We explore the harvesting of acoustic waves by leveraging a 3D-printed gradient-index phononic crystal (GRIN-PC) lens design. The concept is demonstrated numerically and experimentally for audio frequency range acoustic waves in air. Unit cell design procedure to achieve the required refractive index profile and numerical simulations of the band structure are executed using a high-fidelity finite-element model, followed by 3D simulations of the acoustic wave field for validation of the lens performance. Performance enhancement by focusing acoustic waves is quantified along with the level of anisotropy in the resulting 3D lens design. Additionally, a fully coupled multiphysics framework is developed to cover acoustic-structure interaction, piezoelectric coupling, as well as electrical load impedance. Finite-element simulations include the GRIN-PC lens and the harvester components along with basic electrical load to quantify the electrical power. In the full numerical simulations, design parameters such as the unit cell design, aperture of the lens, directional effects and anisotropy are explored in detail. Specifically, efforts are summarized on the unit cell design to minimize the directional sensitivity, toward making the lens close to omnidirectional.

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