Shell-type acoustic metasurface and arc-shape carpet cloak

Recent Progress in Resonant Acoustic Metasurfaces

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.

Nonplanar metasurface for perfect absorption of sound waves

We propose a sound-absorbing nonplanar metasurface by considering locally different incidence angles along the metasurface. Perfect sound absorption is realized with the aid of hybrid resonance between two different subwavelength Helmhwoltz resonators comprising a unit cell. We theoretically investigate the effect of incidence angles on the sound absorption of the unit cells, and present a design method of the nonplanar metasurface that achieves perfect absorption by considering locally different incidence angles along the metasurface. The perfect absorption of plane sound waves on nonplanar surfaces is numerically demonstrated at the target frequency of 1 kHz. The numerical results show that at least 99.8% of the incident wave energy is absorbed by the designed metasurfaces with a thickness of λ/24. A nonplanar metasurface is fabricated via three-dimensional printing, and perfect sound absorption is experimentally validated at the target frequency of 1 kHz. Furthermore, we design nonplanar metasurfaces that can perfectly absorb cylindrical sound waves when a line source is located near the metasurface. While previous sound-absorbing metasurfaces focused only on planar surfaces, the proposed method achieves perfect sound absorption on nonplanar surfaces, expanding the range of practical applications in various industrial areas.

Elastic wave cloak and invisibility of piezoelectric/piezomagnetic mechanical metamaterials

In this paper, a piezoelectric cloaking mechanism is proposed, which makes the enclosed piezomagnetic cylinder invisible to elastic shear horizontal (SH) waves. Based on the scattering cancellation technique, the piezoelectric cloaking mechanism and dynamic stress concentration factor (DSCF) is obtained by the plane wave expansion method. A nonlinear ray trajectory equation for SH waves is derived based on the nonlinear transformation. Furthermore, piezoelectric effects on both cloaking mechanism and dynamic stress concentration are analyzed. The numerical results show that the scattering cancellation can be attributed to the cloak density, and the piezoelectric property can enhance the object's invisibility. The piezoelectric cloaking design can be applied to reduce the DSCF in some frequency regions, which means that it can change the stress distribution. It means that piezoelectric scattering cancellation can enhance both the cloaking results and structural strength of the mechanical metamaterials. This study is expected to have significance for the development and design of elastic wave metamaterials.

Generation of Non-aliased Two-dimensional Acoustic Vortex with Enclosed Metasurface

Two-dimensional (2D) acoustic vortex allows new physics and applications different from three-dimensional counterparts, yet existing mechanisms usually have to rely on active array composed of transducers which may result in complexity, high cost and, in particular, undesired spatial aliasing effect. We propose to generate 2D acoustic vortex inside an enclosed metasurface illuminated by axisymmetric wave carrying no orbital angular momentum. We derive the criterion on unit size for eliminating spatial aliasing effect which is challenging for conventional active approaches and design a membrane-based metasurface to implement our mechanism. The performance of our strategy is demonstrated via precise production of different orders of non-aliased vortices regardless of center-to-center alignment, with undistorted Bessel-like pattern extending to the whole inner region. We anticipate our design with simplicity, compactness, precision and flexibility to open up possibility to design novel vortex devices and find important applications in diverse scenarios such as on-chip particle manipulations.

Expanding the strong absorption band by impedance matched mosquito-coil-like acoustic metamaterials

A mosquito-coil-like acoustic artificial structure consisting of a spiral channel and a perforated plate with excellent impedance matching is proposed, which can realize strong sound absorption within a certain frequency range. Due to the difficulty in matching the impedance of the single-hole structure with that of the sound propagation medium, the sound absorption should be poor. To overcome this shortcoming caused by the mismatched impedance, some multi-hole microstructures are designed. Moreover, since single-chamber labyrinth can only achieve single-frequency perfect sound absorption, a labyrinthine channel is divided into several chambers with each length distributing by an arithmetic progression gradient. The sound absorption bandwidth can be extended by synergetic coupling resonance among multiple chambers. By selecting different structural parameters including the number of holes, the width of the labyrinthine channel, and the depth of labyrinthine channel, sound absorption of these mosquito-coil-like structures is investigated. The results suggest that the multi-hole structures are helpful in improving the impedance matching, while the synergetic coupling resonance among multiple chambers ensures that the sound absorption coefficient of the structure can be maintained at a high level within a certain frequency range. In addition, some mosquito-coil-like sound absorption structures are fabricated by 3D printing, then the sound absorptions under vertical sound incident conditions are measured, and the strong sound absorption ability in a wide band is experimentally demonstrated. Finally, a method is proposed for adjusting the sound absorptions by proportionally zooming in or out the structure, by which the sound absorptions of the acoustic structure can be effectively shifted to lower or higher frequencies.

Ultrathin acoustic cloaking by a conformal hybrid metasurface

Ultrathin acoustic cloaking of obstacles with arbitrary shape is achieved by a conformal hybrid metasurface, which is composed of an outer layer of phase-control metasurface (PCM) and an inner layer of near-zero-index metasurface (NZIM). Here, the PCM and NZIM are discretized into two types of labyrinth elements. The NZIM is functionally equivalent to an equiphase area and can guide the waves around the obstacle, while the PCM can perpendicularly transfer the incident waves to the NZIM and then control the emergent waves from NZIM to propagate along the original incident direction. The efficient cloaking by hybrid metasurface tightly covered on the edges of the square and circular obstacles is demonstrated, with a total thickness only 0.62 times of operating wavelength.