Acoustic Metastructure Research Articles

Acoustic metamaterials characterization via laser plasma sound sources

Phononic crystals and acoustic metamaterials are expected to become an important enabling technology for science and industry. Currently, various experimental methods are used for evaluation of acoustic meta-structures, such as impedance tubes and anechoic chambers. Here we present a method for the precise characterization of acoustic meta-structures that utilizes rapid broadband acoustic pulses generated by point-like and effectively massless laser plasma sound sources. The method allows for broadband frequency response and directivity evaluations of meta-structures with arbitrary geometries in multiple sound propagation axes while also enabling acoustic excitation inside the structure. Experimental results are presented from acoustic evaluations of various phononic crystals with band gaps in the audible range, notably also in the very low frequencies, validating the predictions of numerical models with high accuracy. The proposed method is expected to boost research and commercial adoption of acoustic metamaterials in the near future.

Meta-structure based on coupled resonators for low-frequency broadband sound absorption

This work investigates and explains the behavior of an acoustic meta-structure based on different coupled Helmholtz resonators (HR). The analytical formulation of the Maa model is used to represent both the acoustic impedance and the sound absorption mechanism of the absorber. The structures are numerically modeled using finite element method and validated experimentally in an impedance tube considering sound waves of normal incidence. The absorber samples were manufactured in ABS (acrylonitrile butadiene styrene) polymer material with the additive manufacturing technique by extrusion of material. The results of the impedance tube, numerical and theoretical tests showed agreement and the sound absorber showed a sub-wavelength scale with perfect sound absorption ( 0.97) and broadband of 292 Hz in the low frequency region (200 - 600 Hz). We deduced that the excellent experimental result obtained is due to the high resolution used in the sample manufacturing (± 0.1 mm) which minimized manufacture errors. Finally, the influences of the main parameters of the HR panel (width and depth of the micro-slit) on the sound absorption coefficient of the absorber were also analyzed.

Geometric design and performance analysis of a foldcore sandwich acoustic metastructure for tunable low-frequency sound absorption

Acoustic metamaterial structures have received extensive attention for sound and vibration engineering applications from the scientific community in recent years. However, the real-life application of conventional acoustic metamaterial structures is frequently limited by fixed frequency bands and increased structural thicknesses in low-frequency noise reduction. In this study, we introduce an origami-based acoustic metamaterial structure that consists of a Miura-ori foldcore, along with a perforated and an unperforated panel. The proposed Miura-ori foldcore sandwich acoustic metastructure (MOF-SAM) exhibits adjustable low-frequency sound absorption capacities due to the foldability of the origami foldcore. Moreover, we employ numerical methods to investigate the sound absorption properties of the MOF-SAM, quantified by the sound absorption coefficient. The results indicate that the structure has a single absorption peak which is superior to that of acoustic structures composed of conventional honeycomb cores. The dissipation of acoustic energy is due to the structural vibrations of the metastructure and the losses in the folding process of the origami foldcore. The numerical results of this study show that the proposed sound absorption mechanism enables tunable low-frequency sound absorption. The geometric design and periodicity of the origami unit fragments offer multiple distinct absorption peaks and thus tunable acoustic performance. These findings of this study are expected to inspire novel designs for next-generation acoustic devices.

A novel broadband underwater sound absorption metastructure with multi-oscillators

Underwater absorptive metamaterials are rapidly developed duo local resonance mechanisms, but they still suffer from the disadvantages of poor hydrostatic resistance. The poor acoustic absorption performance at high hydrostatic pressure mainly attributes to the decreased damping coefficient and increased modulus of substrate at high hydrostatic pressure. Acoustic metastructures with helical oscillators are proposed and optimized with simulating annealing algorithm, where the helical oscillators are beneficial for easing the stress in the polymer substrate and resulting in good acoustic absorption abilities. Four acoustic absorptive metastructures without any oscillators (pure polymer substrate), with cylindrical oscillators, with helical oscillators, with both cylindrical and helical oscillators are proposed and experimentally verified in this study. All three metastructures with oscillators demonstrate excellent acoustic absorption coefficient with the absence of the hydrostatic pressure, while the metastructures with helical oscillators exhibit better hydrostatic pressure resistance compared with the metastructures with cylindrical oscillators. Besides, the coupling of helical oscillators and helical oscillators enhanced the acoustic performance furtherly. It is revealed that the acoustic metamaterials with a combination of cylindrical oscillator and helical oscillator exhibit superior performance under hydrostatic pressure up to 3 MPa in the frequency range of 0.5kHz∼10 kHz compared with the others, demonstrating a new avenue for underwater acoustic metamaterials application.

