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Higher-order topological insulators (HOTIs) capable of hosting multi-dimensional topological states have been considered as a significant platform for wave regulation. Here, based on a "breathing" kagome lattice composed of coupled tube resonators, we demonstrate the topological phase transition induced by tuning the nearest-neighbor interactions of the tubes. Crucially, beyond the nontrivial bulk polarization, we unveil that the behaviors of the higher-order topological states are also affected by the boundary conditions, which enriches the design of the desired HOTIs. Along these lines, we propose two prototypes of acoustic topological devices and experimentally exhibit two promising applications of the topological corner states for frequency-selected energy harvesting in different conditions. Our work extends the applications of the higher-order topological states in resonant acoustic systems and provides diverse routes for designing topological devices on a subwavelength scale. [ABSTRACT FROM AUTHOR]

We report a broadband acoustic energy harvesting metasurface consisting of periodic arrangements of coupled Helmholtz resonators. Theoretical analysis and numerical and experimental results show that a high output voltage can be obtained at a wide band (460 Hz–680 Hz) under the push-pull effect on the piezoelectric composite sheet (PCS) between the two coupled cavities, which is because the acoustic pressure phases in the two coupled cavities are nearly opposite to each other. Each output can be directly connected in parallel without any rectification circuit because the output voltage from every PCS has the same phase on account of the symmetry of the proposed metasurface. The proposed design has an efficient and easy-to-integrate structure, and it can be used in broad applications for acoustic energy harvesting devices and acoustic insulators. [ABSTRACT FROM AUTHOR]