Acoustic Metastructure Research Articles

Broadband subwavelength tunable valley edge states induced by fluid filling acoustic metastructure

Topological acoustic insulators demonstrate unusual characteristics in manipulating sound wave, which attract much attention from researchers. However, most of the recent researches are based on passive system, hampering their dispersion tunability. In this paper, a broadband subwavelength tunable fluid filling acoustic topological metastructure is studied. It is composed of perforated cells with tunable water height in the hole, which enables the dispersion of the edge state to be tuned. The inversion symmetry is broken by expanding and shrinking the adjacent holes in the unit cell. Thus, the valley Hall states with opposite Chern number form at the K point in the Brillouin zone. The edge states emerge at the boundary of the different valley Hall phases. The robustness of the edge states is verified by the straight and Z-shaped waveguide. Furthermore, the dispersion of the edge state can be altered continuously by raising and reducing the water height, giving rise to broadband variable topological states, which greatly expands the bandwidth from 40 Hz to 1033 Hz. This work offers a new method to control the topological states and shows great potential for practical application.

A foldable underwater acoustic meta-structure with broadband sound absorption at low frequency

An underwater absorber consisting of a microperforated panel, foldable channel and rubber coating with perfect low-frequency sound absorption, broadband absorption and strong resistance to deformation is presented. The theoretical prediction and simulation analysis are in good agreement. It is demonstrated that sound energy is mainly dissipated in the rubber coating due to waveform conversion at the coupling boundary. A meta-structure with low-frequency and broadband absorption is realized by optimizing the structural parameters. Moreover, at a relatively regulated low-frequency wavelength, the spatial folded structure enables a deep subwavelength dimension. The proposed meta-structure has wide potential applications in underwater noise control.

Ultra-Broadband Bending Beam and Bottle Beam Based on Acoustic Metamaterials

We report the realization of an ultra-broadband bending beam based on acoustic metamaterials by the theoretical prediction and the numerical validation. The proposed structure is composed of a series of straight tubes with spatially modulated depths. We analytically derive the depth profile required for the generation of an ultra-broadband bending beam, and examine the performance of the metastructure numerically. The design is then extended for the generation of a three-dimensional bottle beam. The transverse trapping behaviours on small rigid objects by the bottle beam are investigated based on the force potential. Our work will help the further study of broadband acoustic meta-structures, and may also find applications in a variety of fields such as ultrasound imaging, health monitoring and particle manipulations.

멤브레인 타입 음향 메타 구조의 흡음 분석

In this study, an acoustic metastructure, combined with a thin membrane and a rectangular cavity, was designed to reduce the noise of mechanical structures from 2000 Hz to 4000 Hz. Finite element method (FEM) analysis was performed through COMSOL multiphysics and the sound-absorbing structure was evaluated using the sound absorption coefficient from the 2-microphone method. Subsequently, the acoustic impedance of this metastructure was analyzed to explain the absorption mechanism of the thin film. The target frequency was controlled by carefully designing the size of the cavity to effectively reduce noise in a specific frequency region. Additionally, to determine the sound absorption effect of the thin film, the difference in the sound pressure level owing to the rectangular duct was measured.

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 3.3MHz and high-sensitivity wireless switch/control (threshold ≈0.55MPa) of micro-light-emitting diode/motor at 1MHz, 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.

Acoustic research at the National University of Singapore

Noise pollution is a major problem in many major cities in particular a small island state like Singapore with residential buildings very close to the major trunk roads and expressways. There are therefore increased interest in designs and solutions that will contribute to better sonic ambience. In this presentation, I will focus on the recent research and investigation in related to sonic crystals, acoustic metamaterials, noise barriers, and soundscape. The focus on sonic crystal research is on the use of sonic crystals in the form of sonic windows and sonic cage that would provide noise mitigation in addition to natural ventilation and day light. The design would also include the incorporation of Helmholtz’s resonators, labyrinth and micro-perforated features into the sonic crystal structures. The focus on acoustic metamaterials would be on ventilated acoustic meta-structures incorporating Helmholtz’s resonators and labyrinth or coiled-up space features. For soundscape research, a novel calibration method has been put forward for making the typical smartphone to be accurate enough as a sound level meter for the measurement of environmental noise. Some studies in terms of cabin noise of subway systems, aircraft cabins, high speed trains would be presented.

