An Overview on Acoustic Metamaterials

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.

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.

We present an analog of the quantum-mechanical reversed fractional stimulated Raman adiabatic passage (f-STIRAP) in an acoustic system by designing three-cavity couplers with space-varying coupling strengths, which are further structured to construct acoustic multifunctional metamaterials. The agreement between analytic and simulated results verifies that the propagation of the acoustic wave mimics the evolution of reversed f-STIRAP in quantum optics. The constructed acoustic metamaterials are of optional functions, beam focusing, beam splitting, or asymmetric transmission with switchable features, depending on the wavelength of the incident acoustic wave. Our work extends the application realm of STIRAP, which may have profound impacts on exploring quantum technologies for promoting advanced acoustic devices.

This paper proposes a auditory system based on microphone array and composed of three parts: noise suppression, sound source localization (SSL) and beamforming. The ego-noise is harmful to the auditory system embedded in a small robot and very hard to be suppressed. We have proposed a method to design active acoustic metamaterials (AAMMs) that perform extremely well in the ego-noise suppression in a small robot. In this case, our research mainly focuses on the issues of reliability and real time on auditory system algorithms. A 2-stage SSL algorithm is adopted in our system to reduce the complexity. Furthermore, the matrix inversion can be processed in parallel with signal sampling, which means very low latency between the output and the input.

A multiple scattering formulation of two-dimensional (2D) acoustic metamaterials is presented. This approach is comprehensive and can lead to frequency-dependent effective parameters (scalar bulk modulus and tensorial mass density), as it is possible to have not only positive or negative ellipsoidal refractive index, but also positive or negative hyperbolic refractive index. The correction due to multiple scattering interactions is included in the theory and it is demonstrated that its contribution is important only for lattices with high filling fractions. Since the surface fields on the scatterers are mainly responsible for the anomalous behavior of the resulting effective medium, complex scatterers can be used to engineer the frequency response. Anisotropic effects are also discussed within this formulation and some numerical examples are reported. A homogenization theory is also extended to electromagnetic wave propagation in 2D lattices of dielectric structures, where Mie resonances are found to be responsible for the metamaterial behavior.

Dissipative diatomic acoustic metamaterials for broadband asymmetric elastic-wave transmission

Thermoacoustic waveguides are systems of hollow tubes and thermally graded porous segments that can operate as active materials where acoustic waves receive energy from an external heat source. This work demonstrates that by adjusting the pore geometry several unique low-frequency propagation features arise from the complex-valued band structure of periodic thermoacoustic waveguides that reflect into the acoustic pressure field within finite-length systems. Numerical methods have been employed to model waveguides with porous segments constituted by cylindrical inclusions (parallel pins). In periodic structures, a critical frequency emerges where the sign of the refractive index in one direction of propagation changes, thus zero- and negative-unidirectional refractive index, unidirectional energy transport, and amplification/attenuation crossover effects may take place. On the other hand, the study of the acoustic pressure field shows that, for wave packets with either direction of propagation, finite-length waveguides may behave as active acoustic metamaterials with zero- or negative-refractive index. The acoustic pressure field in the waveguide, generated by an upstream source, may exhibit increasing amplitude and phase recovery farther away from the source, mimicking the field created by a downstream source, propagating upstream in a non-active medium.

Characteristics of Acoustic Metamaterials

Design of low-frequency and broadband acoustic metamaterials with I-shaped antichiral units

Three-dimensional fractal structure with double negative and density-near-zero properties on a subwavelength scale

