Acoustic wave focusing by 2.5D graded index lens

Abstract

Three dimensional sound convergence in air underpins applications ranging from localized acoustic experiences to levitation. Most current solutions, however, are challenging to 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 of focusing in 3D emission of a standard loudspeaker at audio frequencies, which we call a 2.5D lens. First, we use finite-element simulations to describe the underpinning theory in terms of the band structure and equifrequency contours. We then describe how we manufactured two lenses, one of which has a focal length smaller than the wavelength, and compare pressure with simulations. Our measurements not only show good agreement with simulations but also highlight how the efficiency of such lenses strongly depends on how the sound is delivered to them.