Ashok chakra-structured meta-structure as a perfect sound absorber for broadband low-frequency sound

We present thin acoustic meta-structures with subwavelength dimensions through which almost perfect sound absorption is achieved in the low-frequency domain. Our overall strategy builds on the fact that the sound absorption capabilities of the meta-structures primarily depend on the geometric dimensions and can easily be reconfigured as per requirements through a change of geometry. To analyze various possibilities, we optimize the geometric structure through hybrid regression analysis using the genetic algorithm approach and finite element-based numerical simulations so that the geometry is tuned for high attenuation of acoustic signals over a broad range of frequencies. Both theoretical and experimental data show good parity and are able to establish the meta-structure nature of the assembly with respect to different frequency bands in the low frequency domain.

Sound energy enhancement via impedance-matched anisotropic metamaterial

Sound energy enhancement realized by electronic circuit designs plays a vital part in many fields, including acoustic communication, medical imaging, acoustic positioning and sound energy harvesting. The same effect, actually, can be achieved based on anisotropic acoustic meta-structures without electronic components, which has rarely been studied before. Fabricated by 3D printing technology, a two-way spiral-shaped metamaterial and its two-dimensional array are designed in the study, which is capable to realize sound energy enhancement. Due to the impedance matching design, the meta-structure shows almost opposite transmission behaviors in orthogonal directions. The sound wave compression and resonance effects come from the high-refractive-index medium, leading to enlarged acoustic signals inside the waveguide. For the array with an inside point sound source, an energy enhancement phenomenon occurs in the wave emission area outside the structure. Through experimental results and numerical simulations, it is revealed that the sound intensity of the emission wave can be increased more than fifteen-fold compared with the insulation condition in the range of 4.8–5.8 kHz, when the anisotropic metamaterial array is applied. The flexibility of the anisotropic meta-structure and corresponding array designs enables versatile applications demanding excellent sound signal and broadband performance.

저주파수에서 동작하는 가벼운 음향 메타구조 설계

저주파수에서 동작하는 가벼운 음향 메타구조 설계

Labyrinthine acoustic metastructures enabling broadband sound absorption and ventilation

There is growing interest in the development of path coiling-based labyrinthine acoustic metamaterials for realizing extraordinary acoustical properties such as low-to-mid frequency sound absorption. We present a subwavelength labyrinthine acoustic metastructure (≤3 cm) exhibiting a superior sound absorption with a high bandwidth (more than one octave in the range of 400–1400 Hz). The metastructure is orchestrated of multiple labyrinthine unit cells of different configurations in a hexagonal array, and broadband absorption has been achieved by the dissipation of incident propagating sound waves inside the labyrinthine zigzag channels. Furthermore, the unique design of the metastructure allows for simultaneous air circulation for facilitating natural ventilation and sound absorption. The proposed unique designs may find potential applications in architectural acoustics and noise shielding where simultaneous natural ventilation and noise mitigation are required.

Tailoring edge and interface states in topological metastructures exhibiting the acoustic valley Hall effect

In this study, we investigate the acoustic topological insulator or topological metastructure, where an acoustic wave can exist only in an edge or interface state instead of propagating in bulk. Breaking the structural symmetry enables the opening of the Dirac cone in the band structure and the generation of a new band gap, wherein a topological edge or interface state emerges. Further, we systematically analyze two types of topological states that stem from the acoustic valley Hall effect mechanism; one type is confined to the boundary, whereas the other type can be observed at the interface between two topologically different structures. Results denote that the selection of different boundaries along with appropriately designed interfaces provides the acoustic waves in the band gap range with abilities of one-way propagation, dual-channel propagation, immunity from back-scattering at sharp corners, and/or transition between propagation at interfaces and boundaries. Furthermore, we show that the acoustic wave propagation paths can be tailored in diverse and arbitrary ways by combing the two aforementioned types of topological states.

