Illusion for Airborne Sound Source by a Closed Layer with Subwavelength Thickness.

The past decade witnesses considerable efforts to design acoustic illusion cloak that produces the desired scattered field for a specific object illuminated by an external field. Yet the possibility of generating acoustic illusion directly for a sound source still remains unexplored despite the great fundamental and practical significance, and previous transformation acoustics-based designs need to have bulky sizes in terms of working wavelength. Here we propose to produce arbitrary illusion for an airborne sound source with no need to resort to coordinate transformation method. Based on an inherently different mechanism that uses acoustic metasurface to provide azimuthally-dependent local phase delay to the radiated wavefront, we shrink the thickness of the single layer enclosing the source to subwavelength scale without modulating the shape of layer. The performance of our scheme is demonstrated via distinct phenomena of virtually shifting the source location and introducing angular momentum. Numerical results verify our theoretical predictions, showing the extraordinary capability of the presented device to freely manipulate the radiation pattern of a simplest point source, making it acoustically appearing like another arbitrarily complicated source. Our findings open new avenues to the design and application of acoustic illusion devices and may have deep implications in many diverse fields such as architectural acoustics and biomedical engineering.