Symmetry-broken metamaterial for blocking, cloaking, and supertunneling of sound in a subwavelength scale

Metamaterials offer extraordinary possibilities for manipulating the propagation of the sound wave in a subwavelength scale. However, the design of acoustic metamaterials remains challenging with traditional strategies, employing two different types of acoustic resonators simultaneously or using specific substructures with multiple resonances. Here we design a symmetry-broken metamaterial comprising of only Helmholtz resonators whose periodical spatial arrangements are broken. The symmetry-broken metamaterials form a hollow hexagonal resonant absorber with two significant resonances. One is the monopolar resonance presenting a collective in-phase pattern motion independent of angle. The other is the dipolar resonance originating from the multiple scattering of symmetry-broken metamaterials. By concentrating the sound energy in peaks of their modes, the hollow hexagonal resonant absorber with extremely small filling ratio can be effectively used to block the propagation of the sound wave in a low-frequency range between monopolar and dipolar resonances. Numerical results also show that the symmetry-broken metamaterials with careful arrangement can be applied to the sound cloaking and the sound supertunneling. Published by AIP Publishing.