Observation of higher-order topological acoustic states protected by generalized chiral symmetry.

Topological systems are inherently robust to disorder and continuous perturbations, resulting in dissipation-free edge transport of electrons in quantum solids, or reflectionless guiding of photons and phonons in classical wave systems characterized by topological invariants. Recently, a new class of topological materials characterized by bulk polarization has been introduced, and was shown to host higher-order topological corner states. Here, we demonstrate theoretically and experimentally that 3D-printed two-dimensional acoustic meta-structures can possess nontrivial bulk topological polarization and host one-dimensional edge and Wannier-type second-order zero-dimensional corner states with unique acoustic properties. We observe second-order topological states protected by a generalized chiral symmetry of the meta-structure, which are localized at the corners and are pinned to 'zero energy'. Interestingly, unlike the 'zero energy' states protected by conventional chiral symmetry, the generalized chiral symmetry of our three-atom sublattice enables their spectral overlap with the continuum of bulk states without leakage. Our findings offer possibilities for advanced control of the propagation and manipulation of sound, including within the radiative continuum.

Acoustic metastructure for effective low-frequency acoustic energy harvesting

In this study, a multifunctional acoustic metastructure is proposed to achieve both effective low-frequency sound isolation and acoustic energy harvesting. A metallic substrate with proof mass is adopted to generate the local resonant phenomenon for the purpose of overcoming the drawbacks of the previous rubber film-based acoustic metastructure; the latter usually requires an elaborate tension process. Numerical simulations show that the proposed structure exhibits excellent noise isolation performance in the low-frequency band. Meanwhile, the incident sound energy can be converted into electrical energy with the help of an added piezoelectric patch. Numerical simulation results indicate that the harvested energy can reach the mW level. The parameters’ influence on the metastructure’s vibro-acoustic and energy harvesting performance are discussed in detail. An optimized configuration is selected and used for experimental study. It is demonstrated that 0.21 mW electrical power at 155 Hz can be harvested by the proposed metastructure under 114 dB sound pressure excitation.

Double negative acoustic metastructure for attenuation of acoustic emissions

Acoustic metamaterials hold great potential for attenuation of low frequency acoustic emissions. However, a fundamental challenge is achieving high transmission loss over a broad frequency range. In this work, we report a double negative acoustic metastructure for absorption of low frequency acoustic emissions in an aircraft. This is achieved by utilizing a periodic array of hexagonal cells interconnected with a neck and mounted with an elastic membrane on both ends. An average transmission loss of 56 dB under 500 Hz and an overall absorption of over 48% have been realized experimentally. The negative mass density is derived from the dipolar resonances created as a result of the in-phase movement of the membranes. Further, the negative bulk modulus is ascribed to the combined effect of out-of-phase acceleration of the membranes and the Helmholtz resonator. The proposed metastructure enables absorption of low frequency acoustic emissions with improved functionality that is highly desirable for varied applications.

Ultrathin Ashoka Chakra like acoustic metastructure as a sound absorber

Absorption of undesirable sound is a fundamental issue with broad applications in a variety of situations, such as in a conference room, cinema hall, aircraft cabin noise, etc. A variety of sound absorbers, e.g., micro-slit, concrete walls, porous media, etc., are used for noise control but are required to have a thickness comparable to the working wavelength. Here, we report a novel ultrathin “Ashoka Chakra” like acoustic metastructure that can be utilized as a sound absorber. The developed acoustic metastructure demonstrates broad bandwidth and high absorption characteristics. Each unit cell contains 24 hollow spokes through which acoustic wavefront can move freely. Despite its simplicity, this approach provides tunability of the metastructure functionality such as operating frequency range by altering the structure dimensions. Finite element based simulations were carried out in order to optimize the metastructure’s dimensions to achieve maximum absorption. The efficacy of the metastructure is validated through excellent agreement between simulation and experimental measurements. Realization of the proposed acoustic metastructure can show promising applications in the field of sound absorption.

