Ultra-sparse metasurface for high reflection of low-frequency sound based on artificial Mie resonances

Digitally Controlled Piezoelectric Metamaterial for Low-Frequency and High-Efficiency Sound Absorption

This study proposes a membrane-type metamaterial with digitally controlled piezoelectric actuation for low-frequency sound absorption applications. The hybrid structure integrates an aluminum membrane functionally bonded with programmable piezoelectric patches (PZTs) and a sealed air cavity. Two innovative control strategies-Resistance Enhancement and Resonance Enhancement-dynamically adjust circuit impedance to maximize electromechanical energy conversion efficiency, thereby optimizing absorption at targeted frequencies. These strategies are implemented via a real-time digital feedback system. A coupled piezoelectric-structural-acoustic model is established to characterize the system's transfer function, with validation through both finite element simulations and impedance tube experiments. Numerical and experimental results demonstrate nearly complete absorption around the resonant frequency, and the bandwidth can be further broadened through multi-resonance superposition. Theoretical analysis confirms that the active control strategies simultaneously modulate the acoustic impedance components (resistance and reactance), thereby optimizing electromechanical energy conversion efficiency. This work establishes a novel active-control methodology for low-frequency and high-efficiency noise mitigation.

Achieving High-Performance Transcranial Ultrasound Transmission Through Mie and Fano Resonance in Flexible Metamaterials

Transcranial ultrasound holds great potential in medical applications. However, the effective transmission of ultrasound through the skull remains challenging due to the acoustic impedance mismatch, as well as the non-uniform thickness, and the curved surface. To overcome these challenges, this work introduces an innovative Mie-resonance flexible metamaterial (MRFM), which consists of periodically arranged low-speed micropillars embedded within a high-speed flexible substrate. The MRFM generates Mie-resonance, which couples with the skull to form Fano resonance, thereby enhancing ultrasound transmittance through the skull. Simulation results demonstrate that the proposed resonance solution significantly increases transcranial ultrasound transmittance from 33.7% to 75.2% at 0.309 MHz. For the fabrication of the MRFM, porous nickel foam is used as the Mie micropillars, and agarose hydrogel serves as the flexible substrate. Experimental results demonstrate enhanced ultrasound transmittance from 20.6% to 73.3% at 0.33 MHz with the MRFM, which shows good agreement with the simulation results, further validating the effectiveness of the design. The simplicity, tunability, and flexibility of the MRFM represent a significant breakthrough, addressing the limitations of conventional rigid metamaterials. This work lays a solid theoretical and experimental foundation for advancing the clinical application of transcranial ultrasound stimulation and neuromodulation.

State of the Art of Low-Frequency Acoustic Modulation: Intensity Enhancement and Directional Control

High-intensity low-frequency acoustic sources with directivity play a significant role in various fields such as medical treatment, underwater communication, and environmental monitoring. However, the long wavelengths, strong penetration, and their tendency to easily diffract of low-frequency acoustic waves make it challenging to achieve directional control and intensity enhancement. Thanks to the development of acoustic metamaterials, acoustic devices can now effectively manipulate low-frequency acoustic waves at subwavelength scales with excellent acoustic performance. Currently, the directional control and intensity enhancement of low-frequency acoustic waves mainly concentrate on source design and the modulation of propagation processes. These techniques employ acoustic resonance, focusing, and other phase control methods to achieve energy concentration and directional control of low-frequency acoustic waves. Nevertheless, existing low-frequency acoustic wave control techniques still face issues such as low energy efficiency, poor directional control, and limited controllable bandwidth. This paper systematically reviews methods for achieving high-intensity emission and directional control of low-frequency acoustic waves, comprehensively compares the advantages and disadvantages of various technologies, and discusses how to extend these methods to lower acoustic frequency bands, aiming to provide new insights for the development of miniaturized, efficient, and accurately directional ultra-low frequency acoustic devices.

Multi-Physical Lattice Metamaterials Enabled by Additive Manufacturing: Design Principles, Interaction Mechanisms, and Multifunctional Applications

Lattice metamaterials emerge as advanced architected materials with superior physical properties and significant potential for lightweight applications. Recent developments in additive manufacturing (AM) techniques facilitate the manufacturing of lattice metamaterials with intricate microarchitectures and promote their applications in multi-physical scenarios. Previous reviews on lattice metamaterials have largely focused on a specific/single physical field, with limited discussion on their multi-physical properties, interaction mechanisms, and multifunctional applications. Accordingly, this article critically reviews the design principles, structure-mechanism-property relationships, interaction mechanisms, and multifunctional applications of multi-physical lattice metamaterials enabled by AM techniques. First, lattice metamaterials are categorized into homogeneous lattices, inhomogeneous lattices, and other forms, whose design principles and AM processes are critically discussed, including the benefits and drawbacks of different AM techniques for fabricating different types of lattices. Subsequently, the structure-mechanism-property relationships and interaction mechanisms of lattice metamaterials in a range of physical fields, including mechanical, acoustic, electromagnetic/optical, and thermal disciplines, are summarized to reveal critical design principles. Moreover, the multifunctional applications of lattice metamaterials, such as sound absorbers, insulators, and manipulators, sensors, actuators, and soft robots, thermal management, invisible cloaks, and biomedical implants, are enumerated. These design principles and structure-mechanism-property relationships provide effective design guidelines for lattice metamaterials in multifunctional applications.

