Figure 1. Coiling subunits and their acoustic responses.

(a) Schematic of the proposed coiling subunit, where the length l and the number n of the horizontal bars are adjustable parameters. (b) A typical set of the phase shift (red) and normalized amplitude (blue) spectra. (c) Phase (red) and amplitude (blue) responses varying with the numbered eight subunits optimized at 2.55 kHz. Here both (a) and (b) are exemplified by the subunit #8, and the data in (b) and (c) are collected from the outgoing far-field responses for a periodical array of identical subunits, excited by a plane wave along +y direction.