Thermoviscous effect on sound propagation in acoustic metastructures

Thermoviscous dissipation plays an important role in sound propagation through acoustic metastructures for a variety of applications. The thermoviscous effects rely on the resonances in the structures of subwavelength features. Our analyses and numerical simulations reveal that thermoviscous dissipation can significantly reduce the transmission of sound waves through acoustic metastructures of hybrid resonances in certain scenarios even when the thickness of the viscous boundary layer is much smaller than the width of the slit in the structure [Jiang et al., J. Acous. Soc. Am. (2017)]. To further explore how the resonances affect the significance of the losses, we analyze and simulate sound propagation in acoustic metamaterials of various configurations, including a slit connected to one or multiple resonators at the side of the slit or at the end of the slit. We take into account both wall friction and thermoviscous diffusivity in both the slits and the resonators. The results are compared with that without the resonators to gain insight into the role of the resonances. Optimal designs for both minimal dissipation and maximal absorption are addressed.

Sound energy harvesting using a doubly coiled-up acoustic metamaterial cavity

A new sound energy harvesting system that comprises an acoustic metastructure and a double-clamped piezoelectric bimorph plate is presented. A subwavelength-scale acoustic metastructure is specially designed by utilizing a doubly coiled-up configuration for the strong confinement of the sound energy inside the acoustic metamaterial cavity in the low frequency range. The amplified sound pressure in the acoustic metamaterial cavity then substantially increases the vibratory motions of the piezoelectric bimorph plate. For the purpose of generating more electricity from the sound, the resonant frequency of the acoustic metamaterial cavity is tuned to the fundamental frequency of the piezoelectric bimorph plate. The numerical and experiment results show that the proposed sound energy harvesting system increase a sound pressure level (SPL) by up to ∼16 dB, and yields an output voltage that is 6.32 times higher than that of the conventional piezoelectric energy harvesting plate. For the incident SPL of 100 dB, the maximum output power is measured as approximately 0.345 μW at a resonant frequency of 600 Hz.

Rotating the directivity of sound radiation by acoustic meta-structures

Radiation directivity is an important measure of sound fields radiated from acoustic sources. Manipulation of the directivity plays a significant role in many situations ranging from audio and auditorium acoustics to medical ultrasound applications. An acoustic meta-structure is presented here to rotate the directivity of sound radiation from sources surrounded by the meta-structure with an anisotropic acoustic property. The meta-structure consists of an array of platelets, regularly arranged in several concentric rings. The platelets’ orientation is determined from transformation acoustics model. Numerical simulations of sound fields with the structure reveal that the angle of sound directivity rotated by the structure is independent of sound frequency and source location. The rotation is verified through experimental measurements with the meta-structure fabricated by thermoplastics materials via 3D printing. The rotation angle varies over a broad range through tuning geometry parameters of the meta-structure. The meta-structure with its extreme performance can hence provide various applications in the effective control of radiation directivity in acoustic engineering.

Acoustic meta-structures based on periodic acoustic black holes

This paper presents a class of three-dimensional fully isotropic acoustic metamaterials that support the development of load-bearing thin-walled structural elements with engineered wave propagation characteristics, i.e. the meta-structures. This class of metamaterials is based on the concept of Acoustic Black Hole (ABH) that is an element able to bend and, eventually, trap acoustic waves. A periodic lattice of ABHs enables wave propagation characteristics comparable with resonant metamaterials but without the need and the fabrication complexity associated with the use of either multi-material or locally resonant inclusions. The dispersion characteristics show the existence of several interesting phenomena such as strong mode coupling, zero group velocity points on the fundamental modes, and Dirac points at the center of the Brillouin zone. Numerical simulations, conducted on a meta-structure made of a thin metal plate with an embedded slab of ABH material, show the existence of a variety of wave propagation effects (e.g., collimation and bi-refraction) in an extended operating range. Such an engineered material can pave the way to the design of thin-walled adaptive structures with fully passive wave management capabilities.