Acoustic Wave Manipulation Research Articles

Valley-locked waveguide transport in acoustic heterostructures

Valley pseudospin, labeling the pair of energy extrema in momentum space, has been attracting attention because of its potential as a new degree of freedom in manipulating electrons or classical waves. Recently, topological valley edge transport of sound, by virtue of the gapless valley-locked edge states, has been observed in the domain walls of sonic crystals. Here, by constructing a heterostructure with sonic crystals, a topological waveguide is realized. The waveguide states feature gapless dispersion, momentum-valley locking, immunity against defects, and a high capacity for energy transport. With a designable size, the heterostructures are more flexible for interfacing with the existing acoustic devices than the domain wall structures. Such heterostructures may serve as versatile new devices for acoustic wave manipulation, such as acoustic splitting, reflection-free guiding and converging.

An Acoustic Metamaterial-Based Sensor Capable of Multiband Filtering and Amplification

Dispersive acoustic metamaterials (AMMs) with graded refractive index is promising in acoustic sensing enhancement by providing acoustic rainbow trapping (ART) effect. However, previous solutions to this type of AMMs are based on 2D unit cells that require infinite or very large size in the third dimension; they cannot integrate with conventional acoustic sensors without deteriorating the ability of acoustic wave manipulation. In order to advance their practical applications, this paper proposes an axisymmetric, microphone embedded AMM device with gradient profile as well as gradient plate thickness and gradient gap width along the axis. Both numerical simulations and experiments are performed to examine the ART effect of the proposed AMM device and demonstrate its excellent capacity of acoustic pressure amplification and multi-band acoustic filtering. Deploying a microphone in the gap does not disturb the acoustic field, so the proposed AMM sensor experimentally achieves remarkable acoustic gains over one order of magnitude. Sound waves of different frequencies are picked up by microphones at different gaps, realizing the real-time spatially splitting of the sound spectrum. The proposed AMM provides excellent immunity to broadband noise and a little weak suppression of the single tone noise while significantly amplifying acoustic signals of a given frequency.

Multifunction switching by a flat structurally tunable acoustic metasurface for transmitted waves

In this paper a flat structurally tunable acoustic metasurface is constructed based on the helical unit cell. The length of the acoustic channel can be tuned by the screw-in depth of the helix. Accordingly, the wave phase for the transmitted acoustic wave can be tuned and the wavefront can be manipulated. Then multifunctions such as anomalous refraction, point focusing, beam focusing and self-bending can be realized and switched just by screwing in or out the helixes. At the same time, the broadband operating frequency is also realized. The experiments for anomalous refraction and point focusing are also performed, and the results show that the designed metasurface is effective. The present studies have important applications in dynamic manipulation of acoustic waves by metasurfaces.

Experimental studies on surface-wave isolation by periodic wave barriers

Periodic structures often fabricated at the micro/nano-scale have opened new vistas in acoustic and electromagnetic wave manipulations due to band-gap properties. This study shows theoretically and experimentally that the propagation of destructive surface waves can also be controlled artificially using periodic wave barriers that modify the global properties of the background soil. To depict the attenuation property of surface waves, this study provides a complete dispersion analysis, the complex dispersion relation. Results show that, remarkably, the attenuation zone for surface waves includes not only the band gap but also other evanescent surface modes. This result agrees well with that in the experimental transmission spectra for both periodic trench barriers and periodic pile barriers. Due to their broadband attenuation performance, periodic wave barriers are expected to apply to vibration isolation in a wide range of systems.

Enhanced negative-index collimation by overlapped second band of square lattice sonic crystals

Acoustic wave collimation and enhancement are systematically investigated in the overlapped second and third bands of a two-dimensional square lattice sonic crystals by analyzing the band structure, equifrequency contour and field distribution. We find the collimation stems from a small negative-index of the second band, and an additional concave entrance can improve the collimation performance in the far field region. But for the third band, negative, zero, positive refraction and wave splitting may occur at the same time due to the simultaneous excitation of multiple Bloch modes, which ultimately destroy the collimation beam of the second band in the near field region. This work provides an insight into the collimation properties of the overlapped bands of sonic crystals with potential application in acoustic directional emission and wave manipulation.

