Active Acoustic Metamaterials Research Articles

Acoustic Pulling with a Single Incident Plane Wave

The rapid development of active acoustic metamaterials provides an alternative degree of freedom to manipulate scattering fields and acoustic radiation forces. With the multipole expansion method, acoustic radiation forces can be expanded in a series of coupling between adjacent pairs of scattering-field coefficients. Here, systems from two dimensional to one dimensional are explicitly considered. When shining a single acoustic plane wave, a necessary requirement to achieve negative force is that the particle has to be sufficiently active. A criterion of achieving the most negative or positive force by tuning the adjacent terms ``in phase'' or ``out of phase'' is derived. Thus, with the required scattering coefficients, the effective density and modulus corresponding to the largest pulling-pushing forces can be retrieved directly and function as a guide in the design of active metamaterials in one-dimensional systems. Our study can bridge the theoretical requirements of radiation force with the practical design of metamaterials.

Pneumatically-Actuated Acoustic Metamaterials Based on Helmholtz Resonators.

Metamaterials are periodic structures which offer physical properties not found in nature. Particularly, acoustic metamaterials can manipulate sound and elastic waves both spatially and spectrally in unpreceded ways. Acoustic metamaterials can generate arbitrary acoustic bandgaps by scattering sound waves, which is a superior property for insulation properties. In this study, one dimension of the resonators (depth of cavity) was altered by means of a pneumatic actuation system. To this end, metamaterial slabs were additively manufactured and connected to a proportional pressure control unit. The noise reduction performance of active acoustic metamaterials in closed- and open-space configurations was measured in different control conditions. The pneumatic actuation system was used to vary the pressure behind pistons inside each cell of the metamaterial, and as a result to vary the cavity depth of each unit cell. Two pressures were considered, P = 0.05 bar, which led to higher depth of the cavities, and P = 0.15 bar, which resulted in lower depth of cavities. The results showed that by changing the pressure from P = 0.05 (high cavity depth) to P = 0.15 (low cavity depth), the acoustic bandgap can be shifted from a frequency band of 150–350 Hz to a frequency band of 300–600 Hz. The pneumatically-actuated acoustical metamaterial gave a peak attenuation of 20 dB (at 500 Hz) in the closed system and 15 dB (at 500 Hz) in the open system. A step forward would be to tune different unit cells of the metamaterial with different pressure levels (and therefore different cavity depths) in order to target a broader range of frequencies.

Sharkskin-Inspired Magnetoactive Reconfigurable Acoustic Metamaterials.

Most of the existing acoustic metamaterials rely on architected structures with fixed configurations, and thus, their properties cannot be modulated once the structures are fabricated. Emerging active acoustic metamaterials highlight a promising opportunity to on-demand switch property states; however, they typically require tethered loads, such as mechanical compression or pneumatic actuation. Using untethered physical stimuli to actively switch property states of acoustic metamaterials remains largely unexplored. Here, inspired by the sharkskin denticles, we present a class of active acoustic metamaterials whose configurations can be on-demand switched via untethered magnetic fields, thus enabling active switching of acoustic transmission, wave guiding, logic operation, and reciprocity. The key mechanism relies on magnetically deformable Mie resonator pillar (MRP) arrays that can be tuned between vertical and bent states corresponding to the acoustic forbidding and conducting, respectively. The MRPs are made of a magnetoactive elastomer and feature wavy air channels to enable an artificial Mie resonance within a designed frequency regime. The Mie resonance induces an acoustic bandgap, which is closed when pillars are selectively bent by a sufficiently large magnetic field. These magnetoactive MRPs are further harnessed to design stimuli-controlled reconfigurable acoustic switches, logic gates, and diodes. Capable of creating the first generation of untethered-stimuli-induced active acoustic metadevices, the present paradigm may find broad engineering applications, ranging from noise control and audio modulation to sonic camouflage.

