Search Results (38)

Acoustic metamaterials (AMs) are artificially structured materials composed of subwavelength units that enable acoustic phenomena not achievable with conventional materials and structures. This review defines and presents a distinct category referred to as soft acoustic metamaterials (SAMs), which use soft materials or reconfigurable structures to achieve enhanced acoustic functionality. These systems make use of the inherent flexibility of their materials or the deformability of their geometry to support passive, active, and adaptive functions. To capture this structural and functional diversity, we introduce a three-dimensional classification that considers geometry, acoustic control mechanisms, and functional responsiveness as interrelated aspects. The geometry is classified into two-dimensional metasurfaces and three-dimensional bulk structures. The control mechanisms include local resonance, phase modulation, attenuation, and structural reconfiguration. The response type refers to whether the system behaves passively, actively, or adaptively. Using this approach, we provide an overview of representative implementations and compare different design approaches to highlight their working principles and application areas. This review presents a structured classification for soft acoustic metamaterials and offers a foundation for future research, with broad potential in intelligent sound systems, wearable acoustics, and architectural applications. Full article

Phase gradient acoustic metasurfaces often exhibit pronounced structural dependence in imaging applications, with significant performance variations arising from differences in the negative effective parameters of resonant unit cells. However, the relationship between imaging performance and negative effective parameters near resonance frequencies—particularly in multi-band nested structures—remains insufficiently studied. To address this knowledge gap, this work combines effective parameter theory with local resonance characteristics to construct a comparative model investigating how negative effective mass density and modulus influence the imaging quality of single-band and dual-band nested metasurfaces in series and parallel configurations. The results demonstrate that (1) for single-band structures, imaging performance positively correlates with the absolute value of negative effective parameters; (2) in dual-band configurations, smaller inter-band differences in negative parameter values yield more stable imaging; and (3) series-type nested structures exhibit superior reflection imaging performance compared to parallel-type structures, though with marginally reduced design flexibility. This study elucidates the fundamental mechanisms through which negative parameters govern acoustic metasurface imaging and provides theoretical foundations for designing multi-band acoustic devices. Full article

This study presents the design and numerical validation of a grating-type labyrinthine acoustic metasurface capable of full 0–2π phase modulation with high transmission efficiency. By tuning the tooth length of the subwavelength unit cells, precise control of the transmission phase is achieved while maintaining a high transmission coefficient across the operational bandwidth. The proposed metasurface structure is evaluated through comprehensive finite element simulations using COMSOL Multiphysics 6.0 at a center frequency of 4000 Hz. The following five core wavefront manipulation functionalities are demonstrated: complete phase modulation, anomalous refraction, planar wave focusing, cylindrical-to-plane wave conversion, and cylindrical wave focusing. Each functionality is validated across a 400 Hz frequency range to confirm robust broadband performance. The metasurface exhibits minimal phase degradation and maintains high spatial coherence across varying frequencies, highlighting its potential for applications in acoustic beam steering, imaging, and wavefront engineering. Full article

Acoustic metasurfaces play a key role in building acoustics, noise control, and acoustic cloaking by regulating the acoustic wave scattering characteristics through subwavelength structures. The design of diffusely reflecting metasurfaces aims to achieve a uniform distribution of a scattered field, which is essentially a high-dimensional nonconvex optimization problem that needs to balance the computational efficiency in the synergistic optimization of the spatial arrangement of cells and the angular response. In traditional methods, a heuristic algorithm is prone to local optimization, and it is difficult to balance the global search and local adjustment. And full-wave simulation is time consuming and seriously restricts the design efficiency. Therefore, the hybrid tornado-gradient descent optimization algorithm (VDGD) is proposed in this paper. It uses a two-stage collaborative optimization approach to refine the reflection angle distribution of acoustic metasurfaces, thereby enhancing the uniformity of the diffuse acoustic field. The Tornado Optimization Algorithm (TOA) was initially employed to introduce global perturbations to the randomly initialized design. Local optimization can be avoided by gradually decreasing the perturbation magnitude, which reduces the standard deviation of the sound field from about 5.81 dB to about 4.07 dB. Then, the gradient descent is used for local fine adjustment to further reduce the standard deviation to about 1.91 dB. Experimental results show that the VDGD algorithm outperforms the seven classical and up-to-date optimization algorithms in improving scattering uniformity. This method achieves an effective balance between global search and local fine tuning, providing an efficient and flexible optimization strategy for metasurface design, which can bring application support for intelligent acoustic devices and sound field regulation. Full article