Engineering the acoustic field with a Mie scatterer for microparticle patterning

The utilization of acoustic fields offers a contactless approach for microparticle manipulation in a miniaturized system, and plays a significant role in medicine, biology, chemistry, and engineering. Due to the acoustic radiation force arising from the scattering of the acoustic waves, small particles in the Rayleigh scattering range can be trapped, whilst their impact on the acoustic field is negligible. Manipulating larger particles in the Mie scattering regime is challenging due to the diverse scattering modes, which impacts the local acoustic field. The rapid movement of free-moving Mie scatterers in an acoustic standing wave field makes it difficult to study the interaction between a sound field and a Mie scatterer in an engineering context. Here, a combined approach that integrates theoretical analysis and experimental investigation was developed to explore the influence of a Mie scatterer on the acoustic field by fabricating an acoustic trapping device featuring a fixed Mie scatterer at its center. We demonstrate that an insonified Mie scatterer can operate as an acoustic emitter in water, enabling dynamic and versatile modulation of the total acoustic field. Such a scatterer can interact with one or multiple incident propagating acoustic waves, leading to the generation of a localized standing wave field in the vicinity of the scatterer. This local field can be controlled by the relative location of the scatterer with respect to the incident field leading to control over the transformation from an incident 1D acoustic field into a 2D acoustic field. This control paves the way for localized and multi-scale micro-object manipulation.

Shape optimized acoustic metagratings for anomalous refraction under strong thermoviscous effects

The recent development of microacoustic metagratings opens up promising possibilities for manipulating acoustic wavefronts passively, particularly in applications such as flat acoustic lenses and ultra-high frequency ultrasound imaging. The emergence of two-photon polymerization has made it feasible to precisely manufacture microscopic structures, as required when metagratings are scaled to MHz frequencies in airborne ultrasound. Nevertheless, the downsizing process presents another hurdle as the increased thermoviscous effects result in substantial losses that must be considered during the design phase. In this study, we propose two designs for microacoustic metagratings that refract a normally incident wave towards -35 ° at 2 MHz, consisting of single-body and two-body meta-atoms. The designs are created by employing shape optimization techniques that incorporate the linearized Navier-Stokes equations in every iteration starting from a neutral geometry. This ensures that the evolution of geometric key features responsible for anomalous refraction fully accounts for thermoviscous effects, as would be the case during evolution in nature where the full set of physics is always active. Subsequently, we experimentally evaluate the effectiveness of these metagratings by employing a capacitive micromachined ultrasonic transducer as the sound source and an optical microphone as the detector, covering a frequency range from 1.8 to 2.2 MHz. Our findings confirm the single-body geometry reported in the literature and show an alternative geometry for two-body design, showcasing the successful utilization of two-photon polymerization for manufacturing microscopic acoustic metamaterials.

An efficient multiscale method for subwavelength transient analysis of acoustic metamaterials

A reduced-order homogenization framework is proposed, providing a macro-scale-enriched continuum model for locally resonant acoustic metamaterials operating in the subwavelength regime, for both time and frequency domain analyses. The homogenized continuum has a non-standard constitutive model, capturing a metamaterial behaviour such as negative effective bulk modulus, negative effective density and Willis coupling. A suitable reduced space is constructed based on the unit cell response in a steady-state regime and the local resonance regime. A frequency domain numerical example demonstrates the efficiency and suitability of the proposed framework.This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 2)'.

Multi-Order Asymmetric Acoustic Metamaterials with Broad Bandgaps at Subwavelength Scales

Noise manipulation at the subwavelength scale remains a challenging problem. To obtain better broadband sound isolation within the subwavelength range, a class of asymmetric acoustic metamaterials (AAMs) based on rotation is proposed, and this class of AAMs can further improve subwavelength sound isolation performance by introducing multi-orders. The influences of changing the alternate propagation length of the coiled channel and the square cavity in the unit cell on the band frequency distribution and the omnidirectional band structure were investigated. The effective parameters are calculated with the S-parameter retrieval method, and the generation and change mechanisms of the bandgaps were elucidated. The calculation of sound transmission characteristics showed that, in the asymmetric mode, the overall sound isolation performance of the structure was greatly improved, and the relative bandwidth expanded as the alternate propagation length of the coiled channel and square cavity increased. The omnidirectional bandgaps from the first-order to the third-order AAMs occupied 63.6%, 75.96%, and 76.84% of the subwavelength range, respectively. In particular, the first bandgap moves to the low frequency and becomes wider. Both the experimental results and numerical analyses consistently showed that disrupting structural symmetry enhances acoustic metamaterials for superior broadband sound isolation, inspiring broader applications for asymmetry in this field.

Acoustic Forceps Based on Focused Acoustic Vortices with Different Topological Charges

For enhanced energy concentration with improved flexibility for object manipulation, a focused acoustic vortex (FAV) is designed using a sector planar piston transducer array and acoustic lens that can produce the effective concentration of the acoustic field to perform the focusing function. Compared to the Gaussian beam, which tends to cause the object to deviate from the axis of acoustic propagation, FAVs can form a central valley region to firmly bind the objects, thus preventing off-target effects. The heat energy in the paraxial region is transferred to the vortex center in the form of heat transfer so that the temperature-sensitive liposomes captured can quickly release drugs, which has a good effect on targeted drug administration. The focused acoustic wave stopped acting on the tissue (gel) for 2 s, the temperature of the vortex center continued to rise, reaching 41.5 °C at the moment of 3.7 s, at which point the liposomes began to release the drug. The FAVs capture the drug and use its thermal effect to achieve accurate and rapid treatment. The simulation results show that the drug release temperature of temperature-sensitive liposomes can be achieved by controlling the action time of the vortices. This study provides a reliable theoretical basis for the clinical application of targeted drugs.