Conventional porous materials are ineffective at absorbing low-frequency sounds, while acoustic resonators typically provide sound absorption within a narrow frequency band. These limitations present a significant challenge for controlling noise in various applications. This work investigates the acoustic properties of an active electro-mechano-acoustic material capable of broadband sound absorption. Previous works have relied on sensors and digital signal processing to externally control the material's sound absorption. Here, we develop an analytical electro-mechano-acoustic model of a system comprising a non-ideal sound source within an impedance tube and the active material. This model not only provides insights into the system's underlying physics but also yields an expression for the material's apparent reflection coefficient, thereby linking the acoustic, mechanical, and electrical parameters of the system components with the acoustic behaviour of the electroacoustic absorber. Using this expression, we derive the optimal input voltage characteristics required to control sound absorption without relying on wave sensing, external signal processing, or controllers. Theoretical predictions are successfully validated through numerical and experimental data. This work has potential applications in active acoustic treatments and tunable acoustic metamaterials for wave control.

In this paper, the band gaps in a mass-spring structure composed of resonator with elastic support are investigated. The band structure, effective parameters, and transmission spectra of the acoustic metamaterials are calculated numerically. The numerical results show that the elastic support and resonator in the structure generate the first and the second band gap, respectively. The effective parameters are analyzed to reveal the nature of this phenomenon and calculated to obtain the edge frequencies of band gaps. The transmission spectra are in accordance with the complex band structure. It is found that the attenuation factor is non-zero in band gap, and it determines the shape and magnitude of transmission ratio in band gap. The influences of the structural parameters on the band structure and attenuation factor are investigated numerically. Numerical results show that the band structure can be modulated by changing the structural parameters, and the topological structure will be changed when those parameters change greatly.

Multiresonant Layered Acoustic Metamaterial (MLAM) solution for broadband low-frequency noise attenuation through double-peak sound transmission loss response

Classical waves, including elastic waves (acoustic waves) and electromagnetic waves (optical waves and microwaves), are described by conventional wave-propagation functions. Elastic waves were the first waveforms to be understood in condensed matter and have a wide range of applications from industry to defense, from healthcare to entertainment. In 1987, the photonic crystal was proposed to describe the propagation of optical waves in refraction index-modulated periodic structures analogous to the propagation of electrons in real crystals. This situation recalls the classical work by Brillouin. Brillouin considered elastic waves in periodic strings, electromagnetic waves in electrical circuits, and electrons in crystals as a system, resulting in some important common concepts, such as the Brillouin zone, band gap, etc., which are generally shared by the various forms of waves: electrons as scalar waves, optical waves as vector waves, and elastic waves as tensor waves. Following on from photonic crystals, the concept of phononic crystals was conceived with elastic waves propagating in periodic structures modulated with periodic elastic moduli and mass densities. These artificially structured materials possess a number of important properties, such as band gaps, band edge states <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> , and the ability to slow the velocity of sound (slow wave effect). Furthermore, by creating artificially designed structures on a deep subwavelength scale, artificial acoustic `atoms' can be purposely engineered into acoustic metamaterials to dramatically change the excitation and propagation of acoustic waves, and thus give rise to subdiffraction-limited resolution and its related myriad novel effects, such as negative, negative elastic modulus, and negative mass density. Finally, Acoustic metamaterials and phononicc crystal are a newly emerging field, which have inherently abnormal and interesting physical effects that are important to basic research, and offer potential for applications in everyday life that might revolutionize acoustic materials.

We present the theoretical analysis of a periodic structure based on a transformational design of an axisymmetric system from a two-dimensional (2D) Sonic Crystal (SC). Applying an axial rotation of a 2D SC, we obtain a three dimensional (3D) axisymmetric structure made up of toroidal scatterers. Based on the propagating properties of the 2D system, we interpret the scattering produced by the 3D axisymmetric structure, and one can also use the homogenization approach in the long wavelength regime to design a refractive media with controlled effective parameters. We use both the multiple scattering theory, for the analysis of the 2D systems, and the finite elements methods, for the case of 3D axisymmetric structures. This system, due to the axial symmetry, could be useful to manage the radiation properties of sources presenting that symmetry. Moreover it may be useful by transforming in scale to different sizes, and as a consequence, to be applied at different ranges of frequencies.