Metastructure “Trap” Coating by Acoustic Confinement Effect for Antibacterial Sonothermal Therapy

AbstractAntibacterial sonothermal therapy (ASTT) is attractive for treating implant‐associated infection, which addresses the limitations of poor penetration of photothermal therapy. However, achieving implant surface‐specific ultrasound heating is incredibly challenging. Inspired by the porous acoustic metastructure in noise reduction, the acoustic metastructure would be a new strategy for attaining ASTT. Herein, an “acoustic confinement effect” is proposed to “trap” ultrasound for the metastructure TiO2 porous coating on titanium implants. The metastructure coating accomplishes specific acoustic absorption and efficient acoustic‐to‐thermal conversion, surpassing the non‐specific heating of conventional coatings. The theoretical calculation and experiment of metastructure porous coating prove the acoustic‐to‐thermal conversion mechanism, which is also validated in metastructure porous coatings on medical polyetheretherketone, indicating it is universal. Excitingly, this acoustic metastructure “trap” can capture bacteria and local sonothermal enhancement, exhibiting superior ASTT antibacterial rates (S. aureus, 99.69%; E. coil, 99.58%) at low‐power ultrasound. In addition, the metastructure TiO2 coating mimics human bone and demonstrates outstanding osseointegration. This study sheds substantial light on developing the acoustic responsive surface, broadening a new direction of acoustic metamaterials in medical applications.

Nonreciprocal Intelligent Soundproof Barrier with Active Nonlocal Acoustic Metastructure

Sound insulation is an important issue in daily life and has applications in various scenarios. Conventional acoustic passive metamaterial barriers are designed as a reciprocal sound insulation. However, for some specific application scenarios, unidirectional acoustic insulation is required, which is an enormous challenge for breaking the reciprocity of sound transport with conventional passive metamaterials. Here, a soundproof barrier with nonlocal acoustic metastructure is proposed, which consists of a microcontroller to connect a pair of spatially separated microphones and speakers, to tailoring the acoustic wave transmission for realizing nonreciprocal acoustic insulation. The theoretical calculation and experiment measurement indicated that the active acoustic barrier with a selectable working frequency can realize the suppressive effect in one direction and the enhancive effect in the other direction for an incident acoustic signal. Particularly, the function of an active soundproof barrier can be freely switched to forward, backward, and bidirectional sound insulation by tailoring the nonlocal distance of the digital meta‐atom. Furthermore, this acoustic metastructure is extended from a one‐dimentional waveguide to a two‐dimentional plane to illustrate that it is still valid in an open‐field environment. The proposed customizable nonreciprocal soundproof barriers provide an avenue for many applications in the specific scenarios that require unidirectional sound insulation and high air permeability.

Low-frequency broadband sound absorption of the metastructure with extended tube resonators and porous materials

A composite acoustic metastructure consisting of double porous materials and resonators with extended tubes is proposed to seek low-frequency broadband sound absorption, considering the low-frequency absorption of resonators and medium–high frequency absorption of porous materials. Based on the double porosity theory and finite element simulation, the sound absorption performance of the composite metastructure is investigated, and the numerical results are verified by experiments. It has been demonstrated that the metastructure with porous materials arranged vertically or horizontally can exhibit a sound absorption coefficient greater than 0.5 at 265–2000 Hz and greater than 0.8 at 760–2000 Hz, which is significantly superior to the existing sound absorption structure with resonator and porous material with the same thickness. The dependence of the sound absorption performance on the geometric parameters of the extended tubes and the porous materials is revealed. On the premise of keeping the cross-section shape of the extended tube and the arrangement of the porous material unchanged, sound absorption performance at low frequencies depends on the diameter, length, and porosity of the extended tubes, whereas sound absorption performance at medium–high frequencies is primarily determined by the percentage of porous materials. Finally, an improved multiple population genetic algorithm (IMPGA), improved by introducing a weight factor function, is used to optimize the parameters of the composite metastructure in a finite space with a thickness of 50 mm, and the sound absorption coefficient was greater than 0.5 in even lower and broader frequency range of [225,2000] Hz. Additionally, the IMPGA can be adjusted to achieve broadband sound absorption within the target frequency range. It provides a new exploration for acoustic material design for low-frequency broadband sound absorption.

