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.

A sound diffuser is proposed based on sonic crystals, structures formed by a periodic distribution of cylindrical scatterers in a host medium, which is usually air. The diffuser is a so-called biperiodic structure, as formed by two arrays of sonic crystals with slightly different periodicities. Large diffusivity at low frequencies is achieved when the typical scale of the blocks is much larger than the periodicity of the crystals, determined by its lattice constant. An interpretation of the low frequency behavior of the diffuser is given in the homogenization limit in terms of multiple reflections and interference between the fields scattered by the different blocks. It is also shown that sonic crystal based diffusers enhance time spreading in comparison with other conventional diffusers.

Helmholtz resonators (HRs) have the advantage of extending and improving their insulating capacity when used as scatterers in noise barriers made of periodic media, such as sonic crystals (SCs). However, the interaction between multiple Bragg scattering and local resonance phenomena can increase or decrease the insulation of the barrier depending on its design. In the present work, we numerically investigate the factors that determine how such interferences occur and the specific conditions to increase the insertion loss of sonic crystal noise barriers (SCNBs) made of cylindrical scatterers with HRs. Two factors are crucial for the variation of the isolation of the barrier in the Bragg-bandgap (Bragg-BG): the orientation of the resonator mouth with respect to the incident wave, and the resonance frequency of the resonator with respect to the central frequency of the Bragg-BG. Based on this phenomenon, we propose a sonic crystal noise barrier consisting of scatterers with two Helmholtz resonators. The insertion loss of the structure is determined numerically and shows an increase of 20 dB at the BG compared to a conventional barrier with cylindrical scatterers.

A theoretical and experimental study of the propagation of sound beams in and behind two-dimensional sonic crystals at frequencies close to the band edges is presented. Beam focusing is predicted and discussed. We evaluate, by analytical numerical methods, the main focusing characteristics, such as the focal distance, the width of beam waist, and the beam quality at the waist. The field distribution is shown to depend strongly on the beam size and frequency. Experiments were performed on narrow sources radiating in ultrasound regime, although the results are extendable to arbitrary frequencies.