Sound focusing by a broadband acoustic Luneburg lens

The high-performance and aberration-free broadband acoustic lens holds promise for extensive applications, yet remains challenged. In this work, a scheme is proposed, and the experimental demonstration of a planar acoustic Luneburg lens capable of focusing broadband sound ranging from 1 to 3 kHz (relative bandwidth approaching to 100%) in an aberration-free manner is presented. Concretely, plane sound within the frequency range incident from one side can be concentrated on a same point on the opposite edge of the Luneburg lens. The demanded refractive indexes of the lens are obtained from the component space coil acoustic metamaterials, which can easily manipulate the refractive index by adjusting a structural parameter. The prototype of the proposed Luneburg lens is fabricated by three-dimensional printing technology and experimentally characterized in a two-dimensional acoustic measuring platform. The measured results are consistently in good agreement with those from the numerical simulations. Finally, the proposed Luneburg lens is employed to construct a wide-angle acoustic reflector, which can produce a strong echo propagating in the direction exactly opposite to the incident wave. These results facilitate potential possibilities for developing more acoustic functional devices capable of manipulating broadband sound.