Sound Focusing by Acoustic Luneburg Lens
  Luneburg lens is a gradient index lens that focuses the incoming wave on the opposite side of the lens without aberration. We developed a two-dimensional acoustic Luneburg lens by changing the refractive index of the medium in the lens. It has a cylindrical shape and is composed of 701 aluminum columns with various radii of less than 1cm. It focuses the incoming sound wave on the edge of the opposite side of the lens without aberration as well in the frequency range of 1,000 - 3,200Hz. It increases the amplitude of the incoming sound wave by 3 - 4 times, and the amplification corresponds to the sound level difference of 10 - 15dB. The ability of the acoustic Luneburg lens as sonar was checked by VU meters in the air. Acoustic Luneburg lens has a simple structure and easy to build. It could be a strong candidate of a next generation of sonar. 
Sang-Hoon Kim
cond-mat.mtrl-sci