Performance mechanisms and stability criteria for highly sub-wavelength, broadband nonreciprocal non-local acoustic metamaterials

Active acoustic metamaterials have received considerable attention recently due in large part to their ability to generate nonreciprocal wave transmission. We have shown previously how the spatial separation of acoustic sensors and sources can be leveraged to generate highly sub-wavelength, broadband nonreciprocal behavior in an acoustic waveguide. We showed that by transmitting pressure signals from one location along the waveguide to a loudspeaker at another location, we could achieve a peak isolation of 35 dB with a Q10dB factor of just over 0.25. Here, we discuss the mechanisms by which this nonreciprocity is achieved and the conditions required for the system to be stable. We show how conclusions regarding the performance and stability characteristics of an idealized plane wave acoustic model of our system must be adjusted when accounting for the controlling electronics and loudspeaker dynamics used in the experimental testing.Active acoustic metamaterials have received considerable attention recently due in large part to their ability to generate nonreciprocal wave transmission. We have shown previously how the spatial separation of acoustic sensors and sources can be leveraged to generate highly sub-wavelength, broadband nonreciprocal behavior in an acoustic waveguide. We showed that by transmitting pressure signals from one location along the waveguide to a loudspeaker at another location, we could achieve a peak isolation of 35 dB with a Q10dB factor of just over 0.25. Here, we discuss the mechanisms by which this nonreciprocity is achieved and the conditions required for the system to be stable. We show how conclusions regarding the performance and stability characteristics of an idealized plane wave acoustic model of our system must be adjusted when accounting for the controlling electronics and loudspeaker dynamics used in the experimental testing.

Recent Advances in Active Acoustic Metamaterials

Since its first demonstration of an acoustic metamaterial in the early 21st century, it is widely used for sound wave manipulation purposes in many applications such as aerospace, automotive, defense, marine, etc. However, the traditional acoustic metamaterials display acoustic characteristics for restricted use because of their fixed structures. For real-world applications, the active sound wave manipulation is desirable. In recent years, active acoustic metamaterials (AAMs) have garnered attention owing to their unique design and material characteristics, which result in various dynamic responses against the incoming sound wave. This paper aims to provide an overview of the fundamental concept of active metamaterials, describing the multiple tuning mechanisms and design strategies, and highlighting their potential applications. The current fabrication challenges and future outlook in this promising field are also discussed.

Optimal vibration suppression in adaptable acoustic metamaterials by a complex wavenumber

Acoustic metamaterials are composite materials exhibiting effective properties and acoustic behavior not found in traditional materials. Through periodic subwavelength resonant inclusions, acoustic metamaterials enable steering, cloaking, lensing, and frequency band control of acoustic waves. A common drawback of acoustic metamaterials is that the properties are limited to narrow frequency bands. Investigation of practical active and adaptable acoustic metamaterials is valuable in achieving wider operation frequency bands. Here, a one dimensional metamaterial is analyzed using a finite element method. The purpose of the metamaterial is to minimize vibration. From the finite element method formulation, a complex wavenumber is calculated. The unit cell considered is active and adaptable via piezoelectric actuators attached to negative capacitance shunt circuits. Therefore, the stiffness of the unit cell is tunable through selection of the circuit parameters. Choosing the negative capacitance shunt circuit elements to maximize the attenuative part of the complex wavenumber leads to maximum vibration suppression. So far, it has been found that maximizing the attenuative part of the wavenumber gives unrealizable specifications for the shunt circuit. To prevent this, the goal of this work is to constrain the optimization problem using experimentally obtained circuit stability bounds. The experimental stability results and optimization results will be presented. Acoustic metamaterials are composite materials exhibiting effective properties and acoustic behavior not found in traditional materials. Through periodic subwavelength resonant inclusions, acoustic metamaterials enable steering, cloaking, lensing, and frequency band control of acoustic waves. A common drawback of acoustic metamaterials is that the properties are limited to narrow frequency bands. Investigation of practical active and adaptable acoustic metamaterials is valuable in achieving wider operation frequency bands. Here, a one dimensional metamaterial is analyzed using a finite element method. The purpose of the metamaterial is to minimize vibration. From the finite element method formulation, a complex wavenumber is calculated. The unit cell considered is active and adaptable via piezoelectric actuators attached to negative capacitance shunt circuits. Therefore, the stiffness of the unit cell is tunable through selection of the circuit parameters. Choosing the negative capacitance shunt circuit ...