Machine learning assisted intelligent design of meta structures: a review

In recent years, the rapid development of machine learning (ML) based on data-driven or environment interaction has injected new vitality into the field of meta-structure design. As a supplement to the traditional analysis methods based on physical formulas and rules, the involvement of ML has greatly accelerated the pace of performance exploration and optimization for meta-structures. In this review, we focus on the latest progress of ML in acoustic, elastic, and mechanical meta-structures from the aspects of band structures, wave propagation characteristics, and static characteristics. We finally summarize and envisage some potential research directions of ML in the field of meta-structures.

Broadband Waterborne Multiphase Pentamode Metastructure with Simultaneous Wavefront Manipulation and Energy Absorption Capabilities.

Acoustic metastructures are artificial structures which can manipulate the wavefront in sub-wavelength dimensions, and previously proposed acoustic metastructures have been mostly realized with single materials. An acoustic metastructure with composite structure is proposed for underwater acoustic stealth considering both wavefront manipulation and sound absorption. The unit cells of the metastructure are composed of a metallic supporting lattice, interconnecting polymer materials and mass balancing columns. With the gradual modulations of equivalent physical properties along the horizontal direction of metastructure, the incident acoustic wave is reflected to other directions. Meanwhile, the polymer material inside the unit cells will dissipate the acoustic wave energy due to inherent damping properties. With the simultaneous modulations of reflected wave direction and scattering acoustic amplitude, significant improvement of the underwater stealth effect is achieved. Compared with single-phase metastructure, the Far-Field Sound Pressure Level (FFSPL) of multiphase metastructure decreases by 4.82 dB within the frequency range of 3 kHz~30 kHz. The linearized mean stress for multiphase metastructure is only 1/3 of that of single-phase metastructure due to it having much thicker struts and much more uniform stress distribution under the same hydrostatic pressure. The proposed composite structure possesses potential applications due to its acceptable thickness (80 mm) and low equivalent density (1100 kg/m3).

Reconfigurable metatiles with circular maze-like space-coiling-based acoustic metastructure for low-to-mid frequency sound attenuation

This study presents a compact, lightweight, and reconfigurable acoustic metatile for sound mitigation applications. The metatile prototype is designed based on a circular maze-like acoustic metastructure, which utilizes a space-coiling technique for enhanced acoustic performance in low-to-mid frequencies. The proposed labyrinthine acoustic metadisk structure comprises a central hollow front face sheet and two coiling-up backing cavities. Experimental results show that the metadisk has high absorption peaks of 0.81 and 0.75 at 574 and 1436 Hz, respectively, and exhibits high sound transmission loss (STL) values ( ≥25 dB) in separate wide frequency bands between 100–580 Hz and 820–1600 Hz. Based on the metadisk samples, metatiles are constructed, which are 30.5 × 30.5 cm 2 in size. The acoustic performance of these metatiles is investigated in two distinct scenarios. Two prototypes of the acoustic metatile are presented: the metacage and the metapanel. The metacage is a cubical box-shaped structure constructed using five metatiles, which showed a wideband insertion loss of ≥10 dB in 200–16 000 Hz and ≥20 dB over 400–5000 Hz. On the other hand, the metapanel is constructed using nine metatiles, which exhibited STL values of >20 dB over 125–5000 Hz and had a sound transmission class rating of 34. The study highlights the potential of circular maze-like space-coiling-based acoustic disk metastructures to be reconfigured into metatiles and assembled into a metacage or metapanel for practical sound mitigation applications.

Scalable exponentially complex representations of logical phi-bit states and experimental demonstration of an operable three phi-bit gate using an acoustic metastructure

Logical phi-bits are nonlinear acoustic modes analogous to qubits and supported by an externally driven acoustic metastructure. A correspondence is established between the state of three correlated logical phi-bits represented in a low-dimensional linearly scaling physical space and their state representation as a complex vector in a high-dimensional exponentially scaling Hilbert space. We show the experimental implementation of a nontrivial three phi-bit unitary operation analogous to a quantum circuit. This three phi-bit gate operates in parallel on the components of the three phi-bit complex state vector. While this operation would be challenging to perform in one step on a quantum computer, by comparison, ours requires only a single physical action on the metastructure.

