  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.
