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.

We present an experimental demonstration of the subdiffractive propagation (self-collimation) of an ultrasound beam in a two-dimensional sonic crystal formed by a square array of steel cylinders inmersed in water. Measurements show that the diffractive spreading of a narrow beam is strongly reduced along the spatially modulated direction of the crystal. The effect of the finite crystal length is theoretically analyzed, resulting in a frequency shift of the subdiffractive point in good correspondence with the experimental results.

We present the experimental demonstration of self-collimation (subdiffractive propagation) of an ultrasonic beam inside a three-dimensional (3D) sonic crystal. The crystal is formed by two crossed steel cylinders structures in a woodpilelike geometry disposed in water. Measurements of the 3D field distribution show that a narrow beam, which diffractively spreads in the absence of the sonic crystal, is strongly collimated in propagation inside the crystal, demonstrating the 3D self-collimation effect.