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Landau levels, previously proposed and verified in condensed matter systems, are conventionally achieved by introducing an external magnetic field that interacts with electrons. In phononic systems, people have proposed the method of applying strain to structures to form artificial synthetic magnetic fields, which in turn induces the emergence of Landau levels. While most of the current implementations about Landau levels are based on three-dimensional (3D) Weyl systems, the experimental realization of chiral Landau levels in two-dimensional (2D) Dirac acoustic systems remains an open and interesting topic. In this work, we present an innovative approach to generate the chiral Landau levels within a 2D acoustic system by introducing an in-plane artificial pseudomagnetic field. Through breaking the spatial parity symmetry and opening the Dirac cones, we introduce position-dependent effective mass terms to Hamiltonian and confirm the existence of chiral Landau levels by simulations and experiments. Furthermore, We verify the strong robustness of the zeroth Landau level to different kinds of defects. This work provides a feasible way to realize chiral Landau levels in 2D acoustic systems and suggests potential applications in other 2D artificial structures. [ABSTRACT FROM AUTHOR]

Objective: This study presents a novel membrane-type acoustic metamaterial (MAM) with loaded masses of asymmetric dimensions and arrangements. The transmission losses (TL) of the proposed structures were investigated theoretically. The two types of loaded ornaments investigated were arc masses and platelets. The dependence of TL characteristics on mass positions was also investigated. Method: An analytical model based on the Galerkin method was established to predict the TL behavior. Finite element (FE) simulations were conducted to verify the analytical results. Results and Conclusions: In comparison with traditional MAMs of symmetric loaded mass, asymmetric masses are more effective in increasing the number of TL peaks as well as reducing the overall weight of the structure. The TL peak frequency coincides with the frequency of the unbound mass and steep phase change. A good agreement between theoretical and FE results was observed. [ABSTRACT FROM AUTHOR]

Recent advances in machine learning have revolutionized numerous research domains by extracting the hidden features and properties of complex systems, which are not otherwise possible using conventional ways. One such development can be seen in designing smart materials, which intersects the ability of microfluidics and metamaterials with machine learning to achieve unprecedented abilities. Microfluidics involves generating and manipulating fluids in the form of liquid streams or droplets from microliter to femtoliter regimes. However, analysis of such fluid flows is always tiresome and challenging due to the complexity involved in the integration and detection of various chemical or biological processes. On the other hand, acoustic metamaterials manipulate acoustic waves to achieve unparalleled properties, which is not possible using natural materials. Nonetheless, the design of such metamaterials relies on the expertise of specialists or on analytical models that require an enormous number of expensive function evaluations, making this method extremely complex and time-consuming. These complexities and exorbitant function evaluations of both fluidic and metamaterial systems embark on the need for the support of computational tools that can identify, process, and quantify the large amounts of intricacy, thus machine learning techniques. This review discusses the shortcomings of microfluidics and acoustic metamaterials, which are overcome by neoteric machine learning approaches for building smart materials. The following review ends by providing the importance and future perspective of integrating machine learning and optimization approaches with microfluidic-based acoustic metamaterials to build smart and efficient intelligent next-generation materials. [ABSTRACT FROM AUTHOR]

Controlling the absorption and diffusion of sound in the audible range constitutes an exciting field of research. Acoustic absorbers and diffusers perform extraordinarily well at high frequencies with sizes comparable to the wavelength of the working frequency. Conversely, efficient low-frequency attenuators demand large volumes leading to unpractical sizes, and there is now interest in determining whether the size of the resonator can be reduced while not compromising – or perhaps even decreasing – the working frequency. One viable approach is through the use of metamaterials to enable the control of device dynamics such that heavy sub-wavelength attenuation can be efficiently realised. To achieve this goal, the theoretical (including a mathematical model and the use of finite element analysis) and experimental characterisation of 3D-printed membrane-coupled Helmholtz resonator (HR) acoustic metamaterials (AMMs) is explored. The results reveal good agreement between theory and experiments, and show that membrane-coupled HR AMMs feature heavy sub-wavelength acoustic attenuation (λ/55) while also showcasing directional responses under open air conditions. These features are explained by the interplay between resonator size, membrane characteristics, and the presence of two acoustic ports. It is anticipated that, together with recent advances on smart AMMs, these systems will foster new progress in the development of dynamic AMMs for wideband attenuation. [ABSTRACT FROM AUTHOR]

Acoustic metamaterials have been widely studied for their excellent ability of vibration isolation. In this article, a 2D square lattice acoustic metamaterial structure with multiembedded cores is proposed. Based on Bloch's theorem and finite element method, the band structures and bandgap properties of the structure are calculated and analyzed, and the vibration isolation ability of the structure is investigated. The effects of structural parameters on the bandgap property are analyzed. Thus, the anisotropy of the structure is also analyzed by the contours of phased velocity and group velocity. The results show that the structure has significant bandgap properties and vibration isolation. The second‐order similarity ratio has significant effects on the bandgap property. Via increasing the second‐order similarity ratio, the total bandgap width and low‐frequency bandgap width are widened. In addition, the structure has significant anisotropy, and the energy of elastic wave propagates at the direction of 45° and 135°. The research in this article supplements the design and research of 2D lattice acoustic metamaterials and makes a beneficial exploration for future engineering applications. [ABSTRACT FROM AUTHOR]

