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Spatial gradients in refractive index have been used extensively in acousticmetamaterial applications to control wave propagation through phase delay. Thisstudy reports the design and experimental realization of an acoustic gradientindex lens using a sonic crystal lattice that is impedance matched to waterover a broad bandwidth. In contrast to previous designs, the underlying latticefeatures refractive indices that are lower than the water background, whichfacilitates propagation control based on a phase advance as opposed to a delay.The index gradient is achieved by varying the filling fraction of hollow,air-filled aluminum tubes that individually exhibit a higher sound speed thanwater and matched impedance. Acoustic focusing is observed over a broadbandwidth of frequencies in the homogenization limit of the lattice, withintensity magnifications in excess of 7 dB. An anisotropic lattice designfacilitates a flat-faceted geometry with low backscattering at 18 dB below theincident sound pressure level. Three dimensional Rayleigh-Sommerfeldintegration that accounts for the anisotropic refraction is used to accuratelypredict the experimentally measured focal patterns.

Transparent gradient index lens for underwater sound based on phase advance