Nonreciprocal metamaterial with programmable nonlinear virtual resistors

Emphasis is placed on the development of a class of active acoustic diodes and metamaterials in an attempt to control the flow and distribution of acoustic energy in acoustic cavities and systems. The proposed active nonreciprocal acoustic metamaterial (ANAM) cell consists of only one-dimensional acoustic cavity provided with active flexible boundaries. These boundaries are made from piezoelectric bimorphs with the inner layers which interact directly with cavity acting as sensors for monitoring the pressures of the propagating acoustic waves. The outer layers of the bimorphs provide the necessary control actions to an array of programmable nonlinear shunted resistors. These resistors are programmable and are designed in such a manner to introduce simultaneous nonlinear damping and cubic hardening stiffness effects. A lumped-parameter model of the ANAM cell is developed to control the nonreciprocal characteristics of the cell by the selection of the proper balance between the nonlinear damping and stiffness effects. Lyapunov stability criterion is used to generate the structure of such a balanced control strategy. The Harmonic Balance Method is used to predict the limit cycle behavior of the ANAM, the backbone characteristics and the stable limits of the control gains. Numerical examples are presented to demonstrate the effectiveness of the proposed ANAM in tuning and programming the directivity, flow, and distribution of acoustic energy propagating though the metamaterial.

Active acoustic metamaterial with tunable effective density using a disturbance rejection controller

A class of active acoustic metamaterials (AAMMs) is developed with desirable controlled distributions of effective dynamic densities. The proposed AAMM consists of an array of acoustic cavities separated by piezoelectric boundaries and arranged to form a one-dimensional wave guide. The stiffness of the flexible piezoelectric boundaries is controlled to generate desirable density distribution along the wave guide. In this paper, a disturbance rejection control strategy is formulated to achieve a closed-loop control of the effective density of this class of acoustic metamaterials while rejecting the effect of the wave pressure disturbances. The time response characteristics of a unit cell of the AAMM are investigated for various parameters of the controller in order to demonstrate the merits of the proposed controller. A natural extension of this work is to include active control capabilities to control also the bulk modulus distribution along the metamaterial in order to build practical configurations of acoustic cloaks.

Extraordinary wave phenomena in active acoustic metamaterials

Non-Hermitian sonic metamaterials are artifical acoustic media that exploit gain and loss as an extra degree of freedom to induce novel physical effects that cannot be obtained with lossless or trivial lossy systems. In this talk, we will review our recent theoretical and experimental results about one-dimensional non-Hermitian acoustics, highlighting the ability of active metamaterials to overcome limitations of passive metamaterials by engineering the acoustic power flow at the microscopic scale. We will discuss various experiments employing a set of electroacoustic resonators in a pipe, engineered to provide tailored distributions of acoustic gain or loss, and discuss the potentials of such systems in terms of sound manipulation. Altogether, our work points out the large potential and rich physics of non-Hermitian acoustic metamaterials.

Active nonreciprocal acoustic metamaterials using a switching controller.

A class of active acoustic diodes and metamaterials is developed to control the flow and distribution of acoustic energy in acoustic cavities and systems. Such development departs radically from the currently available approaches where the non-reciprocities are generated by hard-wired designs, favoring one transmission direction which is dictated by the arrangement of the hardware and hence it cannot be reversed, or without the presentation of rigorous control theory analysis. The proposed active nonreciprocal acoustic metamaterial (ANAM) cell consists of a one-dimensional acoustic cavity provided with active flexible boundaries. These boundaries are made from piezoelectric bimorphs interacting with the cavity to monitor the pressures of the propagating acoustic waves. The outer layers of the bimorphs provide the necessary control actions by direct application of the appropriate control voltage on each layer or by proper connection of nonlinearly activated shunted networks of switching resistors. The control of the switching is carried out using a Switching Mode Control (SMC) strategy. In this strategy, a lumped-parameter model of the ANAM cell is developed to predict the nonreciprocal characteristics of the cell by proper selection of the slope of the switching surfaces. Numerical examples are presented to demonstrate the merits of the proposed SMC.

