in 2D system. To date, HOTIs have been theoretically predicted and experimentally realized in elastics, [34,35] microwaves, [36] electric circuits, [37] photonics, [38][39][40] and acoustic systems. [41][42][43][44][45][46] In order to make HOTIs more attractive for real-world applications as in sound wave control, several hurdles must be overcome. For example, most of the reported results focus solely on a single frequency band, whose limitations ought to be overcome in order to provide a broadband response for topologically robust acoustics. Also, the above reported acoustic implementations have, for the most part, been implemented inside waveguides or were designed in an acoustically rigid enclosure, which hinders their capabilities from external insonification. Lastly, in terms of compactness, it is desired to utilize building blocks of the HOTI on a subwavelength scale in order to confine sound in tight areas beyond the diffraction limit.In this work, we design topologically protected acoustic corner states at deep subwavelength scales by constructing a perforated crystal, also known as holey metamaterials. The advantage in using holey metamaterials resides in their high levels of integration and miniaturization at scales much smaller than the sound wavelength. Without being pierced by holes, those metamaterials would not be able to sustain surface-confined wave propagation. With perforations on the other hand, externally incident radiation is able to bind to the structure in the form of "spoof" surface acoustic waves (SAWs), thus enabling sound energy confinement way beyond the classical diffraction limit. [47,48] In addition to breaking the diffraction limit for spoof SAWs, we demonstrate that a topological phase transition, which is derived from an extended 2D Zak phase, can be tuned by simply shrinking or expanding the distance among a group of holes within the unit cell. Beyond measuring corner states within multiple nontrivial bandgaps, the first-order resonance in particular displays the strongest topological sound energy confinement down to a feature size of λ/50. Lastly, we experimentally verify their resilience against defects and design a HOTI device for topological subwavelength imaging, which may be relevant for sound energy focusing and detection. Figure 1a illustrates the schematic of deep-subwavelength acoustic SOTI under consideration, which is realized by a perforated rigid material whose holes are arranged in a square lattice. The perforation depth and the radii of holes are H = 12 cm and r = 0.5 cm, respectively. The lattice constant is a = 4.8 cm.The center-to-center distance between the adjacent holes in the unit cell is defined as R, which is chosen to be R/a = 0.5 for the 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...

We present the mechanism for the asymmetric absorption of acoustic waves in a two-port transparent waveguide system by shunting detuned Helmholtz resonators (HRs) pairs in cascade. Theoretical analysis, numerical simulations, and experimental measurements verify that sound energy is almost totally absorbed (96.1%) at ∼373 Hz when sound waves are incident from one side while it is largely reflected back from the opposite side by judiciously designed HRs to provide manipulated surface impedance matching/mismatching to that of air at the opposite sides of the waveguide. Thus, asymmetric acoustic absorber is achieved at a low frequency. We have further demonstrated the flexibility of this methodology to get non-reciprocal absorption and reflectance in multiband and broadband. Our design advances the concept of asymmetric acoustic manipulation in passive two-port systems and may enable sound-absorbing devices for more versatile applications.

The emerging absorptive metasurface relies on arrays of structured meta-atoms with various geometries for customized sound localization, which can significantly enhance the energy dissipation. However, most of the existing absorbers are for given frequencies at an optimal incident angle. This limitation on the working frequency and incident angle remains a challenging obstacle for their practical applications, in addition to the perfect absorptance demand. Guided by the causality principle, a physical model is established in which the absorptive properties of such systems can be fully controlled by two simple parameters (i.e., leakage factor and loss factor) which are dictated by the geometrical properties of the underlying structures. We demonstrate a subwavelength metasurface absorber which shows near-perfect absorptance (at 95%) in a broad frequency regime from 228 Hz to 319 Hz (wavelength λ from 12.6 to 9.0 times thickness) and even allows 93% reduction with a large incident angle of 60°. We prove that this broadband near-perfect absorption behavior stems from the tunable damping conditions, which can be achieved by coupling an ordinary ultrathin surface sponge coating with an artificial underdamped multiband absorptive system. From the view of the causality principle, the subwavelength near-perfect absorptions originate from the finite working bandwidth. As the research premise, we also demonstrate a λ/21.7-thick, 16.7%-filling ratio ultrasparse absorber with unity absorptance by modulating the displacements between uniformly sized coiled space resonators. The paradigm may pave the way for versatile devices in noise remediation engineering.

The perfect absorption (PA) for low-frequency audible sound waves has been achieved by critically coupling the inherent loss factor to the inherent leakage factor of a system, which is constructed by attaching a deep-subwavelength lossy resonant plate (LRP) to a backed rigid wall closely. We have certified it by using the graphical method in the complex frequency plane. By coupling the LRP to an air cavity in front of the rigid wall, the high efficient (>80%) low-frequency broadband absorption is obtained from 99.1 Hz to 294.8 Hz. Here, the thickness of LRP is only 1/13.5 of the relevant wavelength at 294.8 Hz. The impedance analyses further demonstrate that the impedances are perfectly matched between the system and the surrounding background medium at PA.

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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. In asymmetric absorption state through coupling of artificial sound soft boundary with preposed MHR, sound energy is almost totally absorbed in multiple frequency ranges when sound waves are incident from one side while it is largely reflected back from the opposite side. Interestingly, the original asymmetric absorber would turn into symmetric bidirectional absorber if one post MHR concatenates after the soft boundary. Using combination of identical MHRs, we demonstrate function selective asymmetric/symmetric absorber in multi-bands, highlighting the potential to use MHRs in the design of diverse devices for more versatile applications.

Metamaterial absorbers have recently been developed to act as efficient sound absorption components of subwavelength dimensions. However, the working frequency has so far been mainly limited to a single narrow band. Here, we demonstrate a multiband quasi-perfect absorber constructed by a double-channel Mie resonator (DMR) in a unique configuration. By attentively tuning the leakage factor to match the loss factor at multi-order monopolar and dipolar resonances of DMR simultaneously, a series of absorptive peaks with near-unity absorptances have been achieved in both numerical simulation and the experimental measurement. Our approach gives a simple platform for extending the response of metamaterial devices from the single band to the multiband without superimposing resonant elements in multiple configurations, which allows us to envision acoustic devices with versatile applications.

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. Here we engineer topologically valley-projected edge states in the form of spoof surface acoustic waves that confine along the surface of a subwavelength honeycomb lattice composed of 330-mL soda cans. The inversion symmetry is broken through injecting a certain amount of water into one of the two cans in each unit cell, which gaps the Dirac cone and ultimately leads to the topological valley-Hall phase transition. Dual-frequency ranges of the valley-projected edge states below the sound line are observed, which originate from the first-order and second-order resonances, respectively. These results have the potential to enable promising routes to design integrated acoustic devices based on valley-contrasting physics.