Focusing and directional beaming effects of airborne sound through a   planar lens with zigzag slits
  Based on the Huygens-Fresnel principle we design a planar lens to efficiently realize the interconversion of the point-like source and Gaussian beam in the air ambience. The lens is constructed by a planar plate drilled elaborately with a nonuniform array of zigzag slits, where the slit exits act as subwavelength-sized secondary sources carrying desired sound responses. The experiments operated at audible regime agree well with the theoretical predictions. This compact device could be useful in daily life applications, such as for medical and detection purposes. 
Kun Tang, Chunyin Qiu, Jiuyang Lu, Manzhu Ke, and Zhengyou Liu
physics.optics