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%.