Formation of collimated sound beams by three-dimensional sonic crystals
  A theoretical and experimental study of the propagation of sound beams in- and behind three-dimensional sonic crystals at frequencies close to the band edges is presented. An efficient collimation of the beam behind the crystal is predicted and experimentally demonstrated. This effect could allow the design of sources of high spatial quality sound beams. 
A. Cebrecos, V. Romero-Garcia, R. Pico, I. Perez-Arjona, V. Espinosa,   V. J. Sanchez-Morcillo and K. Staliunas
cond-mat.mtrl-sci physics.flu-dyn