Acoustic focusing by a double layered acoustic grating

Abstract

Acoustic focusing has wide applications in many domains including non-destructive evaluation, medical diagnosis and particle manipulation. In the past decades, due to their outstanding capacity of wave manipulation, acoustic metasurfaces (AMSs) based on various structures, such as coiling-up space structures, Helmholtz-resonator-like structures and membrane-type structures, have been proposed to achieve different functionalities including acoustic focusing. However, given their complex geometries and narrow channels, these structures inevitably cause fabrication difficulties and limit energy transmission efficiency due to the thermoviscous loss. To overcome these drawbacks, a double layered acoustic grating (DLAG) is proposed in the paper to focus acoustic waves with high energy transmission efficiency. The DLAG consists of two layers of rigid panels with periodically perforated subwavelength slits on each layer. The surface coupling approach (SCA) which was proposed to predict the sound radiation in multiple connected spaces is employed to predict acoustic field through the DLAG. The widths of the slits and the cavity between the two layers of the DLAG can be optimized to converge the acoustic energy of an incident plane wave into a predefined focusing region, while the slits’ spacing and the thickness of the DLAG are set to be a fraction of wavelength. Using a 3D printed DLAG with the optimized geometrical size, the acoustic focusing was validated experimentally, which show very good agreements with the prediction from the SCA.