wave propagation path. In the second configuration, the face-mounted piezoelectric panel remains unchanged, while the side-mounted panel is replaced with an active Helmholtz resonator with piezoelectric base panel. A detailed theoretical lumped-parameter model for the two configurations is present, from which the stiffness of both active elements is controlled via charge feedback control to yield arbitrary homogenized effective bulk modulus and density over a very wide frequency range. Numerical examples are presented to demonstrate the performance characteristics of the proposed. The CAAMM presents a viable approach to the development of effective domains with a controllable wave propagation pattern to suit many applications.

Acoustic metamaterials with circular sector cavities and programmable densities

Considerable interest has been devoted to the development of various classes of acoustic metamaterials that can control the propagation of acoustical wave energy throughout fluid domains. However, all the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a class of composite one-dimensional acoustic metamaterials with effective densities that are programmed to adapt to any prescribed pattern along the metamaterial. The proposed acoustic metamaterial is composed of a periodic arrangement of cell structures, in which each cell consists of a circular sector cavity bounded by actively controlled flexible panels to provide the capability for manipulating the overall effective dynamic density. The theoretical analysis of this class of multilayered composite active acoustic metamaterials (CAAMM) is presented and the theoretical predictions are determined for a cascading array of fluid cavities coupled to flexible piezoelectric active boundaries forming the metamaterial domain with programmable dynamic density. The stiffness of the piezoelectric boundaries is electrically manipulated to control the overall density of the individual cells utilizing the strong coupling with the fluid domain and using direct acoustic pressure feedback. The interaction between the neighboring cells of the composite metamaterial is modeled using a lumped-parameter approach. Numerical examples are presented to demonstrate the performance characteristics of the proposed CAAMM and its potential for generating prescribed spatial and spectral patterns of density variation.

Multicell Active Acoustic Metamaterial With Programmable Effective Densities

Considerable interest has been devoted to the development of various classes of acoustic metamaterials. Acoustic metamaterials are those structurally engineered materials that are composed of periodic cells designed in such a fashion to yield specific material properties (density and bulk modulus) that would affect the wave propagation pattern within in a specific way. All the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a new class of composite one-dimensional acoustic metamaterials with effective densities that are programmed to vary according to any prescribed patterns along the volume of the metamaterial. The theoretical analysis of this class of multilayered composite active acoustic metamaterials (CAAMM) is presented and the theoretical predictions are determined for an array of fluid cavities separated by piezoelectric boundaries. These smart self-sensing and actuating boundaries are used to modulate the overall stiffness of the metamaterial periodic cell and in turn its dynamic density through direct acoustic pressure feedback. The interaction between the neighboring layers of the composite metamaterial is modeled using a lumped-parameter approach. One-dimensional wave propagation as well as long wavelength assumptions are adapted in the current analysis. Numerical examples are presented to demonstrate the performance characteristics of the proposed CAAMM and its potential for generating prescribed spatial and spectral patterns of density variation. The CAAMM presents a viable approach to the development of effective acoustic cloaks that can be used for treating critical objects in order to render them acoustically invisible.

Acoustic cloak/anti-cloak device with realizable passive/active metamaterials

Utilizing the coordinate transformation method, together with exchange of variables between Maxwell's equations and the acoustic equations with axial-invariance in cylindrical coordinates, the acoustic parameters (anisotropic density and scalar bulk modulus) for an ideal cloak and an ideal anti-cloak are obtained. An anti-cloak allows the inside object to ‘see’ outside, but to be invisible from outside; whereas a cloak is invisible from outside, but ‘blind’ from inside. Utilizing a scattering algorithm developed in this paper, the pressure field calculation of the cloak/anti-cloak is performed and the concepts and characteristics of the acoustic cloak/anti-cloak are revisited. To be more easily achievable experimentally, a multilayered cloak/anti-cloak model with homogeneous isotropic materials is introduced, and its corresponding pressure distributions are calculated. Also, the total scattering cross-section curves for the multilayered cloak and anti-cloak over a certain frequency range are presented and compared. Finally, an active acoustic metamaterial made up of piezo-diaphragm cavity arrays is designed for the cloak/anti-cloak. Taking into account the coupling between adjacent cavity cells, a multi-control strategy for piezo-diaphragm cavity arrays is exploited, rendering possible wide ranges of effective densities and effective bulk moduli (or acoustic speeds), or even double-negative transformation medium (i.e. both density and bulk modulus parameters are negative). With such sets of active acoustic metamaterials, the cloak and anti-cloak may become both theoretically and experimentally realizable.

Analysis and experimental demonstration of an active acoustic metamaterial cell

Active acoustic metamaterials (AAMM) have been developed to overcome the limited frequency bandwidth characteristics of passive acoustic metamaterials. The AAMM rely in their operation on using piezoelectric active ingredients in a fluid-solid composite structure forming the basic building block of a larger metamaterial periodic arrangement. A prototype of AAMM composite cell is manufactured and active control strategies are implemented on the piezoelectric elements to vary its stiffness in order to control the effective dynamic density of the cell. Acoustic characterization of the developed AAMM cell is carried out by measuring its acoustic impedance and transmission loss and comparing the results with the predictions of a finite element model. The obtained experimental measurements and the predictions of the finite element model are in very good agreement for the considered frequency range. The transfer functions between the reference microphone in the impedance tube and the piezoelectric elements demonstrate the coupling nature inside the cell rendering it to a system of single acoustic properties acting as a single degree of freedom system. The proposed AAMM can be useful in manufacturing the next generation of acoustic cloaks and metamaterials with controllable directivity and dispersion characteristics.

