Acoustic Lens Design
  A survey of acoustic devices for focusing airborne sound is presented. We introduce a new approach to design high quality acoustic lenses based on arrays of cylindrical rigid scatterers in air. A population based stochastic search algorithm is used in conjunction with the multiple scattering theory to optimize a cluster of cylinders that focuses the sound in a prefixed focal point. Various lenses of different sized clusters, for different frequencies and with different focal lengths are presented. In general three focusing phenomena are remarked, focusing due to refraction, diffraction and focusing due to multiple scattering. The dependency on the frequency of the incident sound and the focal distance is analyzed indicating that higher frequencies and smaller focal distances favour larger amplifications in thin lenses based on multiple scattering. Furthermore, the robustness of a designed acoustic lens is studied, examining the focusing effect against errors in the cylinders' positions and their radius. 
Andreas Hakansson, Jose Sanchiz-Dehesa, Lorenzo Sanchis