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.
Highlights
Sonic crystals (“SC”s)19 are a special type of metamaterials, characterized by the periodic arrangement of the same unit cell, scaled and arranged into a background material
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
We show how an extruded 2D hexagonal lattice array of rigid cylinders with gradient diameters can be used to focus
Summary
Sonic crystals (“SC”s)19 are a special type of metamaterials, characterized by the periodic arrangement of the same unit cell, scaled and arranged into a background material (like air or water). The “retrieving method,” for instance, utilizes reflection and transmission coefficients to calculate the effective (acoustic) refractive index of 2D cross-shaped structures and to realize focusing in two dimensions.8–10 Jin et al.11 used multiple soft porous materials to achieve the desired refractive index across the lateral dimension of a lens designed for ultrasonic frequencies.
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