Tunable perfect acoustic absorption at subwavelength thickness has been a prominent topic in scientific research and engineering applications. Although metamaterials such as labyrinthine metasurfaces and coiling-up-space metamaterials can achieve subwavelength low-frequency acoustic absorption, efficiently realizing tunable absorption under uniform and limited size conditions remains challenging. In this paper, we introduce a folded slit to enhance the micro-slit acoustic absorber, effectively improving its low-frequency acoustic absorption performance and successfully achieving a perfect acoustic absorption coefficient of 0.99 at a thickness of only 3.1 cm. By adjusting just two parameters of the folded area, we can efficiently achieve a tunable resonant frequency ranging from 525 to 673 Hz and a tunable acoustic absorption bandwidth of 56.5% to 60.2%, simultaneously maintaining uniform external dimensions. Additionally, the folded-slit absorber demonstrates a broader acoustic absorption bandwidth at lower frequencies, enhancing broadband absorption capabilities in the low-frequency domain. These results hold significant potential for the design of highly efficient, thin and tunable acoustic absorbers. Full article

In this paper, we proposed an ultra-thin composite metasurface for broadband sound absorption, in which a compound Helmholtz structure and porous materials are coupled in a parallel-series arrangement. The Helmholtz structure comprises multiple compound cells with hybrid-damping modes, in which the over-damping and matched-damping impedance are integrated for a lower and broader absorption spectrum. By coupling the porous materials, the metasurface obtains above 85% average absorption over 750–10,000 Hz with a thickness of 31 mm, and the performance below 1600 Hz is significantly enhanced compared to the pure porous materials. This metasurface could possess broad applications in modern equipment considering its extraordinary absorption and compact structure. Full article

This study introduces a novel thermoacoustic (TA) focusing system enhanced by Airy beam-based acoustic metasurfaces, significantly improving acoustic focusing and efficiency. The system integrates a TA emitter, fabricated from carbon nanotube (CNT) films, with a binary acoustic metasurface capable of generating quasi-Airy beams. Through finite element simulations, the system’s heat conduction, acoustic focusing, and self-healing properties were thoroughly analyzed. The results demonstrate that the system achieves superior sub-wavelength focusing, tunable focal length via frequency control, and robust self-healing, even in the presence of obstacles. These findings address current limitations in TA emitters and suggest broader applications in medical ultrasound and advanced technology. Full article

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. Full article

Different types of resonators are used to create acoustic metamaterials and metasurfaces. Recent studies focused on the use of multiple resonators of the dipole, quadrupole, octupole, and even hexadecapole types. This paper considers the theory of an acoustic metasurface, which is a flat surface with a periodic arrangement of multipole resonators. The sound field reflected by the metasurface is determined. If the distance between the resonators is less than half the wavelength of the incident plane wave, the far field can be described by a reflection coefficient that depends on the angle of incidence. This allows us to characterize the acoustic properties of the metasurface by a homogenized boundary condition, which is a high-order tangential impedance boundary condition. The tangential impedance depending on the multipole order of the resonators is introduced. In addition, we analyze the sound absorption properties of these metasurfaces, which are a critical factor in determining their performance. The paper presents a theoretical model for the subwavelength case that accounts for the multipole orders of resonators and their impact on sound absorption. The maximum absorption coefficient for a diffuse sound field, as well as the optimal value for the homogenized impedance, are calculated for arbitrary multipole orders. The examples of the multipole resonators, which can be made from a set of Helmholtz resonators or membrane resonators, are discussed as well. Full article

Existing research in metasurface design was based on trial-and-error high-intensity iterations and requires deep acoustic expertise from the researcher, which severely hampered the development of the metasurface field. Using deep learning enabled the fast and accurate design of hypersurfaces. Based on this, in this paper, an integrated learning approach was first utilized to construct a model of the forward mapping relationship between the hypersurface physical structure parameters and the acoustic field, which was intended to be used for data enhancement. Then a dual-feature fusion model (DFCNN) based on a convolutional neural network was proposed, in which the first feature was the high-dimensional nonlinear features extracted using a data-driven approach, and the second feature was the physical feature information of the acoustic field mined using the model. A convolutional neural network was used for feature fusion. A genetic algorithm was used for network parameter optimization. Finally, generalization ability verification was performed to prove the validity of the network model. The results showed that 90% of the integrated learning models had an error of less than 3 dB between the real and predicted sound field data, and 93% of the DFCNN models could achieve an error of less than 5 dB in the local sound field intensity. Full article

