  It is demonstrated that sonic crystals (periodic structures of sound
scatterers) can be used to design acoustic barriers that attenuate efficiently
broadband noise. Traffic noise is chosen here as an example in which our design
procedure is applied. The structures consist of cylindrical units containing
rubber crumb, a sound absorbing material. An optimization algorithm is
developed to obtain the material distribution and the dimensions of the sonic
crystal giving the best attenuation properties for this noise. The good
agreement found between predictions and measurements for a barrier (3m height)
characterized in a transmission room gives strong support to our proposal.
