Acoustic Wave Manipulation Research Articles

Research advances in acoustic metamaterials and metasurface

Acoustic metamateiral (AM) is an artificially structured material with the unique properties that cannot be found in nature materials, such as negative refraction, slab focusing, super-resolution imaging, cloaking, inverse Doppler effect, etc. In this paper we first review the research advances in AM in recent 20 years and then mainly discuss the properties of the meta-atom AM (MAAM), meta-molecule AM (MMAM), meta-atom cluster AM, and meta-molecule cluster AM. The MAAM consists of local resonant meta-atoms, whose resonant frequency is related to the geometry size of the structure. The MAAM presents the transmission dip and inversed phase near the resonant frequency. The meta-atoms discussed in the paper contain the split hollow sphere and hollow tube (HT), which can be used to realize the AM with single negative modulus and AM with single negative mass density near the frequency, respectively. The effective parameter of the MAAM is calculated from the transmission and reflection data in experiment according to the homogeneous-medium theory. By combining the two kinds of meta-atoms together, the assembled two-layered composite AM presents a transmission peak similar to the electromagnetic metamaterial in the overlapping resonant frequency region. The effective parameters calculated by experimental data demonstrate that the composite AM could realize simultaneously negative modulus and negative mass density near the peak frequency. In the double-negative band, this kind of double-negative AM can faithfully distinguish the acoustic sub-wavelength details (/7). Furthermore, by coupling the two kinds of meta-atoms in a structure, we design a flute-like meta-molecule structure of perforated hollow tube, which can be used to fabricate double-negative AM in high or low frequency band. The experimental results also show that the double-negative AM has the properties of flat focusing and negative refraction effect. Based on the weak interaction of the meta-atoms, the meta-atom cluster AM can be fabricated by arraying different sized meta-atoms. The meta-atom cluster AM composed of different sized meta-atoms of SHSs can realize multi-band or broadband negative modulus, and the different sized meta-atoms of HTs can realize broadband negative mass density. Similarly, the meta-molecule cluster AMs are constructed with seven kinds of flute-like perforated hollow tubes, which can overcome the limitations of arbitrary broadband negative bulk modulus and mass density to provide a region of inverse Doppler effects. It is also shown that the inverse frequency shift values will be enhanced with the increase of frequency. As the resonant unit can realize the effect of discontinuous phase, it can be used to design acoustic metasurface (AMS) to control the acoustic wavefronts at will and realize the anomalous manipulation of acoustic waves. Finally, we introduce the research status and tendency of AMS in coming years.

Steering elastic SH waves in an anomalous way by metasurface

Metasurface, which does not exist in nature, has exhibited exotic essence on the manipulation of both electromagnetic and acoustic waves. In this paper, the concept of metasurface is extended to the field of elastic SH waves, and the anomalous refractions of SH waves across the designed elastic SH wave metasurfaces (SHWMs) are demonstrated numerically. Firstly, a SHWM is designed with supercells, each supercell is composed of four subunits. It is demonstrated that this configuration has the ability of deflecting the vertical and oblique incident waves in an arbitrary desired direction. Then, a unique SHWM with supercell composed of only two subunits is designed. Numerical simulation shows its ability of splitting the vertical and oblique incident waves into two tunable transmitted wave beams, respectively. In the process of steering SH waves, it is also found that two kinds of leakages of transmitted waves across the designed SHWM will occur in some particular situations, which will affect the desired transmitted wave. The mechanisms of the leakages, which are different from that of the common high-order diffraction mentioned in existing literatures, are revealed. The current study can offer theoretical guidance not only for designing devices of directional ultrasonic detection and splitting SH waves but also for steering other kinds of classical waves.