Bubble-based metamaterials have been extensively studied both theoretically and experimentally thanks to their simple geometry and their ability to manipulate acoustic waves. The latter is partly dependent on the structural characteristics of the metamaterial and partly dependent on the incident acoustic wave. Initially, the selection of specific structural characteristics is explained by presenting the Fourier transformations of the reflected waves for different arrangements of a bubbly meta-screen subject to Gaussian excitation. Next, the numerical study focuses on the changes induced to the response of a bubbly meta-screen, subject to different excitation pulses. For complex frequency excitation the bubbles delay to return to their equilibrium position for a couple of moments, hence the energy is stored in the system during those moments. This research provides a new strategy to actively control the response of a bubbly meta-screen and seeks to inspire future studies towards further optimization of the incident pulse based on the functionalities in need. [ABSTRACT FROM AUTHOR]

In this work, the attenuation of surface seismic waves from large-scale phononic metamaterials is numerically studied. The proposed metamaterials consist of rectangular trenches that form either I-shaped or T-shaped cavities embedded at the ground surface. The numerical investigation includes the study of the response of the proposed structures for different values of their geometric parameters. In addition, modifications of the proposed structures where heavy cores coated with a soft material were considered in the cavities were also numerically studied. For a more realistic numerical approach, the transmission spectrum of a selected large-scale phononic metamaterial was also investigated in a suitable half-space numerical scheme. The results of the present research showed that the studied large-scale metastructures could be a very promising potential candidate for seismic shielding applications for the protection of existing urban or countryside structures. The proposed metamaterials are low in cost and easy to construct for the protection of existing buildings, critical infrastructures, or even entire urban areas without need for any kind of intervention at them, therefore providing an effective solution in the field of seismic isolation. [ABSTRACT FROM AUTHOR]

Featured Application: The Flexible Soundproofing Metapanel (FSM) developed in this study has potential applications where lightweight, flexible, and effective noise control solutions are crucial. This study addresses the critical challenge of developing lightweight, flexible soundproofing materials for contemporary applications by introducing an innovative Flexible Soundproofing Metapanel (FSM). The FSM represents a significant advancement in acoustic metamaterial design, engineered to attenuate noise within the 2000–5000 Hz range—a frequency band associated with significant human auditory discomfort. The FSM's novel structure, comprising a box-shaped frame and vibrating membrane, was optimized through rigorous finite element analysis and subsequently validated via comprehensive open field tests for enclosure-type soundproofing. Our results demonstrate that the FSM, featuring an optimized configuration of urethane rubber (Young's modulus 6.5 MPa) and precisely tuned unit cell dimensions, significantly outperforms conventional mass-law-based materials in sound insulation efficacy across target frequencies. The FSM exhibited superior soundproofing performance across a broad spectrum of frequency bands, with particularly remarkable results in the crucial 2000–5000 Hz range. Its inherent flexibility enables applications to diverse surface geometries, substantially enhancing its practical utility. This research contributes substantially to the rapidly evolving field of acoustic metamaterials, offering a promising solution for noise control in applications where weight and spatial constraints are critical factors. [ABSTRACT FROM AUTHOR]

Low-frequency noise absorbers often require large structural dimensions, constraining their development in practical applications. In order to improve space utilization, an acoustic metamaterial with a spatial double helix, called a spatial double helix resonator (SDHR), is proposed in this paper. An analytical model of the spatial double-helix resonator is established and verified by numerical simulations and impedance tube experiments. By comparing the acoustic absorption coefficients of the spatial double-helix resonator, it is shown that the results of the analytical model, the numerical model, and the experiments are in good agreement, proving the accuracy of the theoretical model. The effects of different structural parameters on the peak sound absorption coefficient and resonance frequency are quantitatively revealed. The impedance variation law of the model is obtained, and the resistance and reactance distributions at the resonance frequency are analyzed. In the optimization model, the Back Propagation (BP) network is used to construct the mapping between the structural parameters and the resonance frequency and sound absorption coefficient, and this is used as the constraints of the equation, which is combined with Wild Horse Optimization (WHO) to establish the BP-WHO optimization model to minimize the volume of the spatial double helix resonator. The results show that, for a given noise frequency, the optimized structural parameters enhance the space utilization without affecting the performance of the space double helix resonator. [ABSTRACT FROM AUTHOR]

