Gradient-index phononic crystals for omnidirectional acoustic wave focusing and energy harvesting

The design, fabrication, and analysis of omnidirectional gradient-index (GRIN) phononic crystals (PnCs) for acoustic wave focusing and energy harvesting have been demonstrated both numerically and experimentally. Despite that omnidirectional functionality is a key factor to alleviate the directivity dependence issues, the concept has not yet been incorporated into acoustic energy harvesting. In this work, a symmetrical GRIN PnC structure consisting of cylinders with variation in filling fractions has been presented to tailor the spatial acoustic refractive index, thus enforcing the acoustic waves in any direction toward the targeted center area for focusing purposes. Both a numerical simulation and experimental validation confirm substantial sound energy amplification of the designed GRIN PnC over a broad frequency range from 250 Hz to 1 kHz. Notably, the maximum sound amplification occurs at the hybrid resonant frequency of the GRIN PnC structure and the acoustic duct system used to generate incident plane waves. Numerical simulation reveals that the cavity resonance and the refraction of the GRIN PnC mainly contribute to enhanced sound amplification in addition to the reflection from the acoustic duct. The GRIN PnC structure coupled with the acoustic duct system leads to enhanced harvesting output performance when integrated with a piezoelectric energy harvesting device.