Superscattering of water waves

Inspired by the concept of superscattering in optics, we for the first time theoretically predict and experimentally demonstrate the superscattering phenomenon in water waves. The subwavelength superscatterer is constructed by multi-layered concentric cylinders with an inhomogeneous depth profile. The superscatterer breaks the long-held single-channel scattering limit by several times and thus significantly enhances the total scattering strength. The underlying mechanism originates from the near degeneracy of the resonances of multiple channels. We fabricate the superscatterer prototype and experimentally measure the near-field patterns, which are consistent with theoretical prediction and numerical simulation. Our study opens a new avenue to strengthen water-wave scattering and deepen the understanding in water waves, which can be useful for ocean energy harvesting and harbor protection.

3D Printed Graphene-Based Metamaterials: Guesting Multi-Functionality in One Gain

Advanced functional materials with fascinating properties and extended structural design have greatly broadened their applications. Metamaterials, exhibiting unprecedented physical properties (mechanical, electromagnetic, acoustic, etc.), are considered frontiers of physics, material science, and engineering. With the emerging 3D printing technology, the manufacturing of metamaterials becomes much more convenient. Graphene, due to its superior properties such as large surface area, superior electrical/thermal conductivity, and outstanding mechanical properties, shows promising applications to add multi-functionality into existing metamaterials for various applications. In this review, the aim is to outline the latest developments and applications of 3D printed graphene-based metamaterials. The structure design of different types of metamaterials and the fabrication strategies for 3D printed graphene-based materials are first reviewed. Then the representative explorations of 3D printed graphene-based metamaterials and multi-functionality that can be introduced with such a combination are further discussed. Subsequently, challenges and opportunities are provided, seeking to point out future directions of 3D printed graphene-based metamaterials.

Nanobubble size distribution measurement by interactive force apparatus under an electric field

Nanobubbles have been applied in many fields, such as environmental cleaning, material production, agriculture, and medicine. However, the measured nanobubble sizes differed among the measurement methods, such as dynamic light scattering, particle trajectory, and resonance mass methods. Additionally, the measurement methods were limited with respect to the bubble concentration, refractive index of liquid, and liquid color. Here, a novel interactive force measurement method for bulk nanobubble size measurement was developed by measuring the force between two electrodes filled with bulk nanobubble-containing liquid under an electric field when the electrode distance was changed in the nm scale with piezoelectric equipment. The nanobubble size was measured with a bubble gas diameter and also an effective water thin film layer covered with a gas bubble that was estimated to be approximately 10 nm based on the difference between the median diameter of the particle trajectory method and this method. This method could also be applied to the solid particle size distribution measurement in a solution.

Ultra-sparse near-perfect sound absorbers

There is a trade-off between the sparseness of an absorber array and its sound absorption imposed by wave physics. Here, near-perfect absorption (99% absorption) is demonstrated when the spatial period of monopole-dipole resonators is close to one working wavelength (95% of the wavelength). The condition for perfect absorption is to render degenerate monopole-dipole resonators critically coupled. Frequency domain simulations, eigenfrequency simulations, and the coupled mode theory are utilized to demonstrate the acoustic performances and the underlying physics. The sparse-resonator-based sound absorber could greatly benefit noise control with air flow and this study could also have implications for electromagnetic wave absorbers.

Total acoustic transmission in a honeycomb network empowered by compact acoustic isolator

In recent years, acoustic metamaterials have exhibited extraordinary potential for manipulating the propagation of sound waves. However, it has been a challenge to control the propagation of sound waves through arbitrary pathways in a network. In this work, we designed a compact three-port isolator that can produce giant acoustic nonreciprocity by introducing actively controlled CNT films to the device without altering the geometric symmetry of it. This concept is subsequently applied to construct a 4 × 7 honeycomb network, in which, total transmission of sound wave in arbitrary pathway can be slickly achieved. Unlike the acoustic topological insulator, which only supports total transmission of arbitrary pathway in the band gap, our method provides more degrees of freedom and can be realized at any frequency. This ability opens up a new method for routing sound waves and exhibits promising applications ranging from acoustic communication to energy transmission.

Influencing Effects of Fabrication Errors on Performances of the Dielectric Metalens

Despite continuous developments of manufacturing technology for micro-devices and nano-devices, fabrication errors still exist during the manufacturing process. To reduce manufacturing costs and save time, it is necessary to analyze the effects of fabrication errors on the performances of micro-/nano-devices, such as the dielectric metasurface-based metalens. Here, we mainly analyzed the influences of fabrication errors in dielectric metasurface-based metalens, including geometric size and shape of the unit element, on the focusing efficiency and the full width at half maximum (FWHM) values. Simulation results demonstrated that the performance of the metasurface was robust to fabrication errors within a certain range, which provides a theoretical guide for the concrete fabrication processes of dielectric metasurfaces.

A Flexible Meta-Curtain for Simultaneous Soundproofing and Ventilation

We demonstrate a flexible meta-curtain that can simultaneously block the propagation of sound waves of selected frequencies and let air flow through freely. Such a meta-curtain is assembled by two soft and perforated polyvinyl chloride films with an optimized distance between them. The total thickness of the meta-curtain is 1.16 cm and the holes on it have a diameter of 5 cm. The functionality of soundproofing is bestowed by the resonances formed between the films, which is verified by band structure analysis, numerical simulations, and experimental measurements. We experimentally observed sound transmission loss with a peak of 50 dB near 1700 Hz and an average of 26 dB from 1000 Hz to 1760 Hz, which is consistent with the numerical results. Attributing to the softness of the films and the robustness of the resonance, this meta-curtain retains its functionality even at deformations such as bending. Our work paves a way toward soundproof structures with the advantages of ventilation, flexibility, and light weight.

