Broadband acoustic double-zero-index cloaking with coupled Helmholtz resonators

Acoustic double-zero-index metamaterials (DZIM) characterized by extremely large phase velocity along with no phase changes of the wave propagation inside the materials have received tremendous attention due to the fascinating physics and potential applications. However, due to the requirement of the degeneracy of dipolar and monopolar resonances and the available resonance-induced losses, the realization of highly efficient and broadband near-zero index metamaterials is still facing challenges. Here we report that by coupling two identical Helmholtz resonators with a connecting channel, acoustic DZIM can be realized. Owing to the presence of a connecting tube, the system can generate the dipolar mode that is independently tunable and the monopolar mode that is virtually unchanged. It thereby makes the mass density (rho) and the reciprocal of the bulk modulus (1/B) simultaneously crossing zero possible. We numerically obtain the transmission and phase, and then calculate the effective mass density and bulk modulus, which agree remarkably well with the experimental results. Finally, we successfully cloak a rectangle block inside a two-dimensional waveguide grafted by the designed acoustic DZIM array of unit cells. A broadband cloaking is experimentally demonstrated at 1865-1925 Hz, which can offer potential possibilities for vast practical applications.