Class Of Acoustic Metamaterials Research Articles

Recent progress in acoustic metamaterials with ventilation

To develop sustainable cities, natural ventilation is being explored as one of the ways to reduce energy consumption and carbon dioxide emissions. Recently, acoustic metamaterials with ventilation have garnered attention for their potential to provide both ventilation and noise control. In addition, this class of acoustic metamaterials can be used as a machine enclosure for dissipating thermal load and reducing noise emission. This presentation focuses on the recent progress in acoustic metamaterials with ventilation over the last five years, highlighting three types: metamufflers, metapanels, and metacages. Furthermore, the challenges faced by metacages for real-world applications are presented.

Multi-objective optimal design of mechanical metafilters based on principal component analysis

In this paper, an advanced computational method is proposed, whose aim is to obtain an approximately optimal design of a particular class of acoustic metamaterials, by means of a novel combination of multi-objective optimization and dimensionality reduction. Metamaterials are modeled as beam lattices with internal local resonators coupled with the microstructure through a viscoelastic phase. The dynamics is governed by a set of integro-differential equations, that are transformed into the Z-Laplace space in order to derive an eigenproblem whose solution provides the dispersion relation of the free in-plane propagating Bloch waves. A multi-objective optimization problem is stated, whose aim is to achieve the largest multiplicative trade-off between the bandwidth of the first stop band and the one of the successive pass band in the metamaterial frequency spectrum. Motivated by the multi-dimensionality of the design parameters space, the goal above is achieved by integrating numerical optimization with machine learning. Specifically, the problem is solved by combining a sequential linear programming algorithm with principal component analysis, exploited as a data dimensionality reduction technique and applied to a properly sampled field of gradient directions, with the aim to perform an optimized sensitivity analysis. This represents an original way of applying principal component analysis in connection with multi-objective optimization. Successful performances of the proposed optimization method and its computational savings are demonstrated.

Sound absorption in Hilbert fractal and coiled acoustic metamaterials

We describe here a class of acoustic metamaterials with fractal Hilbert space-filling and coiled geometry with equal tortuosity for noise mitigation. Experiments are performed using a four-microphone impedance tube and benchmarked against non-viscous and viscothermal finite element models related to configurations spanning up to five fractal/geometry orders. We show that the acoustic absorption can be predicted by the resonance of the cavities associated with the tortuous paths. For a given fractal/geometry order, the acoustic absorption at specific frequencies is also enhanced by maximizing the difference between the minimum and maximum fluid particle velocity of the air inside the patterns. These principles can be used to design high-performance acoustic metamaterials for sound absorption over broad frequency ranges.

Effective metamaterial properties for non-local active acoustic media

Acoustic metamaterials are generally composed of complex arrays of subwavelength unit cells that can be cumbersome to analyze and simulate. Consequently, it is common practice to approximate metamaterials as homogeneous media with unique material properties, most commonly negative or near-zero density, negative or near-zero bulk modulus, or both negative or near-zero density and bulk modulus. In addition to easing the computational burden associated with complex metamaterial structures, effective material properties can provide deeper understanding of system behavior, providing additional insight and tools in the development of acoustic metamaterials. Here, we present a method for determining effective material properties for a non-local, active acoustic metamaterial. We contrast these properties with those typically associated with acoustic metamaterials and discuss how such properties are characteristic of a new class of acoustic metamaterials.

Broadband transmission-type coding metamaterial for wavefront manipulation for airborne sound

The recent advent of coding metamaterials, as a new class of acoustic metamaterials, substantially reduces the complexity in the design and fabrication of acoustic functional devices capable of manipulating sound waves in exotic manners by arranging coding elements with discrete phase states in specific sequences. It is therefore intriguing, both physically and practically, to pursue a mechanism for realizing broadband acoustic coding metamaterials that control transmitted waves with a fine resolution of the phase profile. Here, we propose the design of a transmission-type acoustic coding device and demonstrate its metamaterial-based implementation. The mechanism is that, instead of relying on resonant coding elements that are necessarily narrow-band, we build weak-resonant coding elements with a helical-like metamaterial with a continuously varying pitch that effectively expands the working bandwidth while maintaining the sub-wavelength resolution of the phase profile that is vital for the production of complicated wave fields. The effectiveness of our proposed scheme is numerically verified via the demonstration of three distinctive examples of acoustic focusing, anomalous refraction, and vortex beam generation in the prescribed frequency band on the basis of 1- and 2-bit coding sequences. Simulation results agree well with theoretical predictions, showing that the designed coding devices with discrete phase profiles are efficient in engineering the wavefront of outcoming waves to form the desired spatial pattern. We anticipate the realization of coding metamaterials with broadband functionality and design flexibility to open up possibilities for novel acoustic functional devices for the special manipulation of transmitted waves and underpin diverse applications ranging from medical ultrasound imaging to acoustic detections.

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 metamaterials based on phononic crystals.

This talk will review recent results on the topic of sound propagation through periodic arrays of sonic scatterers embedded in a fluid or a gas. These structures, which are called phononic crystals, define in the homogenization limit (i.e., for lattice separations much lower than the sound wavelength) a class of acoustic metamaterials with extraordinary properties. For example, they are solid structures that are used to engineer effective fluid-like systems with anisotropic dynamical mass density, a property that can be tailored by changing the parameters of the structure and the material composition of scatterers. A homogenization method based on multiple scattering theory has been developed and semianalytical expressions for the effective acoustic parameters will be given. By using these expressions it is easy to design novel refractive devices like gradient index sonic lens with a perfect match of impedance with the background. Results for a gradient index lens made of aluminum cylinders in air will be presented. Moreover, metamaterials with mass anisotropic have also been proposed to build broadband acoustic cloaks and a new type of crystals named radial sonic crystals, which are radially periodic structures that verify the Bloch theorem. [Work supported by ONR and by MICIIN of Spain.]

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.

Acoustic metamaterials for new two-dimensional sonic devices

It has been shown that two-dimensional arrays of rigid or fluidlike cylinders in a fluid or a gas define, in the limit of large wavelengths, a class of acoustic metamaterials whose effective parameters (sound velocity and density) can be tailored up to a certain limit. This work goes a step further by considering arrays of solid cylinders in which the elastic properties of cylinders are taken into account. We have also treated mixtures of two different elastic cylinders. It is shown that both effects broaden the range of acoustic parameters available for designing metamaterials. For example, it is predicted that metamaterials with perfect matching of impedance with air are now possible by using aerogel and rigid cylinders equally distributed in a square lattice. As a potential application of the proposed metamaterial, we present a gradient index lens for airborne sound (i.e. a sonic Wood lens) whose functionality is demonstrated by multiple scattering simulations.

Ultrasonic metamaterials with negative modulus

The emergence of artificially designed subwavelength electromagnetic materials, denoted metamaterials, has significantly broadened the range of material responses found in nature. However, the acoustic analogue to electromagnetic metamaterials has, so far, not been investigated. We report a new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance. These materials have an effective dynamic modulus with negative values near the resonance frequency. As a result, these ultrasonic metamaterials can convey acoustic waves with a group velocity antiparallel to phase velocity, as observed experimentally. On the basis of homogenized-media theory, we calculated the dispersion and transmission, which agrees well with experiments near 30 kHz. As the negative dynamic modulus leads to a richness of surface states with very large wavevectors, this new class of acoustic metamaterials may offer interesting applications, such as acoustic negative refraction and superlensing below the diffraction limit.