Acoustic extrodinary transmission based on density-near-zero metamaterials

Acoustic Zero-index metamaterials (ZIMs) have attracted much attention recently due to their rich physics and practical applications. The principle of ε-near-zero electromagnetics metamaterials equals valid for that of density-near-zero (DNZ) acoustic metamaterials. In this paper, we design a simple 2D membrane-network structure behaving as DNZ metamaterials for airborne sound. The lumped-circuit model of the metamaterial unit cell exhibits a significant resonance. Further parameter retrievals find that the effective mass density approaches zero at the frequency near the resonance. By using both full-wave simulations and lumped-circuit simulations, the unique transmission property of the metamaterial slab at the DNZ frequency is demonstrated in which the phase velocity and the wavelength approach infinity. We further investigate the extraordinary sound transmission induced by the proposed DNZ metamaterials such as efficient cloaking effect, perfect transmission in waveguides with sharp bends and wave splitter. Different from the previous schemes, no extreme parameters are required and the unit cell of membrane-network DNZ metamaterial is in deep subwavelength scale. The microstructure of the membrane network is comparatively simple, and its lump-circuit model provides a convenient way to construct the unit cell for achieving DNZ at the designed frequency. The near-zero effective density has been ascribed to the first order of the air-membrane resonance and hence the disappearance of density because the restoring force from the membrane adds a negative term to the effective mass.