Fano-Resonant Metasurface with 92% Reflectivity Based on Lithium Niobate on Insulator

Lithium niobate is an excellent optoelectronic and nonlinear material, which plays an important role in integrated optics. However, lithium niobate is difficult to etch due to its very stable chemical nature, and the microstructure of lithium niobate's metasurface is generally of subwavelength, which further increases its processing difficulty. Here, by using Ar+-based inductively coupled plasma etching and KOH wet etching, we improve the etching quality and fabricate a Fano-resonant metasurface based on lithium niobate on insulator, which has a very high reflectivity of 92% at near-infrared wavelength and the potential of becoming a high-reflectivity film. In addition, to evaluate the practical performance of the metasurface, we constructed a Fabry-Perot cavity by using it as a cavity mirror, whose reflection spectrum shows a finesse of 38. Our work paves the way for the development of functional metasurfaces and other advanced photonic devices based on lithium niobate on insulator.

Microacoustic Metagratings at Ultra-High Frequencies Fabricated by Two-Photon Lithography

The recently proposed bianisotropic acoustic metagratings offer promising opportunities for passive acoustic wavefront manipulation, which is of particular interest in flat acoustic lenses and ultrasound imaging at ultra-high frequency ultrasound. Despite this fact, acoustic metagratings have never been scaled to MHz frequencies that are common in ultrasound imaging. One of the greatest challenges is the production of complex microscopic structures. Owing to two-photon polymerization, a novel fabrication technique from the view of acoustic metamaterials, it is now possible to precisely manufacture sub-wavelength structures in this frequency range. However, shrinking in size poses another challenge; the increasing thermoviscous effects lead to a drop in efficiency and a frequency downshift of the transmission peak and must therefore be taken into account in the design. In this work three microacoustic metagrating designs refracting a normally incident wave toward -35° at 2 MHz are proposed. In order to develop meta-atoms insensitive to thermoviscous effects shape optimization techniques incorporating the linearized Navier-Stokes equations discretized with finite element method are used. The authors report for the first time microscopic acoustic metamaterials manufactured using two-photon polymerization and, subsequently, experimentally verify their effectiveness using an optical microphone as a detector in a range from 1.8 to 2.2 MHz.

Elastic Metagratings with Simultaneous Modulation of Reflected and Transmitted Waves

Elastic metagratings enabling independent and complete control of both reflection and transmission of bulk longitudinal and transverse waves are highly desired in application scenarios such as non-destructive assessment and structural health monitoring. In this work, we propose a kind of simply structured metagrating composed only of elliptical hollow cylinders carved periodically in a steel background. By utilizing the grating diffraction theory and genetic algorithm, we endow these metagratings with the attractive functionality of simultaneous and high-efficiency modulation of every reflection and transmission channel of both longitudinal and transverse waves. Interesting wave-front manipulation effects including pure mode conversion and anomalous deflection along the desired direction are clearly demonstrated through full-wave numerical simulations. Due to its subwavelength thickness and high manipulation efficiency, the proposed metagrating is expected to be useful in the design of multifunctional elastic planar devices.

Low-Frequency Low-Reflection Bidirectional Sound Insulation Tunnel with Ultrathin Lossy Metasurfaces

We report both numerical and experimental constructions of a tunnel structure with low-frequency low-reflection bidirectional sound insulation (BSI). The designed tunnel was constructed from a pair of lossy acoustic metasurfaces (AMs), which consists of six ultrathin coiled unit cells, attached on both sides. Based on the generalized Snell′s law and phase modulations for both AMs, the tunnel with the low-frequency BSI was constructed based on sound reflections and acoustic blind areas created by the AMs. The obtained transmittances were almost the same for sound incidences from both sides and were lower than −10 dB in the range 337–356 Hz. The simulated and measured results agreed well with each other. Additionally, we show that the low-reflection characteristic of the tunnel can be obtained simultaneously by thermoviscous energy loss in coiled channels of the unit cells. Finally, an interesting application of the designed tunnel in an open-window structure with low-frequency low-reflection BSI is further simulated in detail. The proposed tunnel based on the ultrathin lossy AMs has the advantages of ultrathin thickness (about λ/35), low-frequency low-reflection BSI, and high-performance ventilation, which may have potential applications in architectural acoustics and noise control.

Achromatic metasurfaces by dispersion customization for ultra-broadband acoustic beam engineering

Metasurfaces, the ultra-thin media with extraordinary wavefront modulation ability, have shown great promise for many potential applications. However, most of the existing metasurfaces are limited by narrow-band and strong dispersive modulation, which complicates their real-world applications and, therefore require strict customized dispersion. To address this issue, we report a general methodology for generating ultra-broadband achromatic metasurfaces with prescribed ultra-broadband achromatic properties in a bottom-up inverse-design paradigm. We demonstrate three ultra-broadband functionalities, including acoustic beam deflection, focusing and levitation, with relative bandwidths of 93.3%, 120% and 118.9%, respectively. In addition, we reveal a relationship between broadband achromatic functionality and element dispersion. All metasurface elements have anisotropic and asymmetric geometries with multiple scatterers and local cavities that synthetically support internal resonances, bi-anisotropy and multiple scattering for ultra-broadband customized dispersion. Our study opens new horizons for ultra-broadband highly efficient achromatic functional devices, with promising extension to optical and elastic metamaterials.

Acoustic Equivalent Lasing and Coherent Perfect Absorption Based on a Conjugate Metamaterial Sphere

Acoustic conjugate metamaterials (ACMs), in which the imaginary parts of the effective complex mass density and bulk compressibility are cancelled out in the refractive index, possess the elements of loss and gain simultaneously. Previous works have focused on panel ACMs for plane wave incidence. In this paper, we explore the extraordinary scattering properties, including the acoustic equivalent lasing (AEL) and coherent perfect absorption (CPA) modes, of a three-dimensional ACM sphere, where incident spherical waves with specific topological orders could be extremely scattered and totally absorbed, respectively. Theoretical analysis and numerical simulations show that the AEL or CPA mode with a single order can be realized with a small monolayer ACM sphere with appropriate parameters. A huge (relative to incident wavelength) ACM sphere with pure imaginary parameters could support the even- (or odd-) order AEL and odd- (or even-) order CPA modes simultaneously. In addition, the AEL and/or CPA with multiple orders could be realized based on a small multilayered ACM sphere. The proposed ACM sphere may provide an alternative method to design acoustic functional devices, such as amplifiers and absorbers.