Acoustic analog computing based on a reflective metasurface with decoupled modulation of phase and amplitude

The use of metasurfaces has allowed the provision of a variety of functionalities by ultrathin structures, paving the way toward novel highly compact analog computing devices. Here, we conceptually realize analog computing using an acoustic reflective computational metasurface (RCM) that can independently manipulate the reflection phase and amplitude of an incident acoustic signal. This RCM is composed of coating unit cells and perforated panels, where the first can tune the transmission phase within the full range of 2π and the second can adjust the reflection amplitude in the range of 0–1. We show that this RCM can achieve arbitrary reflection phase and amplitude and can be used to realize a unique linear spatially invariant transfer function. Using the spatial Fourier transform (FT), an acoustic analog computing (AAC) system is proposed based on the RCM together with a focusing lens. Based on numerical simulations, we demonstrate that this AAC system can perform mathematical operations such as spatial differentiation, integration, and convolution on an incident acoustic signal. The proposed system has low complexity and reduced size because the RCM is able to individually adjust the reflection phase and amplitude and because only one block is involved in performing the spatial FT. Our work may offer a practical, efficient, and flexible approach to the design of compact devices for acoustic computing applications, signal processing, equation solving, and acoustic wave manipulations.

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.

Phononic Crystal Made of Multilayered Ridges on a Substrate for Rayleigh Waves Manipulation

We present a phononic crystal to achieve efficient manipulation of surface acoustic waves (SAW). The structure is made of finite phononic micro-ridges arranged periodically in a substrate surface. Each ridge is constructed by staking silicon and tungsten layers so that it behaves as one-dimensional phononic crystal which exhibits band gaps for elastic waves. The band gap allows the existence of resonance modes where the elastic energy is either confined within units in the free end of the ridge or the ones in contact with the substrate. We show that SAW interaction with localized modes in the free surface of the ridge gives rise to sharp attenuation in the SAW transmission, while the modes confined within the ridge/substrate interface cause broad band attenuations of SAW. Furthermore, we demonstrate that the coupling between the two kinds of modes within the band gap gives high SAW transmission amplitude in the form of Fano-like peaks with high quality factor. The structure could provide an interesting solution for accurate SAW control for sensing applications, for instance.

Directional Acoustic Wave Manipulation by a Porpoise via Multiphase Forehead Structure

For acoustic waves, in general the source must be much larger than the wavelength to produce directional waves, yet porpoises have efficient biosonar systems that break this size rule. How? This study reveals that the porpoise's forehead is a natural gradient-index material, and key to production and control of directional beams. The whale's compression of the multiphase structure of its forehead effectively manipulates the view angle of the beam. Aside from advancing our knowledge of cetaceans, these results could inspire the development of human-made metamaterials for our own subwavelength applications.

Breaking the fundamental limit of space-coiling metamaterial: 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. Despite the successful demonstration of phase engineering using metasurfaces, amplitude modulation remains overlooked. This work explores the feasibility of simultaneous phase and amplitude modulation using space-coiling metamaterials. In the case of conventional space-coiling metamaterials, we observed a fundamental bound on the transmission coefficient, which precludes full wavefront manipulation. Herein, we present a novel class of metasurfaces featuring a modified space-coiling structure and enabling full acoustic control with simultaneous phase and amplitude modulation. The functionality of this class of metasurfaces, featuring a gradient in channel spacing, has been theoretically and numerically investigated and an equivalent model simplifying the structural behavior is presented. Furthermore, a metasurface featuring this novel geometry has been designed and its functionality in modifying acoustic radiation patterns simulated. The class of acoustic metasurface demonstrated in this work provides a new design methodology enabling complete acoustic wave manipulation, which may find utility in a range of applications including biomedical imaging, acoustic communication and non-destructive testing.

Subwavelength sound screening by coupling space-coiled Fabry-Perot resonators

We explore broadband and omnidirectional low frequency sound screening based on locally resonant acoustic metamaterials. We show that the coupling of different resonant modes supported by Fabry-Perot cavities can efficiently generate asymmetric lineshapes in the transmission spectrum, leading to a broadband sound opacity. The Fabry-Perot cavities are space-coiled in order to shift the resonant modes under the diffraction edge, which guaranty the opacity band for all incident angles. Indeed, the deep subwavelength feature of the cavities leads to avoid diffraction that have been proved to be the main limitation of omnidirectional capabilities of locally resonant perforated plates. We experimentally reach an attenuation of few tens of dB at low frequency, with a metamaterial thickness fifteen times smaller than the wavelength . The proposed design can be considered as a new building block for acoustic metasurfaces having a high level of manipulation of acoustic waves.

