Sound focusing by flat acoustic lenses without negative refraction

Abstract

We present the experimental realization of two flat acoustic lenses generated by inverse design. The lenses consist of aperiodic lattices of aluminum cylinders confined in a rectangular area. They were obtained by a design tool that combines multiple scattering theory and a genetic algorithm. The cylinders’ positions are optimized by the genetic algorithm in order to produce maximum sound amplification at the focal point. Both the focus and the dimension of the lenses are arbitrarily chosen. Our approach is illustrated by measurements in two lenses fabricated with aluminum cylinders $2\mathrm{m}$ long and having five and nine layers, respectively. Sound amplification up to $6.4\mathrm{dB}$ is obtained at the focus. The excellent agreement found between experimental pressure patterns and theoretical simulations supports our tool of design. It is concluded that flat acoustic lenses made of aperiodic distribution of scatterers can produce sound focusing with no need of negative refraction, a property that has been demonstrated in periodic lattices of scatterers.