  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.