Low-Frequency, Open, Sound-Insulation Barrier by Two Oppositely Oriented Helmholtz Resonators

In this work, a low-frequency, open, sound-insulation barrier, composed of a single layer of periodic subwavelength units (with a thickness of λ/28), is demonstrated both numerically and experimentally. Each unit was constructed using two identical, oppositely oriented Helmholtz resonators, which were composed of a central square cavity surrounded by a coiled channel. In the design of the open barrier, the distance between two adjacent units was twice the width of the unit, showing high-performance ventilation, and low-frequency sound insulation. A minimum transmittance of 0.06 could be observed around 121.5 Hz, which arose from both sound reflections and absorptions, created by the coupling of symmetric and asymmetric eigenmodes of the unit, and the absorbed sound energy propagating into the central cavity was greatly reduced by the viscous loss in the channel. Additionally, by introducing a multilayer open barrier, a broadband sound insulation was obtained, and the fractional bandwidth could reach approximately 0.19 with four layers. Finally, the application of the multilayer open barrier in designing a ventilated room was further discussed, and the results presented an omnidirectional, broadband, sound-insulation effect. The proposed open, sound-insulation barrier with the advantages of ultrathin thickness; omnidirectional, low-frequency sound insulation; broad bandwidth; and high-performance ventilation has great potential in architectural acoustics and noise control.

Multichannel Piezo-Ultrasound Implant with Hybrid Waterborne Acoustic Metastructure for Selective Wireless Energy Transfer at Megahertz Frequencies

Ultrasound energy transfer (UET) is developed and integrated into various bioelectronics with diagnostic, therapeutic, and monitoring capabilities. However, existing UET platforms generally enable one function at a time due to the single ultrasound channel architecture, limiting the full potential of bioelectronics that requires multicontrol modes. Here, a multichannel piezo-ultrasound implant (MC-PUI) is presented that integrates a hybrid waterborne acoustic metastructure (HWAM), multiple piezo-harvesters, and a miniaturized circuit with electronic components for selective wireless control via ultrasound frequency switching. The HWAM that utilizes both a 3D-printed air-diffraction matrix and a half-lambda Fabry-Perot resonator is optimized to provide the advantage of ultrasound selectivity at megahertz frequencies. Complying with U.S. Food and Drug Administration regulations, frequency-controlled multifunctional operations, such as wireless charging (≈11.08 µW) at 3.3 MHz and high-sensitivity wireless switch/control (threshold ≈0.55 MPa) of micro-light-emitting diode/motor at 1 MHz, are demonstrated ex vivo using porcine tissue and in vivo in a rat. The developed MC-PUI enhances UET versatility and opens up a new pathway for wireless implant design.

Tunable Fluid-Type Metasurface for Wide-Angle and Multifrequency Water-Air Acoustic Transmission

Efficient acoustic communication across the water-air interface remains a great challenge owing to the extreme acoustic impedance mismatch. Few present acoustic metamaterials can be constructed on the free air-water interface for enhancing the acoustic transmission because of the interface instability. Previous strategies overcoming this difficulty were limited in practical usage, as well as the wide-angle and multifrequency acoustic transmission. Here, we report a simple and practical way to obtain the wide-angle and multifrequency water-air acoustic transmission with a tunable fluid-type acoustic metasurface (FAM). The FAM has a transmission enhancement of acoustic energy over 200 times, with a thickness less than the wavelength in water by three orders of magnitude. The FAM can work at an almost arbitrary water-to-air incident angle, and the operating frequencies can be flexibly adjusted. Multifrequency transmissions can be obtained with multilayer FAMs. In experiments, the FAM is demonstrated to be stable enough for practical applications and has the transmission enhancement of over 20 dB for wide frequencies. The transmission enhancement of music signal across the water-air interface was performed to demonstrate the applications in acoustic communications. The FAM will benefit various applications in hydroacoustics and oceanography.

Three-Dimensional Soundproof Acoustic Metacage

In this Letter, we theoretically propose and experimentally demonstrate a three-dimensional soundproof acoustic cage structure, hereby denoted as an acoustic metacage. The metacage is composed of six acoustic metamaterial slabs with open holes and hidden bypass space coiling tunnels connected to the holes. Band structure analysis reveals a novel physical mechanism to open a low-frequency broad partial band gap via the band folding in other directions, which can also be interpreted by an effective medium with indefinite effective mass density and negative effective modulus. Transmission loss in simulations and in the acoustic impedance tube are administered. Strikingly, we prove that the soundproofing effect of the metacage is robust against the airflow perturbation induced by a fan. Our work paves a road for low-frequency airborne soundproof structures in the presence of ventilation.

Low-frequency multi-order acoustic absorber based on spiral metasurface

In this work, we propose a spiral metasurface for multi-order sound absorption in the low-frequency range (<1000 Hz). By dividing the long channel of the spiral metasurface into a series of tunable sub-cavities and employing recessed necks, the metasurface can quasi-perfectly (>0.95 in experiments) absorb airborne sound at multiple low-frequency orders without being limited by the number of equivalent cavities. Owing to the superior impedance manipulation provided by the spiral metasurface, each absorption order can be tuned flexibly with a constant external shape. By suitably modulating the sub-cavities and the recessed necks, we obtained multi-order high-absorption metasurfaces with dual-chamber, tri-chamber, and four-chamber designs. The ratio of the lowest resonant wavelength to the thickness is as high as 78. The samples, which are fabricated by three-dimensional printing technology, were measured to verify the theoretical results. We also investigate the relationship between the geometric parameters of the recessed necks and the sound absorption performance, which facilitates the more feasibly designed multi-order metasurfaces. The concept can be further applied to broadband absorption with ultra-thin thickness and has potential applications for noise reduction.