Abstract Controlling the absorption and diffusion of sound in the audible range constitutes an exciting field of research. Acoustic absorbers and diffusers perform extraordinarily well at high frequencies with sizes comparable to the wavelength of the working frequency. Conversely, efficient low-frequency attenuators demand large volumes leading to unpractical sizes, and there is now interest in determining whether the size of the resonator can be reduced while not compromising – or perhaps even decreasing – the working frequency. One viable approach is through the use of metamaterials to enable the control of device dynamics such that heavy sub-wavelength attenuation can be efficiently realised. To achieve this goal, the theoretical (including a mathematical model and the use of finite element analysis) and experimental characterisation of 3D-printed membrane-coupled Helmholtz resonator (HR) acoustic metamaterials (AMMs) is explored. The results reveal good agreement between theory and experiments, and show that membrane-coupled HR AMMs feature heavy sub-wavelength acoustic attenuation (λ/55) while also showcasing directional responses under open air conditions. These features are explained by the interplay between resonator size, membrane characteristics, and the presence of two acoustic ports. It is anticipated that, together with recent advances on smart AMMs, these systems will foster new progress in the development of dynamic AMMs for wideband attenuation.

In recent years, acoustic metamaterials, as a sort of novel artificial composite materials, have demonstrated the ability to surpass the limitations of traditional materials through their exceptional acoustic parameter manipulation capabilities. Thus, acoustic metamaterials have shown promising applications in numerous areas such as underwater detection, underwater target identification, acoustic imaging, navigation, and underwater communication. The advancements in metamaterial-based underwater acoustic communication were reviewed, focusing primarily on multiplex communication based on acoustic orbital angular momentum, underwater acoustic communication between specific transmitter and receiver based on beam steering of acoustic metasurface, and water-air trans-medium acoustic communication. The key technologies were summarized, and the current challenges and future prospects of metamaterial-based underwater acoustic communication were outlined.

Abstract Acoustic metamaterials are growing in popularity for sound applications including noise control. Despite this, there remain significant challenges associated with the fabrication of these materials for the sub-100 Hz regime, because acoustic metamaterials for such frequencies typically require sub-mm scale features to control sound waves. Advances in additive manufacturing technologies have provided practical methods for rapid fabrication of acoustic metamaterials. However, there is a relatively high sensitivity of their resonant characteristics to sub-mm deviations in geometry, pushing the limits of additive manufacturing. One way of overcoming this is via active control of device resonance. Here, an acoustic metamaterial cell with adjustable resonance is demonstrated for the sub-100 Hz regime. A functionally superparamagnetic membrane—devised to facilitate the fabrication process by eliminating magnetic poling requirements—is engineered using stereolithography, and its mechanical and acoustic properties are experimentally measured using laser Doppler vibrometry and electret microphone testing, with a mathematical model developed to predict the cell response. It is demonstrated that an adjustable magnetic acoustic metamaterial can be fabricated at ultra-subwavelength dimensions ( $$\le \lambda$$ ≤ λ /77.5), exhibiting adjustable resonance from 88.73 to 86.63 Hz. It is anticipated that this research will drive new innovations in adjustable metamaterials, including wider frequency ranges.

Skyrmions, a stable topological vectorial textures characteristic with skyrmionic number, hold promise for advanced applications in information storage and transmission. While the dynamic motion control of skyrmions has been realized with various techniques in magnetics and optics, the manipulation of acoustic skyrmion has not been done. Here, the propagation and control of acoustic skyrmion along a chain of metastructures are shown. In coupled acoustic resonators made with Archimedes spiral channel, the skyrmion hybridization is found giving rise to bonding and antibonding skyrmionic modes. Furthermore, it is experimentally observed that the skyrmionic mode of acoustic velocity field distribution can be robustly transferred covering a long distance and almost no distortion of the skyrmion textures in a chain of metastructures, even if a structure defect is introduced in the travel path. The proposed localized acoustic skyrmionic mode coupling and propagating is expected in future applications for manipulating acoustic information storage and transfer. [ABSTRACT FROM AUTHOR]

The manipulation of acoustic waves is becoming increasingly crucial in research and practical applications. The coordinate transformation methods and acoustic metamaterials represent two significant areas of study that offer innovative strategies for precise acoustic wave control. This review highlights the applications of these methods in acoustic wave manipulation and examines their synergistic effects. We present the fundamental concepts of the coordinate transformation methods and their primary techniques for modulating electromagnetic and acoustic waves. Following this, we deeply study the principle of acoustic metamaterials, with particular emphasis on the superior acoustic properties of polymers. Moreover, the polymers have the characteristics of design flexibility and a light weight, which shows significant advantages in the preparation of acoustic metamaterials. The current research on the manipulation of various acoustic characteristics is reviewed. Furthermore, the paper discusses the combined use of the coordinate transformation methods and polymer acoustic metamaterials, emphasizing their complementary nature. Finally, this article envisions future research directions and challenges in acoustic wave manipulation, considering further technological progress and polymers' application potential. These efforts aim to unlock new possibilities and foster innovative ideas in the field. [ABSTRACT FROM AUTHOR]