Designing phononic crystal with anticipated band gap through a deep learning based data-driven method

Phononic crystal is a type of artificial heterogeneous material constituted by a periodic repetition of cells. This characteristic provides a possible solution to the accurate manipulation of acoustic and elastic waves. For this reason, phononic crystal is of application potentials in vibration and noise reduction, filtering, acoustic lens, acoustic imaging, and acoustic stealth, etc. It is thus of significance in the fields of information, communication, and medical applications. To design phononic crystal with anticipated manipulation characteristic has become a research hotspot in recent years. However, how to accurately manipulate acoustic and mechanical wave is still a major challenge for existing designing approaches. Assisted by image-based finite element analysis and deep learning, a data-driven approach is proposed in this study for designing phononic crystals. An auto-encoder is trained to extract the topological features from sample images. Finite element analysis is employed to study the band gaps of samples. A multi-layer perceptron is trained to establish the inherent relation between band gaps and topological features. The trained models are ultimately employed to design phononic crystals with anticipated band gaps. Not limited to this material, the proposed method could be further extended to design various structured mechanical materials with specific functionalities.

Asymmetric Absorption in Acoustic Metamirror Based on Surface Impedance Engineering

Angle-dependent asymmetric acoustic absorption attracts growing interest, due to its great importance in various applications where directional responses are required, such as acoustic antennas, sensing, and angle-encoded steganography (concealed messaging). However, studies of angular-asymmetric absorbing structures remain scarce in acoustics. This work demonstrates a planar acoustic metamirror that can realize asymmetric sound absorption by controlling the surface impedance profile. The proposed structure enriches the toolbox for acoustic wave manipulation, and the systematic design method may also be used for $e.g.$ acoustic focusing, acoustic carpet cloaking, and sound steering.

Programmable mode conversion and bandgap formation for surface acoustic waves using piezoelectric metamaterials

A spatially reversible and programmable piezoelectric metamaterial concept is introduced for the manipulation of surface acoustic waves to achieve on-demand wave mode conversion and reflection. The concept uses an array of inductive-shunted piezoelectric elements (with gradually varying inductors in space) attached to the surface of an elastic propagation domain. The value of each inductor directly controls the phase velocity of the Rayleigh wave locally as quantified through unit cell band diagram analysis that guides the design process. By varying the spatial inductance distribution, the proposed piezoelectric metamaterial domain can be programed to convert incident surface waves into bulk shear waves or reflect them completely. The location of surface-to-bulk wave mode conversion or wave reflection can be tailored by means of the inductance distribution, and the directional behavior in space can be reversed. The proposed concept may enable novel surface acoustic wave devices and filters, via digital or analog programmable shunt circuits.

Observation of topological locally resonate and Bragg edge modes in a two-dimensional slit-typed sonic crystal

We present the design, analysis, fabrication and testing of the two topological edge states in a sub-wavelength two-dimensional sonic crystal (SC) system based on the mechanism of local resonance and Bragg scattering. The slit-typed SC consists of artificial meta-molecules embedded in the air with a triangle lattice, which is designed by introducing three coupled Helmholtz resonators. Frequency-tunable edge modes are obtained by adjusting the slit of the structure. Experimental results agree well with the theoretical calculations. Our works may enrich the designing of acoustic topological insulators in a sub-wavelength region and have potential applications in acoustic wave manipulation in general.

Novel acoustic flat focusing based on the asymmetric response in parity-time-symmetric phononic crystals

We present a two-dimensional (2D) parity-time-symmetric (PT-symmetry) phononic crystals (PCs) with balanced gain and loss medium. Using the super cell method of rectangular lattice, we exhibit the thresholdless spontaneous PT-symmetry breaking in the band structure. The numerical results show that the asymmetric scattering properties obviously occur in a non-Hermitian system. At two specific incident frequencies, unidirectional reflectionless and perfect transmission behaviors exist individually in opposite directions, which are accompanied by a phase transition of π. Based on the generalized Snell’s law, combining such a PT-symmetric medium, we design a novel metamaterial crystal for PT-symmetric acoustic flat focusing. Its focus frequency can also be modulated by the gain/loss parameter. The novel flat focusing based on the PT-symmetry that we propose opens a new door for high-dimensional applications of non-Hermitian metamaterials in acoustic wave manipulation.

Floquet topological acoustic resonators and acoustic Thouless pumping.

Constructing the topological states can serve as a promising approach for robust acoustic wave transports and manipulations. Here, the authors develop a scheme to realize acoustic topological states and adiabatic Thouless pumping in acoustic Floquet resonator systems. The directional acoustic wave can be robustly manipulated and pumped adiabatically from one side to the opposite side due to the non-trivial topological nature. The physical mechanism behind these phenomena can be understood by effective one-dimensional Aubry-André-Harper Hamiltonian, with an additional synthetic dimension originating from Floquet spatially periodic modulation. This Aubry-André-Harper acoustic resonator system can be regarded as a projection from a two-dimensional topological Hofstadter model for the integer quantum Hall effect. The authors' scheme provides a promising method for synthesizing acoustic topological states for efficient acoustic wave manipulations. Introducing the topological mechanism to the control wave will become an alternative method besides the conventional effective acoustic parameter methods.