Mathematical operations for acoustic signals based on layered labyrinthine metasurfaces

Acoustic devices such as acoustic diodes, switches, and logic gates can manipulate acoustic signals extraordinarily and show potential in acoustic engineering. However, pioneering works suffer from the limitations of simple functionality, high losses, and geometrical complexity. Here, we present an acoustic analog computing system based on acoustic metasurfaces constructed from layered labyrinthine metamaterials. The system can perform complex mathematical operations, such as spatial differentiation, integration, and convolution on acoustic signals. The proposed system offers a practical, efficient, and flexible solution of designing compact devices for acoustic computing applications, signal processing, and acoustic wave manipulations.

Subwavelength diffractive acoustics and wavefront manipulation with a reflective acoustic metasurface

Acoustic metasurfaces provide useful wavefront shaping capabilities, such as beam steering, acoustic focusing, and asymmetric transmission, in a compact structure. Most acoustic metasurfaces described in the literature are transmissive devices and focus their performance on steering sound beam of the fundamental diffractive order. In addition, the range of incident angles studied is usually below the critical incidence predicted by generalized Snell's law of reflection. In this work, we comprehensively analyze the wave interaction with a generic periodic phase-modulating structure in order to predict the behavior of all diffractive orders, especially for cases beyond critical incidence. Under the guidance of the presented analysis, a broadband reflective metasurface is designed based on an expanded library of labyrinthine acoustic metamaterials. Various local and nonlocal wavefront shaping properties are experimentally demonstrated, and enhanced absorption of higher order diffractive waves is experimentally shown for the first time. The proposed methodology provides an accurate approach for predicting practical diffracted wave behaviors and opens a new perspective for the study of acoustic periodic structures. The designed metasurface extends the functionalities of acoustic metasurfaces and paves the way for the design of thin planar reflective structures for broadband acoustic wave manipulation and extraordinary absorption.

Acoustic Holographic Rendering with Two-dimensional Metamaterial-based Passive Phased Array.

Acoustic holographic rendering in complete analogy with optical holography are useful for various applications, ranging from multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. Conventional approaches rely on a large number of active transducers and phase shifting circuits. In this paper we show that by using passive metamaterials as subwavelength pixels, holographic rendering can be achieved without cumbersome circuitry and with only a single transducer, thus significantly reducing system complexity. Such metamaterial-based holograms can serve as versatile platforms for various advanced acoustic wave manipulation and signal modulation, leading to new possibilities in acoustic sensing, energy deposition and medical diagnostic imaging.

Demonstration of acoustic holographic rendering with two-dimensional metamaterial-based passive phased array

Acoustic holograms in analogy with optical holograms are useful for a variety of applications, such as multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. We previously presented the designs and simulation results of several metamaterial-based acoustic holograms (J. Acoust. Soc. Am. 138, 1751 (2015)). Here, we demonstrate the experimental realization of two of them: one projects a complex pattern on the image plane, while the other focus energy onto multiple circular spots of different sizes. We showcase the holographic reconstruction measurements carried out in an anechoic chamber. The demonstrated passive holograms, without phase-shifting circuitry and transducer arrays, are advantageous for applications where simplicity, robustness, and small footprint are preferred over adaptive control. We also discuss the higher frequency versions of such holograms and their applications in robotic sensing and wireless power transfer. Such metamaterial-based holograms can serve as versatile platforms for various advanced acoustic wave manipulation and signal modulation, leading to new possibilities in acoustic sensing, imaging and energy deposition.