Perspective: Acoustic metamaterials in transition

Acoustic metamaterials derive their novel characteristics from the interaction between acoustic waves with designed structures. Since its inception seventeen years ago, the field has been driven by fundamental geometric and physical principles that guide the structure design rules as well as provide the basis for wave functionalities. Recent examples include resonance-based acoustic metasurfaces that offer flexible control of acoustic wave propagation such as focusing and re-direction; parity-time (PT)-symmetric acoustics that utilizes the general concept of pairing loss and gain to achieve perfect absorption at a single frequency; and topological phononics that can provide one-way edge state propagation. However, such novel functionalities are not without constraints. Metasurface elements rely on resonances to enhance their coupling to the incident wave; hence, its functionality is limited to a narrow frequency band. Topological phononics is the result of the special lattice symmetry that must be fixed at the fabrication stage. Overcoming such constraints naturally forms the basis for further developments. We identify two emergent directions: Integration of acoustic metamaterial elements for achieving broadband characteristics as well as acoustic wave manipulation tasks more complex than the single demonstrative functionality; and active acoustic metamaterials that can adapt to environment as well as to go beyond the constraints on the passive acoustic metamaterials. Examples of a successful recent integration of multi-resonators in achieving broadband sound absorption can be found in optimal sound-absorbing structures, which utilize causality constraint as a design tool in realizing the target-set absorption spectrum with a minimal sample thickness. Active acoustic metamaterials have also demonstrated the capability to tune bandgaps as well as to alter property of resonances in real time through stiffening of the spring constants, in addition to the PT symmetric acoustics that can achieve unprecedented functionalities. These emergent directions portend the transitioning of the field from the stage of novelty demonstrations to imminent applications of some acoustic metamaterials to select real-world problems, supported by an active research endeavor that continues to push the boundary of possibilities.

Active acoustic metamaterials using sensor-driver architecture

Acoustic metamaterials can, in principle, overcome the fundamental limitations of passive structures and can enable novel applications that are hard or impossible with passive media. However, active acoustic metamaterials remain elusive, and the few implementations going beyond basic material parameter tunability lag significantly behind the true potential of active media. We will present a design method that results in active metamaterials whose acoustic response is electronically programmed to cover a very large range of non-linear acoustic responses. Specifically, metamaterials obtained with this approach can be configured to have almost any second and higher harmonic response, making them very effective and versatile non-linear acoustic media. We demonstrate experimentally this design methodology in an application in which an acoustically thin metamaterial funnel responds to incident sound waves by producing collimated second harmonics. Past demonstrations of this type of active metamaterials will also be briefly reviewed.

Active vibration control by a tunable piezoelectric shunt metamaterial

Acoustic metamaterials are composite materials exhibiting effective properties and acoustic behavior not found in traditional materials. Primarily through periodic subwavelength resonant inclusions, acoustic metamaterials can enable steering, cloaking, lensing, and frequency band control of acoustic waves. However, a common drawback of acoustic metamaterials is that effectiveness is limited to narrow frequency bands. Thus, investigation of practical active and adaptable acoustic metamaterials is valuable in achieving wider operation frequency bands. Here, a metamaterial consisting of active tunable piezoelectric shunts is investigated numerically and experimentally from the unit cell level. A physical model of the unit cell is developed using the finite element method. From the finite element model, the wave finite element method is applied to compute the dispersion and forced response of the periodic structure. It is demonstrated that the shunts introduce an additional degree of freedom by which adaptable bending wave attenuation can be accomplished. Since the periodic shunts are only effective at certain frequency bands, a known optimization method is implemented to tune the shunts. Additionally, a new optimization scheme is compared to the existing scheme found in literature.

A sound future for acoustic metamaterials

The field of acoustic metamaterials borrowed ideas from electromagnetics and optics to create engineered structures that exhibit desired fluid or fluid-like properties for the propagation of sound. These metamaterials offer the possibility of manipulating and controlling sound waves in ways that are challenging or impossible with conventional materials. Metamaterials with zero, or negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. And active acoustic metamaterials use external control and power to create effective material properties that are fundamentally not possible with passive structures. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and, critically, converting exciting laboratory experiments into practically useful devices. In this presentation, I will outline the recent history of the field, describe some of the designs and properties of materials with unusual acoustic parameters, discuss examples of extreme manipulation of sound, and finally, provide a personal perspective on future directions in the field.

