Recently, many researchers have investigated acoustic/elastic metamaterials that exhibit an unusual negative effective mass density and/or negative effective modulus. Liu et al. used lead balls coated with silicone rubber to produce a metamaterial with a negative effective mass. Fang et al. applied Helmholtz resonators to yield an acoustic metamaterial with a negative modulus. Yang et al. theoretically and experimentally investigated a membrane-type metamaterial that possesses negative effective mass. Lee et al. used a tube with side holes to produce a negative modulus metamaterial. Huang and Sun proposed a negative Young’s modulus metamaterial using rigid and massless truss members to connect two resonators. A number of acoustic/elastic metamaterials with simultaneously negative effective moduli and mass density have also been investigated.
We classify the application of acoustic/elastic metamaterials into three areas. First, metamaterials can be used to overcome the diffraction limit. Second, metamaterials can be applied to satisfy the material properties required for cloaking in the transformation method. Third, to create an acoustic black hole, metamaterials can be arranged to develop a gradient index shell, and to guide waves into the energy-dissipating metamaterial core.
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