Remote whispering metamaterial for non-radiative transceiving of ultra-weak sound

Transceiving ultra-weak sound typically relies on signal pre-amplification at the transmitting end via active electro-acoustic devices, which inherently perturbs the environment in the form of noise that inevitably leads to information leakage. Here we demonstrate a passive remote-whispering metamaterial (RWM) enabling weak airborne sound at audible frequencies to reach unprecedented signal enhancement without altering the detected ambient soundscape, which is based on the extraordinary scattering properties of a metamaterial formed by a pair of self-resonating subwavelength Mie meta-cavities, constituting the acoustic analogy of Förster resonance energy transfer. We demonstrate efficient non-radiative sound transfer over distances hundreds times longer than the radius of the meta-cavities, which enables the RWM to recover weak sound signals completely overwhelmed by strong noise with enhanced signal-to-noise ratio from -3 dB below the detection limit of 0 dB in free space to 17.7 dB.

A Fano-based acoustic metamaterial for ultra-broadband sound barriers

Ultra-broadband sound reduction schemes covering living and working noise spectra are of high scientific and industrial significance. Here, we report, both theoretically and experimentally, on an ultra-broadband acoustic barrier assembled from space-coiling metamaterials (SCMs) supporting two Fano resonances. Moreover, acoustic hyper-damping is introduced by integrating additional thin viscous foam layers in the SCMs for optimizing the sound reduction performance. A simplified model is developed to study sound transmission behaviour of the SCMs under a normal incidence, which sets forth the basis to understand the working mechanism. An acoustic barrier with 220 mm thickness is then manufactured and tested to exhibit ultra-broadband transmission loss overall above 10 dB across the range 0.44–3.85 kHz, covering completely nine third-octave bands. In addition, unconventional broadband absorption in the dampened barrier (65%) is experimentally observed as well. We believe this work paves the way for realizing effective broadband sound insulation, absorption and sound wave controlling devices with efficient ventilation.

Bilayer ventilated labyrinthine metasurfaces with high sound absorption and tunable bandwidth

The recent advent of acoustic metamaterials offers unprecedented opportunities for sound controlling in various occasions, whereas it remains a challenge to attain broadband high sound absorption and free air flow simultaneously. Here, we demonstrated, both theoretically and experimentally, that this problem can be overcome by using a bilayer ventilated labyrinthine metasurface. By altering the spacing between two constituent single-layer metasurfaces and adopting asymmetric losses in them, near-perfect (98.6%) absorption is achieved at resonant frequency for sound waves incident from the front. The relative bandwidth of absorption peak can be tuned in a wide range (from 12% to 80%) by adjusting the open area ratio of the structure. For sound waves from the back, the bilayer metasurface still serves as a sound barrier with low transmission. Our results present a strategy to realize high sound absorption and free air flow simultaneously, and could find applications in building acoustics and noise remediation.

Multiband asymmetric sound absorber enabled by ultrasparse Mie resonators

On the quest towards efficiently eliminating noises, the development of a subwavelength sound absorber with the capability of free ventilation remains challenging. Here, we theoretically propose and experimentally demonstrate an asymmetric metamaterial absorber constructed by tuned Mie resonators (MRs) with unbalanced intrinsic losses. The lossy MR layer is highly dissipative to consume the sound energy while the lossless one acts as an acoustically soft boundary. Thus, the absorber presents quasi-perfect absorption (95% in experiment) for sound waves incident from the port nearer the dissipative MR and large-amount reflection (71% in experiment) from the opposite port. Moreover, the fluid dynamics investigation confirms the superior character of free air circulation owing to the ultrasparsity (volume filling ratio as low as 5%) of the absorber and its robustness to the velocity of airflows. Due to the multiple-order resonant modes of MR, we further demonstrate the flexibility of a methodology to extend asymmetric absorptions into multibands. Coupled mode analysis is employed to reveal the physical mechanism and further indicates that sparsity can be tuned by attentively controlling the reference leakage factor and intrinsic loss.

Engineering three-dimensional labyrinthine fractal acoustic metamaterials with low-frequency multi-band sound suppression

Acoustic metamaterials are a class of artificially periodic structures with extraordinary elastic properties that cannot be easily found in naturally occurring materials and can be applied to regulate the sound propagation behavior. The fractal configuration can be widely found in the acoustic system, like characterizing the broadband or multi-band sound propagation. This work will engineer three-dimensional (3D) labyrinthine fractal acoustic metamaterials (LFAMs) to regulate the sound propagation on subwavelength scales. The dispersion relations of LFAMs are systematically analyzed by the Bloch theory and the finite element method (FEM). The multi-bands, acoustic modes, and isotropic properties characterize their acoustic wave properties in the low-frequency regime. The effective bulk modulus and mass density of the LFAMs are numerically calculated to explain the low-frequency bandgap behaviors in specific frequencies. The transmissions and pressure field distributions of 3D LFAMs have been used to measure the ability for sound suppression. Furthermore, when considering the thermo-viscous loss on the transmission properties, the high absorptions occur within the multi-band range for low-frequency sound. Hence, this research contributes to potential applications on 3D LFAMs for multi-bands blocking and/or absorption on deep-subwavelength scales.

