Creating novel materials with advantageous properties and superior performance has been a crucial engineering challenge since the early days of mankind. In the past two decades, electromagnetic (EM) metamaterials have demonstrated unique abilities, in the context of manipulating light, which are not readily available in nature. The most essential aspect of achieving the amazing functionalities of metamaterials is subwavelength-scale microstructures, which are designed to introduce local resonance and produce negative effective material properties in a specified frequency range. For photonic crystals, the band gap (a range of frequencies within which waves are prohibited from propagating) is the result of destructive interference between the waves scattered by the periodic distribution of heterogeneous media (Bragg scattering). The local resonance mechanism is of particular interest for the possibility of generating low-frequency band gaps without increasing the lattice constant, and for the possibility of providing the medium with unusual mechanical properties at long wavelengths. Locally resonating acoustic metamaterials (AMMs), as counterparts to EM metamaterials, have been studied intensively due to their strong sound absorption ability along with remarkable acoustic wave manipulation abilities, such as acoustic cloaking, acoustic focusing and imaging, nonreciprocal transmission, and wavefront engineering.
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