Switchable dual-regime focusing via all-angle self-collimation in a locally resonant sonic crystal

We present a switchable Helmholtz resonator-inspired sonic crystal (SHRSC) based on asymmetric multiresonant scatterers capable of exhibiting dual focusing functionalities through all-angle self-collimation (AASC), namely, parallel dual beam focusing (PDBF) and diffraction-limited focusing (DLF). Two distinct flat equifrequency contour frequencies, identified at fAASCLow=870 Hz and fAASCHigh=1810 Hz, enable either DLF and PDBF using only a 3-unit-cell-thick slab. Experimental measurements validate the numerical results of both DLF and PDBF effects, even when the source is positioned as far as 4a (a is periodicity constant) from the SHRSC. Switchable focusing is achieved by rotating the scatterers, which modulates the directional bandgaps and significantly affects acoustic wave transmission. Our SHRSC operates in low audible frequencies, where fewer studies have exploited AASC. The potential applications of proposed asymmetric locally resonant scatterers include acoustic focusing devices and waveguiding elements. Our findings demonstrate a compact and switchable approach to directionally controlled acoustic focusing, combining AASC-driven dual functionality with passive reconfiguration in a thin locally resonant slab.