Asymmetric coding metasurfaces for the controllable projection of acoustic images

The projection of acoustic images is of relevance for a wide range of applications, including acoustic communication and medical imaging. Various acoustic projection devices that use holograms have been proposed; however, they inevitably involve multiple basic elements and lack reconfigurability. In this paper, we demonstrate that an acoustic coding metasurface that consists of a single type of asymmetric acoustic unit (AAU) can be used to project the desired acoustic images. The AAU is constructed using a perforated panel and coating unit cell, which can be tunable in terms of the transmission coefficient and reflection phase asymmetry. Patterned in regular and inverse forms, the AAU can function as the coding elements ``0'' and ``1,'' respectively. We demonstrate, both numerically and experimentally, the efficient projection of complex acoustic images such as the letters ``N,'' ``J,'' and ``U.'' More importantly, we develop a simple composite metasurface system to achieve the controllable encryption and decryption of acoustic images. Our proposed approach provides a design methodology for the coding metasurface and may have applications in acoustic security, information storage, and wave manipulation.

Tunable perfect negative reflection based on an acoustic coding metasurface

Artificial manipulation of acoustic waves is a highly active research area, in which negative reflection is an exotic phenomenon. Emerging acoustic coding metasurfaces (ACMs) show great power in controlling acoustic waves. Here, we propose an ACM, where the state “0” or “1” is achieved by pushing up or pressing down a rigid rectangular strip. The ACM with a tunable coding sequence can adjust the direction of negative reflection as well as suppress the specular reflection. The feasibility of the ACM is demonstrated by totally reflecting the incident acoustic waves from 22° to the directions of −39°, −47°, and −61°. Furthermore, the ACM can spread the incident white noise into its acoustic spectral components like rainbows. This work may open diverse routes toward reconfigurable acoustic metasurfaces that are critical for practical functional devices, which are promising in potential applications including acoustic beam steering, retro-reflection, and surface wave sensing.

Nonplanar acoustic metasurface for focusing

Acoustic metasurfaces have attracted considerable attention in recent years because of their unprecedented manipulation of acoustic waves within subwavelength planar structures. However, planar rigid structures are not compatible with human skin, which possesses dynamically varying and highly unconventional geometries. This limits the applicability of acoustic metasurfaces in a promising industry. In this paper, we describe a nonplanar acoustic metasurface for focusing based on the ability of metasurfaces to impart nontrivial phase shifts. We propose a common mapping method for the pattern design that enables focusing behind the free surface. By cutting hollowed-out patterns on a nonplanar metasurface membrane, forming a specialized transmitted phase distribution, we obtain the approximate constructive interference on the focal spot, thus implementing high-efficiency focusing behind an arbitrary nonplanar metasurface. Specifically, we demonstrate focusing on spherical, conical, and chaotic surfaces and discuss the scope of application of our design rule. Moreover, by applying the pattern to conical surfaces, we design an “umbrella structure” to implement high-efficiency focusing with a dynamic, tunable focal length. The proposed nonplanar metasurface not only expands the application range of metamaterials but also demonstrates the real-world applicability of cutting-edge metamaterial research.

Realizing passive direction-bias for mechanical wave propagation using a nonlinear metamaterial

The possibility of realizing amplitude-activated passive direction-bias in longitudinal elastic wave propagation using a nonlinear acoustic metamaterial is demonstrated. Applying the method of multiple scales, approximate analytical solutions are derived for the dispersion curve and bandgap shifts due to the presence of nonlinear hardening local oscillators within the metamaterial using its lumped parameter effective-mass model. The configuration for a structural waveguide consisting of a tuned combination of a nonlinear and a linear acoustic metamaterial that takes advantage of these shifts to produce amplitude-activated direction-bias in propagation was postulated and verified using discrete element simulations. A numerical routine to generate root profile geometries that enable contact-based hardening response in tip-loaded cantilever beam resonators was developed and implemented. Utilizing a hybrid fabrication process involving additively manufactured and machined components, a prototype test article was constructed. Experiments using a structural waveguide test rig verify the existence and extent of bandgaps and provide evidence for the passive direction-bias phenomenon. Follow-on investigations on configurations with different types of nonlinearities could pave the way for development of additional passive acoustic wave manipulation devices with enriched dynamics.