Electrical modulation and switching of transverse acoustic phonons

We report on the electrical manipulation of coherent acoustic phonon waves in GaN-based nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In such structures, transverse symmetry within the $c$ plane hinders both the generation and detection of the transverse acoustic (TA) modes, and usually only longitudinal acoustic phonons are generated by ultrafast displacive screening of potential gradients. We show that even for $c$-GaN, the combined application of lateral and vertical electric fields can not only switch on the normally forbidden TA mode, but they can also modulate the amplitudes and frequencies of both modes. By comparing the transient differential reflectivity spectra in structures with and without an asymmetric potential distribution, the role of the electrical controllability of phonons was demonstrated as changes to the propagation velocities, the optical birefringence, the electrically polarized TA waves, and the geometrically varying optical sensitivities of phonons.

Downsizing granular crystals and their kin

In their interesting article on granular crystals (Physics Today, November 2015, page 44), Mason Porter, Panayotis Kevrekidis, and Chiara Daraio presented a rich set of recent developments in the field. They noted that switches and Boolean logic elements are among the possible applications of the propagation of nonlinear waves in granular media. Readers might be interested to know that Boolean logic operations have also been performed in related phononic crystals, although in that setting the functions do not rely on nonlinear phenomena.11. J. O. Vasseur, B. Morvan, A. Tinel, N. Swinteck, A. C. Hladky-Hennion, P. A. Deymier, Phys. Rev. B 86, 134305 (2012). https://doi.org/10.1103/PhysRevB.86.134305 ,22. S. Bringuier, N. Swinteck, J. O. Vasseur, J.-F. Robillard, K. Runge, K. Muralidharan, P. A. Deymier, J. Acoust. Soc. Am. 130, 1919 (2011). https://doi.org/10.1121/1.3631627 We strongly agree with the authors that the emerging possibility of downsizing granular crystals to the micron and perhaps submicron scale is full of potential. The richer physics of granular interactions at the smaller scales could serve as a bridge between macroscopic crystals and atomistic crystals. For instance, the rotational degrees of freedom inherent in granular media33. A. Merkel, V. Tournat, V. Gusev, Phys. Rev. Lett. 107, 225502 (2011). https://doi.org/10.1103/PhysRevLett.107.225502 offer unique opportunities to break the time-reversal symmetry of the dynamical equations. That could lead to topologically constrained wave propagation and the possibility of information-processing components that exploit the phase of waves instead of their amplitude.44. P. A. Deymier, K. Runge, N. Swinteck, K. Muralidharan, C. R. Méc. 343, 700 (2015). https://doi.org/10.1016/j.crme.2015.07.003 We believe, and we hope the authors will agree, that small-scale granular crystals and their phononic-crystal and acoustic-metamaterial cousins constitute a new frontier in the manipulation of acoustic waves and their utilization in technological progress.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. O. Vasseur, B. Morvan, A. Tinel, N. Swinteck, A. C. Hladky-Hennion, P. A. Deymier, Phys. Rev. B 86, 134305 (2012). https://doi.org/10.1103/PhysRevB.86.134305 , Google ScholarCrossref, ISI2. S. Bringuier, N. Swinteck, J. O. Vasseur, J.-F. Robillard, K. Runge, K. Muralidharan, P. A. Deymier, J. Acoust. Soc. Am. 130, 1919 (2011). https://doi.org/10.1121/1.3631627 , Google ScholarCrossref, ISI3. A. Merkel, V. Tournat, V. Gusev, Phys. Rev. Lett. 107, 225502 (2011). https://doi.org/10.1103/PhysRevLett.107.225502 , Google ScholarCrossref, ISI4. P. A. Deymier, K. Runge, N. Swinteck, K. Muralidharan, C. R. Méc. 343, 700 (2015). https://doi.org/10.1016/j.crme.2015.07.003 , Google ScholarCrossref, ISI© 2016 American Institute of Physics.

Pentamode material for underwater acoustic wave control

Pentamode materials, made of conventional solids through microstructure de-sign, may have acoustic property of complex fluid. Its superior capacity for underwater acoustic wave manipulation stimulates recently an intense research activity. In this review, pentamode materials and their recent progress are explained from di?erent angles: basic concept, microstructure design, acoustic applications and fabrication technology, in order to provide an overall view on this kind of special material.

