Nondiffractive sonic crystals
  We predict theoretically the nondiffractive propagation of sonic waves in periodic acoustic media (sonic crystals), by expansion into a set of plane waves (Bloch mode expansion), and by finite difference time domain calculations of finite beams. We also give analytical evaluations of the parameters for nondiffractive propagation, as well as the minimum size of the nondiffractively propagating acoustic beams. 
Isabel Perez-Arjona, Victor J. Sanchez-Morcillo, Javier Redondo,   Victor Espinosa and Kestutis Staliunas