In many noise scenarios, it is necessary to ensure ventilation and noise suppression. In this paper, a ventilated acoustic metamaterial labyrinth plate (VAMLP), formed by an array of labyrinth cells (LCs), is presented. Each labyrinth cell contains four labyrinth waveguide units (WUs). Based on the impedance series principle, an analytical model of the WU was developed and validated by a numerical model and impedance-tube experiments to determine the sound transmission loss of the WU and the LC. The mechanism of the influence of thermo-viscous loss was quantitatively analyzed, and it was clarified that the VAMLP produced sound absorption due to thermo-viscous loss. The change law of impedance at the entrance of the waveguide was analyzed, revealing the noise-reduction mechanism of the labyrinth unit. Combining a BP network and an improved sparrow search algorithm (ISSA), a BP–ISSA optimization model is proposed to optimize the ventilation capacity of the labyrinth cells. The BP-network model can accurately predict the resonance frequency from the structural parameters to form the fitness function. The ISSA optimization model was constructed using the fitness function as the constraint of an equation. Finally, the combination of structural parameters with optimal ventilation capacity was obtained for a given noise frequency. [ABSTRACT FROM AUTHOR]

Acoustic metamaterials based on out‐of‐plane actuation can flexibly reconfigure arrayed unit cells on demand, to shape sound fields for applications such as beam formation or holography. However, implementing active reconfiguration on the millimeter‐scale is challenging, due to the lack of suitable actuation methods. Besides electronic complexity, current methods suffer from limited actuation range (sub‐millimeter), and discrete steps inhibit smooth sound modulation. Here, a novel fluid‐driven approach for continuous out‐of‐plane actuation is presented. A three dimensionally (3D)‐printed fluidic chip is integrated with an elastomeric membrane, and selective inflation of membrane sections actuates acoustic reflector unit cells according to their shape and position. The compact device enables displacements >5 mm without coupling mechanisms or external power. It is experimentally demonstrate single‐channel and multi‐channel prototypes, including an ultrasonic metasurface built for controllable acoustic focusing at five different locations. The fluidic chips are monolithically printed via digital light processing without internal support material, and the membranes are fabricated by accessible and cost‐effective spacer‐based fabrication. The methods are reproducible and eliminate complex processes (e.g., adhesion of layers) commonly associated with multi‐layered micro/milli fluidic devices. The outlined approaches and concepts in this work can be applied beyond the field of metamaterials, such as for visual displays, or tactile devices. [ABSTRACT FROM AUTHOR]

The presented research delineates an extensive study aimed at obtaining and comparing optimal designs and geometries for one-dimensional periodic acoustic metamaterials to attenuate noise within the audible frequency range of 20 Hz to 20 kHz. Various periodic designs, encompassing diverse geometric parameters and shapes—from Basic-Periodic to Semi-Periodic, Tapered-Diverging, and Tapered-Converging unit cells of repeated patterns—are examined to identify the most effective configurations for this application. A thorough parametric analysis is executed employing FE-Bloch's theorem across these four configurations to determine their bandgaps and to identify the most effective geometry. A normalization process is utilized to extend the domain of the analysis and the range of the system parameters studied in this work, totaling 202,505 design cases. Finally, the optimal design is identified based on achieving the best bandgaps coverage. The study concludes with the presentation of frequency domain acoustic pressure responses at multiple sensing points along the filters, validating the performance and the obtained bandgaps through these optimal geometries. [ABSTRACT FROM AUTHOR]

Rabi‐like splitting effect in acoustics is demonstrated in a hybrid metasurface. The hybrid metasurface is composed of a membrane‐type acoustic metasurface and a Fabry–Perot (F–P) cavity. It is found that the transmission resonance of the F–P cavity is modulated to spectrum with two peaks around the resonant frequencies. The transmission valley in the middle of the two transmission peaks is caused by the antiresonance effect, and the phenomenon is similar to the Rabi‐splitting effect in quantum physics. The results are promising for utilization in designing acoustic wave devices like filtering and acoustic sensors with new mechanisms in hybrid metasurfaces. [ABSTRACT FROM AUTHOR]