Acoustic metasurfaces, as two-dimensional acoustic metamaterials, are a current research topic for their sub-wavelength thickness and excellent acoustic wave manipulation. They hold significant promise in noise reduction and isolation, cloaking, camouflage, acoustic imaging, and focusing. Resonant structural units are utilized to construct acoustic metasurfaces with the unique advantage of controlling large wavelengths within a small size. In this paper, the recent research progresses of the resonant metasurfaces are reviewed, covering the design mechanisms and advances of structural units, the classification and application of the resonant metasurfaces, and the tunable metasurfaces. Finally, research interest in this field is predicted in future. Full article

The transition to sustainable or green(er) cities requires the development and implementation of many innovative technologies. It is vital to ensure that these technologies are themselves as sustainable and green as possible. In this context, smart materials offer excellent prospects for application. They are capable of performing a number of tasks (e.g., repair, opening/closing, temperature measurement, storage and release of thermal energy) without embedded electronics or power supplies. In this short review paper, we present some of the most promising smart material-based technologies for sustainable or green(er) cities. We will briefly present the state-of-the-art in smart concrete for the structural health monitoring and self-healing of civil engineering structures, phase-change materials (PCM) for passive air-conditioning, shape-memory materials (SMA) for various green applications, and meta-surfaces for green acoustics. To better illustrate the potential of some of the solutions discussed in the paper, we present, where appropriate, our most recent experimental results (e.g., embedded SAW sensors for the Structural Health Monitoring of concrete structures). The main aim of this paper is to promote green solutions based on smart materials to engineers and scientists involved in R&D projects for green(er) cities. Full article

Focused ultrasound featuring non-destructive and high sensitivity has attracted widespread attention in biomedical and industrial evaluation. However, most traditional focusing techniques focus on the design and improvement of single-point focusing, neglecting the need to carry more dimensions of multifocal beams. Here we propose an automatic multifocal beamforming method, which is implemented using a four-step phase metasurface. The metasurface composed of four-step phases improves the transmission efficiency of acoustic waves as a matching layer and enhances the focusing efficiency at the target focal position. The change in the number of focused beams does not affect the full width at half maximum (FWHM), revealing the flexibility of the arbitrary multifocal beamforming method. Phase-optimized hybrid lenses reduce the sidelobe amplitude, and excellent agreement is observed between the simulation and experiments for triple-focusing beamforming metasurface lenses. The particle trapping experiment further validates the profile of the triple-focusing beam. The proposed hybrid lens can achieve flexible focusing in three dimensions (3D) and arbitrary multipoint, which may have potential prospects for biomedical imaging, acoustic tweezers, and brain neural modulation. Full article

We numerically and experimentally investigated the behavior of high-frequency underwater ultrasounds reflected by gradient acoustic metasurfaces. Metasurfaces were fabricated with a periodic array of gradient slits along the surface of a steel specimen. The finite element method was adopted for the acoustics–structure interaction problem to design the metasurfaces and simulate the reflected fields of the incident ultrasound. Our metasurfaces yielded anomalous reflection, specular reflection, apparent negative reflection, and radiation of surface-bounded modes for ultrasonic waves impinging on the metasurfaces at different incident angles. The occurrence of these reflection behaviors could be explained by the generalized Snell’s law for a gradient metasurface with periodic supercells. We showed that at some incident angles, strong anomalous reflection could be generated, which could lead to strong retroreflection at specific incident angles. Furthermore, we characterized the time evolution of the reflections using pulsed ultrasound. The simulated transient process revealed the formation of propagating reflected ultrasound fields. The experimentally measured reflected ultrasound signals verified the distinct reflection behaviors of the metasurfaces; strong anomalous reflection steering the ultrasound pulse and causing retroreflection was observed. This study paves the way for designing underwater acoustic metasurfaces for ultrasound imaging and caustic engineering applications using pulsed ultrasound in the high-frequency regime. Full article

Phase change metasurfaces based on VO₂, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO₂ and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO₂ film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO₃ substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO₃ substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO₂/TiO₂/LiNbO₃ composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology. Full article