Metafluids beyond the Bulk Modulus

It is well known that the acoustic properties of fluid are characterized by mass density and bulk modulus. Metafluids, the fluid metamaterials, generalize the natural fluid, which can accommodate extreme and/or negative values of these two parameters. Here, we further show that the metafluids, composed of periodic thin-walled hollow cylinders immersed in fluid, can provide not only the designable effective mass density and bulk modulus, but also a completely new effective parameter, which appears in the wave velocities as a role similar to the shear modulus of solid. The new effective parameter, describing the response of the fluid to the quadrupolar component of waves, is obtained by generalizing the effective medium theory (EMT) to include the second-order effects, which is vanishing and neglected in the conventional EMT, but giant here in the metafluids with built-in quadrupolar resonances. With the discovery of the metafluids of shearlike moduli, our Letter extends the concept of metafluids and will have a great significance in the field of metamaterials.

Dopant-modulated sound transmission with zero index acoustic metamaterials

Zero index metamaterials have shown the ability to achieve total transmission or reflection by embedding particular defects with various effective parameters. Here, we present that tunable sound transmission can be realized by configuring a subwavelength-sized dopant inside zero index acoustic metamaterials. Despite its small spatial signature, the dopant is able to strongly interact with the acoustic waves inside the whole zero index metamaterials. It is due to the essence of the zero effective index that can homogenize the pressure field within the metamaterials. Sound transmission can thus be fully switched on and off by adjusting the dopant's surface impedance. A simple rectangular cavity with varied lengths is proposed to provide the required impedance boundary. Our model of correlating the dopant design with sound transmission performance is validated theoretically and numerically. We further demonstrate the utilization of the proposed design to effectively modulate the sound focusing effect. Such a dopant-modulated sound transmission scheme, with its simplicity and capability, has potential applications in fields like noise control and ultrasonography.

Recent Advances in Acoustic Metamaterials for Simultaneous Sound Attenuation and Air Ventilation Performances

In the past two decades, acoustic metamaterials have garnered much attention owing to their unique functional characteristics, which are difficult to find in naturally available materials. The acoustic metamaterials have demonstrated excellent acoustical characteristics that paved a new pathway for researchers to develop effective solutions for a wide variety of multifunctional applications, such as low-frequency sound attenuation, sound wave manipulation, energy harvesting, acoustic focusing, acoustic cloaking, biomedical acoustics, and topological acoustics. This review provides an update on the acoustic metamaterials’ recent progress for simultaneous sound attenuation and air ventilation performances. Several variants of acoustic metamaterials, such as locally resonant structures, space-coiling, holey and labyrinthine metamaterials, and Fano resonant materials, are discussed briefly. Finally, the current challenges and future outlook in this emerging field are discussed as well.

A Review of Acoustic Impedance Matching Techniques for Piezoelectric Sensors and Transducers

The coupling of waves between the piezoelectric generators, detectors, and propagating media is challenging due to mismatch in the acoustic properties. The mismatch leads to the reverberation of waves within the transducer, heating, low signal-to-noise ratio, and signal distortion. Acoustic impedance matching increases the coupling largely. This article presents standard methods to match the acoustic impedance of the piezoelectric sensors, actuators, and transducers with the surrounding wave propagation media. Acoustic matching methods utilizing active and passive materials have been discussed. Special materials such as nanocomposites, metamaterials, and metasurfaces as emerging materials have been presented. Emphasis is placed throughout the article to differentiate the difference between electric and acoustic impedance matching and the relation between the two. Comparison of various techniques is made with the discussion on capabilities, advantages, and disadvantages. Acoustic impedance matching for specific and uncommon applications has also been covered.

A Review of Acoustic Metamaterials and Phononic Crystals

As a new kind of artificial material developed in recent decades, metamaterials exhibit novel performance and the promising application potentials in the field of practical engineering compared with the natural materials. Acoustic metamaterials and phononic crystals have some extraordinary physical properties, effective negative parameters, band gaps, negative refraction, etc., extending the acoustic properties of existing materials. The special physical properties have attracted the attention of researchers, and great progress has been made in engineering applications. This article summarizes the research on acoustic metamaterials and phononic crystals in recent decades, briefly introduces some representative studies, including equivalent acoustic parameters and extraordinary characteristics of metamaterials, explains acoustic metamaterial design methods, and summarizes the technical bottlenecks and application prospects.

Quasi-Periodic Dendritic Metasurface for Integral Operation in Visible Light

A reflective metasurface model composed of silver dendritic units is designed in this study. The integral property of this metasurface, which consists of an upper layer of dendritic structures, a silica spacer, and a bottom silver substrate was demonstrated at visible wavelengths. The simulation results revealed that the metasurface can perform integral operation in the yellow and red bands; this can be easily generalized to the infrared and communication bands by scaling the transverse dimensions of this metasurface. A dendritic metasurface sample responding to red light was prepared via the bottom-up electrochemical deposition method. The integral operation property of the sample was verified experimentally. This dendritic metasurface, which can perform integral operation in visible light, can be used for big data processing technology, real-time signal processing, and beam shaping, and provides a new method for miniaturized and integrated all-optical signal processing systems.

Acoustic metamaterial capsule for reduction of stage machinery noise

Noise mitigation of stage machinery can be quite demanding and requires innovative solutions. In this work, an acoustic metamaterial capsule is proposed to reduce the noise emission of several stage machinery drive trains, while still allowing the ventilation required for cooling. The metamaterial capsule consists of c-shape meta-atoms, which have a simple structure that facilitates manufacturing. Two different metamaterial capsules are designed, simulated, manufactured, and experimentally validated that utilize an ultra-sparse and air-permeable reflective meta-grating. Both designs demonstrate transmission loss peaks that effectively suppress gear mesh noise or other narrow band noise sources. The ventilation by natural convection was numerically verified, and was shown to give adequate cooling, whereas a conventional sound capsule would lead to overheating. The noise spectra of three common stage machinery drive trains are numerically modelled, enabling one to design meta-gratings and determine their noise suppression performance. The results fulfill the stringent stage machinery noise limits, highlighting the benefit of using metamaterial capsules of simple c-shape structure.

