Acoustic wave focusing by 2.5D graded index lens

Three dimensional sound convergence in air underpins applications ranging from localized acoustic experiences to levitation. Most current solutions, however, are challenging scale-up: they either require complex electronics or intricate geometries. In this paper, we propose a solution based on sonic crystals: an extruded 2D hexagonal lattice array of rigid cylinders with gradient diameters, capable focusing 3D emission standard loudspeaker at audio frequencies, which call 2.5D lens. First, use finite-element simulations describe the underpinning theory terms band structure and equifrequency contours. We then how manufactured two lenses, one has focal length smaller than wavelength, compare pressure simulations. Our measurements not only show good agreement but also highlight efficiency such lenses strongly depends is delivered them.