An elastic medium with inclusions that are small compared to the sound wavelength and differ in density is considered. If the inclusions are resonators that respond equally to the influence of waves coming from different directions, then the effective density of the medium in a certain frequency band becomes negative. If the direction of the dipole moment of the resonators is fixed, then the medium with inclusions has an anisotropic effective density. The Helmholtz equation for such a medium was obtained, and the field of a point source was studied. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Machine learning (ML) and Deep learning (DL) are increasingly pivotal in the design of advanced metamaterials, seamlessly integrated with material or topology optimization. Their intrinsic capability to predict and interconnect material properties across vast design spaces, often computationally prohibitive for conventional methods, has led to groundbreaking possibilities. This paper introduces an innovative machine learning approach for the optimization of acoustic metamaterials, focusing on Multiresonant Layered Acoustic Metamaterial (MLAM), designed for targeted noise attenuation at low frequencies (below 1000 Hz). This method leverages ML to create a continuous model of the Representative Volume Element (RVE) effective properties essential for evaluating sound transmission loss (STL), and subsequently used to optimize the overall topology configuration for maximum sound attenuation using a Genetic Algorithm (GA). The significance of this methodology lies in its ability to deliver rapid results without compromising accuracy, significantly reducing the computational overhead of complete topology optimization by several orders of magnitude. To demonstrate the versatility and scalability of this approach, it is extended to a more intricate RVE model, characterized by a higher number of parameters, and is optimized using the same strategy. In addition, to underscore the potential of ML techniques in synergy with traditional topology optimization, a comparative analysis is conducted, comparing the outcomes of the proposed method with those obtained through direct numerical simulation (DNS) of the corresponding full 3D MLAM model. This comparative analysis highlights the transformative potential of this combination, particularly when addressing complex topological challenges with significant computational demands, ushering in a new era of metamaterial and component design. [ABSTRACT FROM AUTHOR]

Sandwich structures are widely used in various fields. Curved sandwich structures have attracted the attention of researchers owing to their superior stiffness and strength. With the application of the structures in engineering, it is required that the structures can not only meet the needs of mechanical loads but also can cope with thermal, acoustic, optical, and electrical loads. Sound insulation is one of the most common problems in the fields of aerospace, transportation, and architecture, so it is imperative to research new structures with good mechanical and acoustic properties. In this paper, a composite structure which is coupled with curve shell sandwich structure and acoustic metamaterials is proposed to obtain good mechanical and acoustic properties. Based on the harmonic expansion method and principle of virtual work, the theoretical model is established and the sound transmission loss (STL) performance is studied. Then, the effects of the geometry of the structure and material parameters on the STL of the structure are discussed. [ABSTRACT FROM AUTHOR]

Noise pollution, a major concern in modern life, contributes to various mental and psychological problems. In this study, we concentrate on the recent developments in metamaterials, particularly additively manufactured acoustic-absorbing metamaterials. These have demonstrated enormous potential for noise reduction across a broad range of frequencies. The current review attempts to classify these additive-manufactured acoustic absorbing metamaterials as perforated, slotted, cellular, and hybrid. Herein, we present a detailed study of the efficiencies and applicability of each subclass, as well as a critique of the experimental research undertaken within each subclass. The review focuses on the subtle features of their operation by evaluating the effect of varying key parameters on the sound absorption capabilities of these metamaterials. Through this comprehensive review, we aim to highlight the current status of this evolving area and motivate more study and innovation in pursuit of effective noise pollution reduction measures.

Abstract Cryptography is crucial in protecting sensitive information and ensuring secure transactions in a time when data security and privacy are major concerns. Traditional cryptography techniques, which depend on mathematical algorithms and secret keys, have historically protected against data breaches and illegal access. With the advent of quantum computers, traditional cryptography techniques are at risk. In this work, we present a cryptography idea using logical phi-bits, which are classical analogues of quantum bits (qubits) and are supported by driven acoustic metamaterials. The state of phi-bits displays superpositions similar to quantum bits, with complex amplitudes and phases. We present a representation of the state vector of single and multi-phi-bit systems. The state vector of multiple phi-bits system lies in a complex exponentially scaling Hilbert space and is used to encode information or messages. By changing the driving conditions of the metamaterial, the information can be encrypted with exceptional security and efficiency. We illustrate experimentally the practicality and effectiveness of encoding and encryption of a message using a 5 phi-bits system and emphasize the scalability of this approach to an N phi-bits system with the same processing time.

Topological band theory has conventionally been concerned with the topology of bands around a single gap. Only recently non-Abelian topologies that thrive on involving multiple gaps were studied, unveiling a new horizon in topological physics beyond the conventional paradigm. Here, we report on the first experimental realization of a topological Euler insulator phase with unique meronic characterization in an acoustic metamaterial. We demonstrate that this topological phase has several nontrivial features: First, the system cannot be described by conventional topological band theory, but has a nontrivial Euler class that captures the unconventional geometry of the Bloch bands in the Brillouin zone. Second, we uncover in theory and probe in experiments a meronic configuration of the bulk Bloch states for the first time. Third, using a detailed symmetry analysis, we show that the topological Euler insulator evolves from a non-Abelian topological semimetal phase via. the annihilation of Dirac points in pairs in one of the band gaps. With these nontrivial properties, we establish concretely an unconventional bulk-edge correspondence which is confirmed by directly measuring the edge states via. pump-probe techniques. Our work thus unveils a nontrivial topological Euler insulator phase with a unique meronic pattern and paves the way as a platform for non-Abelian topological phenomena. [ABSTRACT FROM AUTHOR]