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.

Acoustic waveguide with virtual soft boundary based on metamaterials

The use of acoustic metamaterials with novel phenomena to design acoustic waveguides with special properties has obvious potential application value. Here, we propose a virtual soft boundary (VSB) model with high reflectivity and half cycle phase loss, which consists of an acoustic propagation layer and an acoustic metamaterial layer with tube arrays. Then the waveguide designed by the VSB is presented, and the numerical and experimental results show that it can separate acoustic waves at different frequencies without affecting the continuity and the flow of the medium in the space. The VSB waveguide can enrich the functions of acoustic waveguides and provide more application prospects.

Parameter Optimization for Composite Structures of Microperforated Panel and Porous Metal for Optimal Sound Absorption Performance

The composite structure of a microperforated panel and porous metal is a promising sound absorber for industrial noise reduction, sound absorption performance of which can be improved through parameter optimization. A theoretical model is constructed for the composite structure of a microperforated panel and porous metal based on Maa’s theory and the Johnson–Champoux–Allard model. When the limited total thickness is 30 mm, 50 mm, and 100 mm respectively, dimensional optimization of structural parameters of the proposed composite structure is conducted for the optimal average sound absorption coefficient in the frequency range (2000 Hz, 6000 Hz) through a cuckoo search algorithm. Simulation models of the composite structures with optimal structural parameters are constructed based on the finite element method. Validations of the optimal composite structures are conducted based on the standing wave tube method. Comparative analysis of the theoretical data, simulation data, and experimental data validates feasibility and effectiveness of the parameter optimization. The optimal sandwich structure with an actual total thickness of 36.8 mm can obtain the average sound absorption coefficient of 97.65% in the frequency range (2000 Hz, 6000 Hz), which is favorable to promote practical application of the composite structures in the fields of sound absorption and noise reduction.

Dual-Band Fano Resonance of Low-Frequency Sound Based on Artificial Mie Resonances

It is reported both experimentally and numerically that dual-band acoustic Fano resonances (AFRs) of low-frequency sound are realized by a compound unit array composed of two types of multiple-cavity unit cells with different inner radii. Eigenmode analyses show that two types of monopolar Mie resonance (MMR) modes can be observed below 650 Hz, which arise from the coupling resonance of the overall structure and the Helmholtz resonance of each resonance cavity, respectively. Based on the MMRs with the out-of-phase characteristic induced by the mutual coupling of the two types of unit cells, the dual-band AFRs, in which the quality factor of the AFR II can exceed 600 when the ratio of the inner radii is closed to 1.0, can be observed. More interestingly, the application of the dual-band AFRs in sound encryption communication is further discussed. The proposed multiple-cavity unit cell and its associated dual-band AFRs provide diverse routes to design multiband sound devices with versatile applications, such as filtering, sensing, and communication.

Low-frequency perfect sound absorption achieved by a modulus-near-zero metamaterial

We have analytically proposed a mechanism for achieving a perfect absorber by a modulus-near-zero (MNZ) metamaterial with a properly decorated imaginary part, in which the perfect absorption (PA) is derived from the proved destructive interference. Based on the analysis, an ultrathin acoustic metamaterial supporting monopolar resonance at 157 Hz (with a wavelength about 28 times of the metamaterial thickness) has been devised to construct an absorber for low-frequency sound. The imaginary part of its effective modulus can be easily tuned by attentively controlling the dissipative loss to achieve PA. Moreover, we have also conducted the experimental measurement in impedance tube, and the result is of great consistency with that of analytical and simulated ones. Our work provides a feasible approach to realize PA (>99%) at low frequency with a deep-wavelength dimension which may promote acoustic metamaterials to practical engineering applications in noise control.

Damped resonance for broadband acoustic absorption in one-port and two-port systems

We demonstrate broadband perfect acoustic absorption by damped resonances through inclusion of lossy porous media. By minimally placing the lossy materials around the necks of single-resonance Helmholtz resonators, where acoustic energy is concentrated, we show an increase in absorption bandwidths (>100% of the resonance frequency). Using the damped resonance, we demonstrate three types of broadband acoustic absorbers in one-port and two-port systems: broadband absorbers (one-port), broadband sparse absorbers (two-port), and broadband duct absorbers (two-port). Our approach for broadband absorption allows to minimize the number of resonances for compact absorbers, while it is beneficial for practical applications owing to the minimum use of porous materials.

Subwavelength acoustic monopole source emission enhancement through dual gratings

Acoustic source emission rate is generally low at low frequencies. In this work, we propose a simple design of ‘LEGO’-type acoustic metamaterial that can significantly enhance the low frequency emission rate of an acoustic monopole source. Such enhancement is resulted from the coupling between resonances of a cavity and a dual grating comprised of two concentric layers of periodically distributed narrow slits. We develop an effective medium model to characterize the enhancement. Because of its simple structure, the metamaterial is easy to fabricate and thus facilitates the applications in various domains such as oil exploration.

Acoustic meta-atom with experimentally verified maximum Willis coupling

Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled.

Willis coupling is an additional degree of freedom, which can enhance acoustic metamaterials, by coupling monopole and dipole excitations. Here, the authors experimentally demonstrate a meta-atom with Willis coupling approaching the theoretical maximum, which is robust to thermo-viscous losses.

Acoustic meta-atom with experimentally verified maximum Willis coupling

Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled.