Theory for tailoring sonic devices: Diffraction dominates over refraction

Acoustic crystal devices, with dimensions on the order of several wavelengths, are studied by using the finite-difference time domain method in the moderately long wavelength propagation regime. From the focusing and imaging process performed by a square shaped lens, it is shown that diffractive effects dominate over those due to refraction. The major role of the device edge diffraction is shown by means of the well known Babinet principle. The first examples of imaging with a sonic plane lens, both with crystal structure and massive, and with an acoustic prism able to change the propagation direction of a plane wave, are presented.