Featured Application: Fault diagnosis for HVAC systems. Acoustic detection of machinery defaults from in-duct measurements is of practical importance in many areas, such as the health assessment of turbines in ventilation systems or engine testing in the surface and air transport sectors. This approach is, however, impeded by the low signal-to-noise ratio (SNR) observed in such environments. In this study, it is proposed to exploit the slow sound effect of Sonic Black Hole (SBH) ducted silencers to enhance the sensing of incident pulse acoustic signals with low SNR. It is found from transfer matrix and finite element modelling that fully opened SBH silencers with perforated skin interfaces are able to substantially enhance an incident pulse amplitude while channeling an air flow. We demonstrate that the graded depths of the SBH cavities provide rainbow spectral decomposition and amplification of the incident pulse frequency components, provided that impedance matching, slow sound, and critically coupled conditions are met. In-duct experiments showed the ability of a 3D printed SBH silencer to simultaneously enhance acoustic sensing and fully trap the pulse spectral components in the SBH cavities in the presence of a low-speed flow. This study opens up new avenues for the development of dual-purpose silencers designed for acoustic monitoring and noise control in duct systems without obstructing the air flow. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Metamaterials are artificial materials with properties depending mainly on their designed structures instead of their materials. Pentamode metamaterials are one type of metamaterial. They have solid structures with fluid-like properties, which can only withstand compressive stresses, not shear stresses. Two-dimensional pentamode metamaterials are easier to manufacture than three-dimensional models, so they have received wide attention. In this review, the properties, manufacturing, and applications of two-dimensional pentamode metamaterials will be discussed. Their water-like properties are their most important properties, and their velocities and anisotropy can be designed. They can be processed by wire-cut electrical discharge machining, waterjet cutting, and additive manufacturing techniques. They have a broad application prospect in acoustic fields such as acoustic stealth cloaks, acoustic waveguides, flat acoustic focusing lenses, pentamode acoustic meta-surfaces, etc. [ABSTRACT FROM AUTHOR]

Directional sensing of underwater acoustic signals is vital in many areas, such as waterborne navigation systems, abyssal signal communication, and underwater sound source localization. Here, we propose an easy-to-make acoustic sensing metamaterial device, which is made of a bubble architecture with a line defect. Both numerical and experimental demonstrations show that an input-direction-sensitive response with an acoustic pressure enhancement factor ~ 13.9 was achieved approximately at the resonant frequency. Furthermore, enhanced waterborne acoustic sensing at different frequencies was realized by adjusting the volume of the defect cavity. This strategy provides an effective and feasible method for underwater weak signal detection and localization. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Test & Measurement Technology is the property of Publishing Center of North University of China and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Skyrmions, a stable topological vectorial textures characteristic with skyrmionic number, hold promise for advanced applications in information storage and transmission. While the dynamic motion control of skyrmions has been realized with various techniques in magnetics and optics, the manipulation of acoustic skyrmion has not been done. Here, the propagation and control of acoustic skyrmion along a chain of metastructures are shown. In coupled acoustic resonators made with Archimedes spiral channel, the skyrmion hybridization is found giving rise to bonding and antibonding skyrmionic modes. Furthermore, it is experimentally observed that the skyrmionic mode of acoustic velocity field distribution can be robustly transferred covering a long distance and almost no distortion of the skyrmion textures in a chain of metastructures, even if a structure defect is introduced in the travel path. The proposed localized acoustic skyrmionic mode coupling and propagating is expected in future applications for manipulating acoustic information storage and transfer.

Low-frequency noise absorbers often require large structural dimensions, constraining their development in practical applications. In order to improve space utilization, an acoustic metamaterial with a spatial double helix, called a spatial double helix resonator (SDHR), is proposed in this paper. An analytical model of the spatial double-helix resonator is established and verified by numerical simulations and impedance tube experiments. By comparing the acoustic absorption coefficients of the spatial double-helix resonator, it is shown that the results of the analytical model, the numerical model, and the experiments are in good agreement, proving the accuracy of the theoretical model. The effects of different structural parameters on the peak sound absorption coefficient and resonance frequency are quantitatively revealed. The impedance variation law of the model is obtained, and the resistance and reactance distributions at the resonance frequency are analyzed. In the optimization model, the Back Propagation (BP) network is used to construct the mapping between the structural parameters and the resonance frequency and sound absorption coefficient, and this is used as the constraints of the equation, which is combined with Wild Horse Optimization (WHO) to establish the BP-WHO optimization model to minimize the volume of the spatial double helix resonator. The results show that, for a given noise frequency, the optimized structural parameters enhance the space utilization without affecting the performance of the space double helix resonator.

