Sound focusing by a broadband acoustic Luneburg lens.

The high-performance and aberration-free broadband acoustic lens holds promise for extensive applications, yet remains challenged. In this work, a scheme is proposed, and the experimental demonstration of a planar acoustic Luneburg lens capable of focusing broadband sound ranging from 1 to 3 kHz (relative bandwidth approaching to 100%) in an aberration-free manner is presented. Concretely, plane sound within the frequency range incident from one side can be concentrated on a same point on the opposite edge of the Luneburg lens.

Ultra-sparse metamaterials absorber for broadband low-frequency sound with free ventilation.

An absorptive device for broadband low-frequency sound with ventilation is essential but challenging in acoustic engineering, which is subjected to the narrow-band limitation and difficulty of balancing high-efficiency absorption and excellent ventilation. Here, we have theoretically and experimentally demonstrated an ultra-sparse (with filling ratio of 53.7%) broadband metamaterial absorber which can efficiently absorb (absorptance  >90%) sound energy ranging from 307 to 341 Hz, while enabling air to flow freely.

Multiband asymmetric sound absorber enabled by ultrasparse Mie resonators.

On the quest towards efficiently eliminating noises, the development of a subwavelength sound absorber with the capability of free ventilation remains challenging. Here, we theoretically propose and experimentally demonstrate an asymmetric metamaterial absorber constructed by tuned Mie resonators (MRs) with unbalanced intrinsic losses. The lossy MR layer is highly dissipative to consume the sound energy while the lossless one acts as an acoustically soft boundary.

Subwavelength broadband sound absorber based on a composite metasurface.

Suppressing broadband low-frequency sound has great scientific and engineering significance. However, normal porous acoustic materials backed by a rigid wall cannot really play its deserved role on low-frequency sound absorption. Here, we demonstrate that an ultrathin sponge coating can achieve high-efficiency absorptions if backed by a metasurface with moderate surface impedance.

Reversed Doppler effect based on hybridized acoustic Mie resonances.

The realization of reversed Doppler effects in double-negative acoustic metamaterials remains challenging. This paper demonstrates the reversed Doppler effect associated with sound wave propagation in negative group velocity in hybridized metamaterial (HM) system using a simple Mie-resonator configuration. Double-negative acoustic parameters act simultaneously on the effective dynamic bulk modulus and mass density within overlapped frequency region of multiple Mie resonances.

Deep-Subwavelength Holey Acoustic Second-Order Topological Insulators.

Higher-order topological insulators (HOTIs) belong to a new class of materials with unusual topological phases. They have garnered considerable attention due to their capabilities in confining energy at the hinges and corners, which is entirely protected by the topology, and have thus become attractive structures for acoustic wave studies and control. However, for most practical applications at audible and low frequencies, compact and subwavelength implementations are desirable in addition to providing robust guiding of sound beyond a single-frequency operation.

Subwavelength Acoustic Valley-Hall Topological Insulators Using Soda Cans Honeycomb Lattices.

Topological valley-contrasting physics has attracted great attention in exploring the use of the valley degree of freedom as a promising carrier of information. Recently, this concept has been extended to acoustic systems to obtain nonbackscattering sound propagations. However, previous demonstrations are limited by the cut-off frequency of 2D waveguides and lattice-scale size restrictions since the topological edge states originate from Bragg interference.

Low-frequency perfect sound absorption achieved by a modulus-near-zero metamaterial.

We have analytically proposed a mechanism for achieving a perfect absorber by a modulus-near-zero (MNZ) metamaterial with a properly decorated imaginary part, in which the perfect absorption (PA) is derived from the proved destructive interference. Based on the analysis, an ultrathin acoustic metamaterial supporting monopolar resonance at 157 Hz (with a wavelength about 28 times of the metamaterial thickness) has been devised to construct an absorber for low-frequency sound. The imaginary part of its effective modulus can be easily tuned by attentively controlling the dissipative loss to achieve PA.

Reconfigurable sound anomalous absorptions in transparent waveguide with modularized multi-order Helmholtz resonator.

Helmholtz resonators offer an ideal platform for advanced sound absorbers, but their utility has been impeded by inherent frequency range limitations and the lack of function reconfiguration. Here, we introduce a multi-order Helmholtz resonator (MHR) that allows multiple monopolar resonant modes theoretically and experimentally. The combination of these modularized MHRs further creates reconfigurable multi-band anomalous absorbers in a two-port transparent waveguide while maintaining undisturbed air ventilation.

Wide-angle asymmetric acoustic absorber based on one-dimensional lossy Bragg stacks.

Based on one-dimensional lossy Bragg stacks, an asymmetric absorber is realized for low-frequency sound waves, that is, perfect absorption can be obtained when sound waves are normally incident from one side while a small absorption can be obtained from the opposite side. Moreover, the asymmetric absorption persists for a wide incident angle of sound waves in the range from 0° to 42° with the absorptive coefficient larger than 90% from one side while less than 20% from the other side. By changing the thickness of the top sublayer, a series of interesting absorption phenomena such as Fano-resonance type absorption are further investigated.