Feedback control of an active acoustic metamaterial

Acoustic metamaterials have recently been of significant interest due to their potential ability to exhibit behavior not found in naturally occurring materials. This extends to the realization of acoustic cloaks, but perhaps of greater industrial impact is their ability to achieve high levels of noise control performance. In particular, previous research has demonstrated the high levels of transmission loss that can be achieved by an array of locally resonant elements. However, these passive metamaterials are inherently limited in performance due to both losses and their static nature. Therefore, there has been an increasing interest in active acoustic metamaterials, which can allow both increased performance and adaptability. Recent work has investigated the integration of active elements into a passive resonator-based metamaterial, and it has been demonstrated using a feedforward control architecture that significant increases in the level and bandwidth of transmission loss are achievable. However, in many practical noise control applications, it is not possible to obtain a time advanced reference signal that is required for a feedforward control implementation. Therefore, this paper will explore the design of a feedback control architecture that is applicable to the active resonator based acoustic metamaterial and demonstrate the potential performance of such a system.

Experimental demonstration of one-dimensional active plate-type acoustic metamaterial with adaptive programmable density

A class of active acoustic metamaterials (AMMs) with a fully controllable effective density in real-time is introduced, modeled, and experimentally verified. The density of the developed AMM can be programmed to any value ranging from −100 kg/m3 to 100 kg/m3 passing by near zero density conditions. This is achievable for any frequency between 500 and 1500 Hz. The material consists of clamped piezoelectric diaphragms with air as the background fluid. The dynamics of the diaphragms are controlled by connecting a closed feedback control loop between the piezoelectric layers of the diaphragm. The density of the material is adjustable through an outer adaptive feedback loop that is implemented by the real-time evaluation of the density using the 4-microphone technique. Applications for the new material include programmable active acoustic filters, nonsymmetric acoustic transmission, and programmable acoustic superlens.

Modeling and design of two-dimensional membrane-type active acoustic metamaterials with tunable anisotropic density.

A two-dimensional active acoustic metamaterial with controllable anisotropic density is introduced. The material consists of composite lead-lead zirconate titanate plates clamped to an aluminum structure with air as the background fluid. The effective anisotropic density of the material is controlled, independently for two orthogonal directions, by means of an external static electric voltage signal. The material is used in the construction of a reconfigurable waveguide capable of controlling the direction of the acoustic waves propagating through it. An analytic model based on the acoustic two-port theory, the theory of piezoelectricity, the laminated pre-stressed plate theory, and the S-parameters retrieval method is developed to predict the behavior of the material. The results are verified using the finite element method. Excellent agreement is found between both models for the studied frequency and voltage ranges. The results show that, below 1600 Hz, the density is controllable within orders of magnitude relative to the uncontrolled case. The results also suggest that simple controllers could be used to program the material density toward full control of the directivity and dispersion characteristics of acoustic waves.

Membrane- and plate-type acoustic metamaterials.

Over the past decade there has been a great amount of research effort devoted to the topic of acoustic metamaterials (AMMs). The recent development of AMMs has enlightened the way of manipulating sound waves. Several potential applications such as low-frequency noise reduction, cloaking, angular filtering, subwavelength imaging, and energy tunneling have been proposed and implemented by the so-called membrane- or plate-type AMMs. This paper aims to offer a thorough overview on the recent development of membrane- or plate-type AMMs. The underlying mechanism of these types of AMMs for tuning the effective density will be examined first. Four different groups of membrane- or plate-type AMMs (membranes with masses attached, plates with masses attached, membranes or plates without masses attached, and active AMMs) will be reviewed. The opportunities, limitations, and challenges of membrane- or plate-type AMMs will be also discussed.

Controlling sound with acoustic metamaterials

Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Active acoustic metamaterials reconfigurable in real time

A major limitation of current acoustic metamaterials is that their acoustic properties are either locked into place once fabricated or are only modestly tunable, tying them to the particular application for which they are designed. We present a design approach that yields active metamaterials whose physical structure is fixed, yet their local acoustic response can be changed almost arbitrarily and in real time by configuring the digital electronics that control the metamaterial acoustic properties. We demonstrate this approach experimentally by designing a metamaterial slab configured to act as a very thin acoustic lens that manipulates differently three identical, consecutive pulses incident on the lens. Moreover, we show that the slab can be configured to simultaneously implement various roles, such as that of a lens and a beam steering device. Finally, we show that the metamaterial slab is suitable for efficient second harmonic acoustic imaging devices capable of overcoming the diffraction limit of linear lenses. These advantages demonstrate the versatility of this active metamaterial and highlight its broad applicability, in particular, to acoustic imaging.