Theory for tailoring sonic devices: Diffraction dominates over   Refraction
  In the moderately long wavelength propagation regime, acoustic crystaldevices which size is of several wave lengths are studied by using thefinite-difference time domain method. It is shown by the focusing and imaging of a square shaped lens that the diffractive effects dominate over the refractive ones. Taking into account the well known Babinet principle, the major role of the device edge diffraction is shown. The first examples of imaging with a sonic plane lens and of an acoustic prism able to change the propagation direction of a plane wave are also presented here. 
N. Garcia, M. Nieto-Vesperinas, E.V. Ponizovskaya and M. Torres