To facilitate the fabrication of acoustic printheads, arrays of spherical acoustic lenses are provided for bringing rf acoustic waves to essentially diffraction limited focii at or near the free surface of a pool of ink. These lenses produce focal patterns which are relatively free of localized amplitude...http://www.google.es/patents/US4751530?utm_source=gb-gplus-sharePatente US4751530 - Acoustic lens arrays for ink printing