This study addresses the critical challenge of developing lightweight, flexible soundproofing materials for contemporary applications by introducing an innovative Flexible Soundproofing Metapanel (FSM). The FSM represents a significant advancement in acoustic metamaterial design, engineered to attenuate noise within the 2000–5000 Hz range—a frequency band associated with significant human auditory discomfort. The FSM’s novel structure, comprising a box-shaped frame and vibrating membrane, was optimized through rigorous finite element analysis and subsequently validated via comprehensive open field tests for enclosure-type soundproofing. Our results demonstrate that the FSM, featuring an optimized configuration of urethane rubber (Young’s modulus 6.5 MPa) and precisely tuned unit cell dimensions, significantly outperforms conventional mass-law-based materials in sound insulation efficacy across target frequencies. The FSM exhibited superior soundproofing performance across a broad spectrum of frequency bands, with particularly remarkable results in the crucial 2000–5000 Hz range. Its inherent flexibility enables applications to diverse surface geometries, substantially enhancing its practical utility. This research contributes substantially to the rapidly evolving field of acoustic metamaterials, offering a promising solution for noise control in applications where weight and spatial constraints are critical factors.

In this work, the attenuation of surface seismic waves from large-scale phononic metamaterials is numerically studied. The proposed metamaterials consist of rectangular trenches that form either I-shaped or T-shaped cavities embedded at the ground surface. The numerical investigation includes the study of the response of the proposed structures for different values of their geometric parameters. In addition, modifications of the proposed structures where heavy cores coated with a soft material were considered in the cavities were also numerically studied. For a more realistic numerical approach, the transmission spectrum of a selected large-scale phononic metamaterial was also investigated in a suitable half-space numerical scheme. The results of the present research showed that the studied large-scale metastructures could be a very promising potential candidate for seismic shielding applications for the protection of existing urban or countryside structures. The proposed metamaterials are low in cost and easy to construct for the protection of existing buildings, critical infrastructures, or even entire urban areas without need for any kind of intervention at them, therefore providing an effective solution in the field of seismic isolation.

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. [ABSTRACT FROM AUTHOR]

For more than a century, segmented displays, such as the seven‐segment display, have been a popular and cost‐effective option. They can display various commonly used characters by controlling a few custom‐shaped binary light emitters. Herein, the acoustic equivalent of segmented displays, which uses heterogeneous sound modulators to generate a limited set of acoustic holographic images, is introduced. Designing segmented acoustic displays is more challenging than optical ones due to the complex relationship between emitted sound and generated holographic images. To address this challenge, a design methodology based on unsupervised learning techniques is proposed. The approach balances the cost of acoustic displays and the quality of the images they generate, resulting in segmented displays that outperform existing general‐purpose ones when generating a finite set of acoustic images. Using simulations and physical fabrication of metamaterial‐based acoustic displays, it is proven that the approach can create segmented acoustic displays that produce high‐quality images at a lower cost. Additionally, the methodology is applied to multifrequency acoustic displays and its scalability is assessed as the number of images increases. The cost reduction through segmentation presented herein is expected to democratize sound manipulators for acoustic displays and other applications like acoustic levitation and noise cancellation. [ABSTRACT FROM AUTHOR]

With the increasing demand for household appliances, people are putting forward higher requirements for their sound quality. In this paper, we apply the theory of acoustic metamaterials to a bladeless fan and propose a curly space-type acoustic metamaterial (CSAM) to optimize the sound quality while ensuring the airflow of the bladeless fan. The acoustic transmission loss of CSAM is calculated by numerical simulations. Based on the hybrid approach to calculating aerodynamic acoustics (CAA) to calculate the aerodynamic noise of the bladeless fan, the ICFD module of Actran is used to convert the CFD simulated data into sound field data. The internal flow field and sound field of the bladeless fan with or without CSAM are compared and analyzed. Finally, an experimental test is done to verify the noise reduction effect and air velocity change after adding CSAM. The analysis shows that the change in the air velocity of the bladeless fan by adding CSAM is not apparent, and the sound pressure level at the monitoring point is reduced. The experimental results show that the noise of the bladeless fan is reduced by 4.9 dB after adding CSAM, and the wind speed at the location of the monitoring point is increased by 0.08 m/s. Without affecting the air velocity, CSAM can change the intensity of the sound source inside the bladeless fan and effectively suppress the aerodynamic noise. It demonstrates the feasibility of acoustic metamaterials to reduce aerodynamic noise. [ABSTRACT FROM AUTHOR]

The acoustic wave transmission manipulation ability is the most important performance for the acoustic metamaterials. To manipulate the acoustic transmission, the combination acoustic metamaterials structures are involved, and the two-directional acoustic penetration cloaking scheme are constructed. The combination acoustic metamaterials include chiral metamaterials and self-collimation metamaterials, the acoustic wave transmission path are manipulated to bypass from the acoustic scattering target in the penetration cloaking, and the target scattering stealth problem are effectively solved. In the end, the acoustic transmission manipulation performances are verified by numerical analysis with a finite-width acoustic metamaterial structure plates. The results provide technical support for design and application for acoustic transmission manipulation with acoustic metamaterials. [ABSTRACT FROM AUTHOR]

