In this project, we investigated the harvesting of sound waves by exploiting a 3D-printed gradient-index phononic crystal lens. We demonstrated the concept numerically and experimentally for focusing audio frequency range acoustic waves in air to enhance sound energy harvesting. We developed a finite-element model using COMSOL to design the unit cell dispersion properties and to construct the 3D lens for wave field simulations. COMSOL simulations were presented to confirm the focusing of incident plane waves and to study the sensitivity of the refractive index profile to the direction of wave propagation. The theoretical predictions are validated experimentally using an in-house scanning microphone setup under speaker excitation, and a very good agreement is observed between the experimental and numerical wave fields.
A circular piezoelectric unimorph harvester is placed at the focal position of the lens, and its performance is characterized with a resistor sweep in the absence and presence of the lens, resulting in more than an order of magnitude enhancement in the harvested power with the lens. The 3D-printed lens presented here substantially enhances the intensity of sound energy via focusing, yielding micro-Watt level power output, which can find applications for wireless sensors and other low-power electronic components.