Robust acoustic wave manipulation of bubbly liquids

Experiments with water–air bubbly liquids when exposed to acoustic fields of frequency ∼100 kHz and intensity below the cavitation threshold demonstrate that bubbles ∼30 μm in diameter can be “pushed” away from acoustic sources by acoustic radiation independently from the direction of gravity. This manifests formation and propagation of acoustically induced transparency waves (waves of the bubble volume fraction). In fact, this is a collective effect of bubbles, which can be described by a mathematical model of bubble self-organization in acoustic fields that matches well with our experiments.

Effect of Brittle Fracture in a Metaconcrete Slab under Shock Loading

A new type of concrete, named metaconcrete, has been developed for the attenuation of shock waves induced by dynamic excitation. Inspired by the metamaterials used for the manipulation of electromagnetic and acoustic waves, this new metamaterial for the mitigation of shock waves utilizes the activation of resonance within engineered inclusions. Metaconcrete replaces the standard stone and gravel aggregates of regular concrete with spherical inclusions consisting of a heavy core coated in a compliant outer layer. Finite-element analyses of metaconcrete slabs for the case of purely elastic constituents reveal trapping of the supplied energy within the inclusions and a reduction in mortar stress, indicating the presence of resonance behavior within the aggregates. Mortar is, however, a brittle material and the fracture properties under dynamic loading should also be considered. Thus, the models used in the elastic analyses are extended by incorporating brittle fracture through the use of an eigenerosion scheme, which erodes elements satisfying an energy-based fracture criterion. The effect of different fracture parameters on the performance of the slab is investigated through parametric studies, looking at the change in slab behavior caused by various aggregate geometry and material configurations. These studies indicate that mechanical energy is captured by the aggregates, reducing the transmission of energy through the slab, the extension of the zone damaged by fracture, and the longitudinal stress within the mortar matrix. The understanding gained from these analyses incorporating fracture characteristics will enable more informed design of metaconcrete aggregates for dynamic loading applications, such as blast shielding and impact protection.

Broadband Acoustic Hyperbolic Metamaterial.

In this Letter, we report on the design and experimental characterization of a broadband acoustic hyperbolic metamaterial. The proposed metamaterial consists of multiple arrays of clamped thin plates facing the y direction and is shown to yield opposite signs of effective density in the x and y directions below a certain cutoff frequency, therefore, yielding a hyperbolic dispersion. Partial focusing and subwavelength imaging are experimentally demonstrated at frequencies between 1.0 and 2.5 kHz. The proposed metamaterial could open up new possibilities for acoustic wave manipulation and may find usage in medical imaging and nondestructive testing.

Controlling an acoustic wave with a cylindrically-symmetric gradient-index system**Project supported by the National Basic Research Program of China (Grant Nos. 2010CB327803 and 2012CB921504), the National Natural Science Foundation of China (Grant Nos. 11174138, 11174139, 11222442, 81127901, and 11274168), NCET-12-0254, and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, China.

We present a detailed theoretical description of wave propagation in an acoustic gradient-index system with cylindrical symmetry and demonstrate its potential to numerically control acoustic waves in different ways. The trajectory of an acoustic wave within the system is derived by employing the theory of geometric acoustics, and the validity of the theoretical descriptions is verified numerically by using the finite element method simulation. The results show that by tailoring the distribution function of the refractive index, the proposed system can yield a tunable manipulation of acoustic waves, such as acoustic bending, trapping, and absorbing.

The anomalous manipulation of acoustic waves based on planar metasurface with split hollow sphere

This paper presents an acoustic metasurface (AMS) model consisting of split hollow sphere (SHS) resonator arrays with the property of negative modulus. It shows that the AMS can imprint phase discontinuities on an acoustic reflected wave as it traverses the interface between two media. By designing suitable phase gradients, the AMS enables the perpendicularly incident acoustic wave to be converted to a surface wave or reflected in any angle. Four kinds of AMSs, which can anomalously manipulate the reflected wave’s direction, are simulated to fulfill the generalized Snell’s law. The results provide an available and simple path to experimentally achieving the AMS.