Poly(methyl methacrylate)-based triply periodic minimal surfaces (TPMS) structures promise great potential in phononic applications, but the complicated TPMS structure induces a design challenge for controlling their properties. Numerical acoustic simulations of seven major PMMA-based TPMS lattice structures are presented for low-frequency sound attenuation applications while varying their relative density. Except for the local resonances in primitive and Neovius-based lattice structures, the acoustic properties of other TPMS structures show a common Bragg bandgap with a central frequency of around 435 Hz and a bandwidth of around 286 Hz, which results from multiple scattering of periodic unit cells. In contrast, the acoustic bandgaps of primitive and Neovius-based lattices have much smaller and larger complete bandgaps, respectively, which are mainly attributed to the local resonances in their geometric cavities with different sizes. Thus, by taking the mechanism of generated bandgaps in the TPMS-based lattice structures into consideration, we can design suitable bandgaps for acoustic applications in the specific frequency range. [ABSTRACT FROM AUTHOR]

Purpose: A new mechanical metamaterial with a periodic fractal honeycomb structure was designed and optimized based on the vibration and noise reduction demand in engineering applications. Methods: The dynamic stiffness matrix of the structure was derived based on the spectral element method, and the frequency response curves were plotted. The feasibility and correctness of the band gap results calculated by the spectral element method were also confirmed by comparison with the finite element method. Results: Based on the finite element method, the energy band structure and vibration transmission characteristics of different order fractal structures are analyzed and calculated, and the effect of a structural fractal on the vibration band gap of different order structures is investigated. The influence of these factors on the band gap is explored by changing the structural fractal ratio and the internal concave angle when designing the optimized structure. Conclusions: The feasibility and correctness of the spectral element method are verified by comparing the calculation results of the finite element and spectral element methods. By comparing the band gaps of fractal structures of different orders, it is found that the band gaps generated by the second-order structures are lower frequency and wider frequency, which are more compatible with the current applications in engineering. By varying the fractal ratio and the internal concave angle of the structure, it is found that the larger the value of the fractal ratio, the lower the frequency at which the band gap is generated. At the same time, the increase of the internal concave angle shifts the band gap of the fractal structure toward the high-frequency range. By optimizing the design, the structural band gap is gradually shifted to the position of low-frequency broadband, and the vibration and noise reduction performance is improved. [ABSTRACT FROM AUTHOR]

Abstract One of the hallmark of topological insulators is having conductivity properties that are unaffected by the possible presence of defects. In this work, by going beyond backscattering immunity and topological invisibility across defects or disorder is obtained. Using a combination of chiral and mirror symmetry, the transmission coefficient is guaranteed to be unity. Importantly, but no phase shift is induced making the defect completely invisible. Many lattices possess the chiral‐mirror symmetry, and the principle is chosen to be demonstrated on an hexagonal lattice model with Kekulé distortion displaying topological edge waves, and analytically and numerically is shown that the transmission across symmetry preserving defects is unity. Then this lattice in an acoustic system is realized, and the invisibility is confirmed with numerical experiments. It is foreseen that the versatility of the model will trigger new experiments to observe topological invisibility in various wave systems, such as photonics, cold atoms or elastic waves.

In many noise scenarios, it is necessary to ensure ventilation and noise suppression. In this paper, a ventilated acoustic metamaterial labyrinth plate (VAMLP), formed by an array of labyrinth cells (LCs), is presented. Each labyrinth cell contains four labyrinth waveguide units (WUs). Based on the impedance series principle, an analytical model of the WU was developed and validated by a numerical model and impedance-tube experiments to determine the sound transmission loss of the WU and the LC. The mechanism of the influence of thermo-viscous loss was quantitatively analyzed, and it was clarified that the VAMLP produced sound absorption due to thermo-viscous loss. The change law of impedance at the entrance of the waveguide was analyzed, revealing the noise-reduction mechanism of the labyrinth unit. Combining a BP network and an improved sparrow search algorithm (ISSA), a BP–ISSA optimization model is proposed to optimize the ventilation capacity of the labyrinth cells. The BP-network model can accurately predict the resonance frequency from the structural parameters to form the fitness function. The ISSA optimization model was constructed using the fitness function as the constraint of an equation. Finally, the combination of structural parameters with optimal ventilation capacity was obtained for a given noise frequency.

The presented research delineates an extensive study aimed at obtaining and comparing optimal designs and geometries for one-dimensional periodic acoustic metamaterials to attenuate noise within the audible frequency range of 20 Hz to 20 kHz. Various periodic designs, encompassing diverse geometric parameters and shapes—from Basic-Periodic to Semi-Periodic, Tapered-Diverging, and Tapered-Converging unit cells of repeated patterns—are examined to identify the most effective configurations for this application. A thorough parametric analysis is executed employing FE-Bloch’s theorem across these four configurations to determine their bandgaps and to identify the most effective geometry. A normalization process is utilized to extend the domain of the analysis and the range of the system parameters studied in this work, totaling 202,505 design cases. Finally, the optimal design is identified based on achieving the best bandgaps coverage. The study concludes with the presentation of frequency domain acoustic pressure responses at multiple sensing points along the filters, validating the performance and the obtained bandgaps through these optimal geometries.