Electrostatic Induction Nanogenerator Boosted by One‐Dimensional Metastructure: Application to Energy and Information Transmitting Smart Tag System (Adv. Sci. 11/2023)

Energy and Information-Transmitting Smart Tag System An energy and information-transmitting smart tag system is designed and demonstrated based on the electrostatic induction nanogenerator. The acoustic one-dimensional metastructure showing resonant behavior boosts the performance of nanogenerators, which is enough to transfer the electrical energy and signal simultaneously in free space. More details can be found in article number 2205141 by Geon-Ju Choi, Sang-Hyun Sohn, and Il-Kyu Park.

Transverse Particle Trapping Using Finite Bessel Beams Based on Acoustic Metamaterials

We investigate the transverse trapping force acting on a small particle on or off the central axis of zero-order finite Bessel beams. Combining the simulated fields with the Gorkov force potential, the transverse trapping behaviors for small objects are analyzed, and the reversal of the trapping behaviors is recovered when varying the paraxiality parameter of the Bessel beam. The results prove the possibility of using axisymmetric Bessel beams to trap both dense and stiff particles as well as light and soft ones. The particles can be trapped at the maximum central pressure in the main lobe and also the maximum pressure of the other transverse locations. A lossy acoustic metastructure with the ability to decouple the phase and amplitude modulations is used to generate the desired sound field, which is used to trap a foam ball as an example. Our research opens a promising avenue to the design and development of simplified acoustic tweezers.

Ultrabroadband and Reconfigurable Transmissive Acoustic Metascreen

AbstractPassive acoustic wave manipulations are severely constrained by the narrow frequency bandwidth of acoustic metastructures. In this research, an unprecedented type of reconfigurable acoustic metascreen is proposed for broadband manipulations of transmitted acoustic waves. The conceived structure is composed of uniquely designed unit cells producing the modulation of the transmitted phase shift within the full 2π range with an excellent impedance matching with the background medium. By rationally arranging the reconfigurable elements within the metascreen based on the corresponding parameter profile, different phenomena and functionalities can be easily realized. As examples, acoustic focusing and acoustic bending are presented to showcase the performance of the proposed metascreen. We indeed numerically and experimentally demonstrate the ultra‐broadband and reconfigurable features of our concept over an astonishing frequency range extending from 3 to 17 kHz, which covers the majority spectrum of the audible range of human hearing. Our work provides a unique and remarkable conceptual design of acoustic metascreen opening a promising and pragmatic route to conceive compact broadband acoustic devices, where wavefront manipulations on broadband sound signals or pulsed signals are required.

A Compact Tunable Broadband Acoustic Metastructure with Continuous Gradient Spiral Channels

Sound‐absorbing structures, an effective measure to reduce vibration and noise, have always been the desirable research hotspot in scientific and engineering communities. However, the mutual restriction between ultrathin thickness and broadband sound absorption is the key issue that hinders its development. Herein, a compact tunable broadband acoustic metastructure with continuous gradient spiral channels is presented, which has the ultrathin thickness and can efficiently realize low‐frequency broadband sound absorption. Coiled‐up individual absorbers are introduced to constitute the acoustic metastructure. The continuous gradient spiral channels are formed by successively decreasing the effective spiral length of labyrinthine cavity to achieve efficient broadband sound absorption. It is showed in the results that the sound‐absorption metastructure coupled with eight individual absorbers maintains the excellent broadband sound‐absorption effect over 0.8 in the frequency range of 487–930 Hz and a deep subwavelength thickness of 37 mm. In addition, the highly efficient low‐frequency broadband sound absorption can be implemented in the target range of 480–990 Hz through optimizing the adjustable parameters such as the effective spiral length, cross‐section height, orifice radius, and the coupling number of single absorbers. Herein, inspirations and threads are provided for the compact tunable broadband sound‐absorption design of acoustic metastructures.

Ultra-broadband sound absorption of a multiple-cavity metastructure with gradient thickness

In order to achieve ultra-broadband perfect sound absorption at low and middle frequencies, a novel acoustic metastructure composed of a multiple-cavity coupled resonator and melamine foam (MF) is proposed. Firstly, the metastructure termed as MPP-AG consisting of microperforated panel (MPP) and multiple annular groove cavities (AG) is investigated. Analytical research and finite element simulation are conducted, and the sound absorption characteristics are verified through the impedance-tube test. The effects of the MPP and backing cavity parameters on the sound absorption and coupling performance of the AG are studied. In Particular, MPP-AG with gradient depth shifts the initial frequency with a sound absorption coefficient greater than 0.5 to a lower 315 Hz at the deepest depth of 95 mm. The MPP-AG is optimized with the Genetic Algorithm and the absorption lower band with an absorption coefficient greater than 0.5 is shifted towards 278 Hz. Furthermore, through filling melamine foam into the MPP-AG, the improved metastructure MPP-AG-MF is established to broaden the absorption bandwidth. A perfect absorption (α>0.8) between 255 and 1600 Hz can be obtained at a depth of 70 mm by the optimization of the MPP-AG-MF with the Genetic Algorithm. This study can provide significant guidance for noise attenuation and sound absorber design, which is promising to be used in aerospace, transportation, construction and mechanical design, etc.