Acoustic metamaterials and additive manufacturing

Acoustic metamaterials (AMM) have captured the attention of a wide range of researchers for their potential to create materials with properties or functionality that exceed naturally occurring materials or conventional composites. Progress in acoustic metamaterials is largely the product of a combination of technological advances that have been made in the past three decades. Specifically, AMM research is a prototypical example of novel concepts in physics converging with advances in technology, primarily additive manufacturing and widespread access to robust computational tools. Additive manufacturing is a key component to this area of research since it allows for rapid build and test cycles as well as the production of elaborate structures for acoustic wave manipulation that follow from rigorous mathematical predictions. This talk will highlight physical concepts central to AMM research and the role that advanced manufacturing processes play in present and future research. It will then discuss the significant technical challenges that must be overcome if AMM are to be brought to their full potential, including the need to fabricate large amounts of subwavelength dynamic microstructure spanning orders of magnitude in length scale.

Inverse design method in acoustic wave manipulation

We present here the involvement of inverse design method with the acoustic wave modulation, to simplify how we get the required material parameters and design novel acoustic devices that can perform certain prescribed operation. As an example, the acoustic cloaking will be discussed. The existing acoustic invisibility design based on coordinate transformation method needs negative index media which can be realized through complicate structures. With inverse design method, we have designed a circular cloak device with positive index materials, that successfully make a circular scatterer acoustically invisible. This inverse design method takes a significant step towards realizing practical cloaking devices and would open up new possibilities for manipulating acoustic wavefront with simplified parameter distribution and optimized properties.

Horn-like space-coiling metamaterials toward simultaneous phase and amplitude modulation

Acoustic metasurfaces represent a family of planar wavefront-shaping devices garnering increasing attention due to their capacity for novel acoustic wave manipulation. By precisely tailoring the geometry of these engineered surfaces, the effective refractive index may be modulated and, consequently, acoustic phase delays tuned. Despite the successful demonstration of phase engineering using metasurfaces, amplitude modulation remains overlooked. Herein, we present a class of metasurfaces featuring a horn-like space-coiling structure, enabling acoustic control with simultaneous phase and amplitude modulation. The functionality of this class of metasurfaces, featuring a gradient in channel spacing, has been investigated theoretically and numerically and an equivalent model simplifying the structural behavior is presented. A metasurface featuring this geometry has been designed and its functionality in modifying acoustic radiation patterns experimentally validated. This class of acoustic metasurface provides an efficient design methodology enabling complete acoustic wave manipulation, which may find utility in applications including biomedical imaging, acoustic communication, and non-destructive testing.

Topologically protected edge states for out-of-plane and in-plane bulk elastic waves

Topological phononic insulators (TPnIs) show promise for application in the manipulation of acoustic waves for the design of low-loss transmission and perfectly integrated communication devices. Since solid phononic crystals exist as a transverse polarization mode and a mixed longitudinal-transverse polarization mode, the realization of topological edge states for both out-of-plane and in-plane bulk elastic waves is desirable to enhance the controllability of the edge waves in solid systems. In this paper, a two-dimensional (2D) solid/solid hexagonal-latticed phononic system that simultaneously supports the topologically protected edge states for out-of-plane and in-plane bulk elastic waves is investigated. Firstly, two pairs of two-fold Dirac cones, respectively corresponding to the out-of-plane and in-plane waves, are obtained at the same frequency by tuning the crystal parameters. Then, a strategy of zone folding is invoked to form double Dirac cones. By shrinking and expanding the steel scatterer, the lattice symmetry is broken, and band inversions induced, giving rise to an intriguing topological phase transition. Finally, the topologically protected edge states for both out-of-plane and in-plane bulk elastic waves, which can be simultaneously located at the frequency range from 1.223 to 1.251 MHz, are numerically observed. Robust pseudospin-dependent elastic edge wave propagation along arbitrary paths is further demonstrated. Our results will significantly broaden its practical application in the engineering field.

Liquid-assisted tunable metasurface for simultaneous manipulation of surface elastic and acoustic waves

A tunable and multi-functional one-dimensional metasurface, which is formed by engraving periodic semi-ellipse grooves on the surface of an aluminum half-space, is proposed in this paper. One characteristic of the metasurface is the manipulation of multi-physical fields, i.e. it could be utilized to manipulate surface elastic and acoustic waves simultaneously. The dispersion curves of the elastic and acoustic waves can be effectively tuned by adding liquids into the grooves. Based on the tunability different applications can be realized by adding different volumes of different liquids into the grooves. As an example, simultaneous rainbow trapping of the surface elastic and acoustic waves is demonstrated in the metasurface. Moreover, a resonant cavity where the elastic and acoustic waves are highly confined is reported. The proposed metasurface paves the way to the design of multi-functional devices for simultaneous control of elastic and acoustic waves.