Stability analysis of active acoustic metamaterial with programmable bulk modulus

Acoustic metamaterials (AMMs) have been considered as an effective means of controllingthe propagation of acoustical wave energy through metamaterials. However, most of thecurrently exerted efforts are focused on studying passive metamaterials with fixed materialproperties. In this paper, the emphasis is placed on the development of a new class ofone-dimensional acoustic metamaterials with effective bulk moduli that are programmed tovary according to any prescribed pattern along the volume of the metamaterial. Acousticcavities coupled with either actively controlled Helmholtz or flush-mounted resonators areintroduced to develop two possible configurations for obtaining active AMMs(AAMMs) with programmable bulk modulus capabilities. The resonators are providedwith piezoelectric boundaries to enable control of the overall bulk modulus of theacoustic cavity through direct acoustic pressure feedback. Theoretical analyses ofthese two configurations of AAMMs are presented using the lumped-parametermodeling approach. The presented analyses are utilized to study the stabilitycharacteristics of the two configurations in an attempt to define their stable regions ofoperation. Numerical examples are presented to demonstrate the performancecharacteristics of the proposed AAMM configurations and their potential forgenerating prescribed spatial and spectral patterns of bulk modulus variation.

Active acoustic metamaterials.

Extensive efforts are being exerted to develop various types of acoustic metamaterials to effectively control the flow of acoustical energy through these materials. However, all these efforts are focused on passive metamaterials with fixed material properties. The emphasis is placed here on the development of a class of acoustic metamaterials with tunable effective densities and bulk modulus in an attempt to enable the control of wave propagation. More importantly, the active metamaterials can be tailored to have increasing or decreasing variation of the material properties along and across the material volume. With such unique capabilities, physically realizable acoustic cloaks can be achieved and objects treated with these active metamaterials can become acoustically invisible. The theoretical analysis of this class of active acoustic metamaterials will be presented and the theoretical predictions are determined for an array of fluid cavities separated by piezoelectric boundaries. These boundaries control the stiffness of the individual cavity and in turn its dynamical density and bulk modulus. Various configurations will be considered to achieve different spectral and spatial control of the density and bulk modulus of this class of acoustic metamaterials. Extensive efforts are now exerted to build and test modules of these active acoustic metamaterials in order to build practical configurations of acoustic cloaks.

An Active Acoustic Metamaterial With Tunable Effective Density

Extensive efforts are being exerted to develop various types of acoustic metamaterials to effectively control the flow of acoustical energy through these materials. However, all these efforts are focused on passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a class of one-dimensional acoustic metamaterials with tunable effective densities in an attempt to enable the adaptation to varying external environment. More importantly, the active metamaterials can be tailored to have increasing or decreasing variation of the material properties along and across the material volume. With such unique capabilities, physically realizable acoustic cloaks can be achieved and objects treated with these active metamaterials can become acoustically invisible. The theoretical analysis of this class of active acoustic metamaterials is presented and the theoretical predictions are determined for an array of fluid cavities separated by piezoelectric boundaries. These boundaries control the stiffness of the individual cavity and in turn its dynamical density. Various control strategies are considered to achieve different spectral and spatial control of the density of this class of acoustic metamaterials. A natural extension of this work is to include active control capabilities to tailor the bulk modulus distribution of the metamaterial in order to build practical configurations of acoustic cloaks.

Multi-cell Active Acoustic Metamaterial with Programmable Bulk Modulus

Considerable interest has been devoted to the development of various classes of acoustic metamaterials that can control the propagation of acoustical wave energy through these materials. However, all the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this article, the emphasis is placed on the development of a new class of composite acoustic metamaterials with effective bulk moduli that are programmed to vary according to any prescribed pattern along the volume of the metamaterial. The composite consists of an acoustic cavity, which is coupled with an array of actively controlled Helmholtz resonator to enable the control of the effective bulk modulus distribution along the cavity. The theoretical analysis of this class of multi-cell composite active acoustic metamaterials (CAAMM) is presented and the theoretical predictions are determined when the Helmholtz resonators are provided with piezoelectric boundaries. These smart boundaries are used to control the overall bulk modulus of the cavity/resonator assembly through direct acoustic pressure feedback. The interaction between the neighboring cells of the composite metamaterial is modeled using a lumped-parameter approach. Numerical examples are presented to demonstrate the performance characteristics of the proposed CAAMM and its potential for generating prescribed spatial and spectral patterns of bulk modulus variation.

The structure of an active acoustic metamaterial with tunable effective density

A new class of one-dimensional active acoustic metamaterials (AAMMs) with programmable effective densities is presented. The proposed AAMM is capable of producing densities that are orders of magnitudes lower or higher than the ambient fluid. Such characteristics are achieved by using an array of fluid cavities separated by piezoelectric diaphragms that are controlled to generate constant densities over wide frequency bands. The piezodiaphragms are augmented with passive electrical components to broaden the operating frequency bandwidth and enable densities higher than the fluid medium to be generated. The use of these components is shown to be essential to maintain the closed-loop compliance of the piezodiaphragm away from the zone of elastic instabilities. The values of the passive components are selected on a rational basis in order to ensure a balance between the frequency bandwidth and control voltage. With this unique structure of the AAMM, physically realizable acoustic cloaks can be implemented and objects treated with these active metamaterials can become acoustically invisible.