Experimental investigation of mechanical, acoustic and hybrid metamaterial designs for enhanced and multi-band electric motor noise dissipation

Acoustic metastructures exhibit an unprecedented ability to arbitrarily manipulate the properties of sound waves, including amplitude and phase modulation which has attracted much attention in both the academic and industrial communities. Design of metastructures has been extensively investigated for the purpose of the independent manipulation of acoustic and elastic waves. It is up to now unclear however if the optimal dissipation approach involves locally resonant structural elements, or locally resonant acoustic cavities or a hybrid mix of both noise abatement mechanisms. To study this problem, we investigate numerically and experimentally the noise reduction performance of acoustic and mechanically locally resonant structures and how they perform when integrated into the same application scenario. Taking the noise emission of an electric motor as the excitation regime for our study, the acoustic and mechanical metamaterial structures with respective C-shape and T-shaped resonators are judiciously designed to maximize noise dissipation within two targeted frequency bands where the motor noise peaks. A hybrid structure with a mix of both resonators is also designed. The results demonstrate that all three metastructure designs improved the noise attenuation performance with an additional sound insertion loss of up to 16 dB when compared to a flat wall structure of equivalent mass. The findings in this work demonstrate that significant noise attenuation can be obtained by incorporating these acoustic and mechanical meta-atoms into a wall architecture which may have great potential for real world applications such as surrounding electric motors which tend to emit monotonic and irritating noise.

Enhancing sound absorption for an acoustic metastructure with extended tubes at ultra-low frequency

A micro-perforated plate (MPP) as a typical acoustic absorber has ideal sound absorption performance at mid-frequency. A novel metastructure with a MPP and multiple sub-cavities (MPP-MSC) is proposed to broaden the sound absorption frequency range based on Helmholtz resonance, and its sound absorption performance is studied theoretically, numerically, and experimentally. Then, the genetic algorithm is adopted to extend the effective sound absorption band by 120 Hz and reduce the structure thickness to less than 61 mm, which is about 40% thinner than the existing structures. Additionally, a perforated plate with extended tubes (PPET) and porous sound absorptive material (PSAM) are introduced into the metastructure to enhance sound absorption performance in the ultra-low and mid-frequency range. Therefore, the composite sound absorber (PP-MPP-MSC) composed of PPET, PSAM, and MPP with superior sound absorption performance is proposed. Specifically, the effective acoustic absorption frequency of the structure can be broadened to 200–1800 Hz with a maximum thickness of 61 mm. It is worth noting that the design and fabrication of PP-MPP-MSC are relatively accessible, and effective sound absorption performance can be achieved in a certain frequency range, which presents the guiding role for current noise reduction applications.

Unfolding engineering metamaterials design: Relaxed micromorphic modeling of large-scale acoustic meta-structures

In this paper, we present a unit cell showing a band-gap in the lower acoustic domain. The corresponding metamaterial is made up of a periodic arrangement of one unit cell. We rigorously show that the relaxed micromorphic model can be used for metamaterials’ design at large scales as soon as sufficiently large specimens are considered. We manufacture the metamaterial via metal etching procedures applied to a titanium plate so as to show that its production for realistic applications is viable. Experimental tests are also carried out confirming that the metamaterials’ response is in good agreement with the theoretical design. In order to show that our micromorphic model opens unprecedented possibilities in metastructural design, we conceive a finite-size structure that is able to focus elastic energy in a confined region, thus enabling its possible subsequent use for optimizing complex structures. Indeed, thanks to the introduction of a well-posed set of micromorphic boundary conditions, we can combine different metamaterials and classical Cauchy materials in such a way that the elastic energy produced by a source of vibrations is focused in specific collection points. The design of this structure would have not been otherwise possible (via e.g., direct simulations), due to the large dimensions of the metastructure, counting hundreds of unit cells.

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.