A broadband acoustic metamaterial with impedance matching layer of gradient index

Narrowband transmission of some acoustic metamaterials limits their device applications. Here, we propose and demonstrate a broadband acoustic metamaterial comprising a space coiling structure by introducing an impedance-matching layer between air and the metamaterial. The impedance-matching layer is achieved by especially designing the parameters of the space coiling structure to form a gradient index. It is found that the metamaterial with the impedance matching layers substantially improves energy transmission in the frequency range of 2-6 kHz. We also show the capability of such a metamaterial to modulate the phase of acoustic waves with high energy transmission up to at least 60%. Published by AIP Publishing.