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.

The aim of this study is to realize an achromatic acoustic gradient-index (GRIN) phononic crystal (PC) lens system with a spatially invariant focal length over a broad operating frequency range. To this end, we propose an approach of introducing thin achromatic coating layers that can be easily assembled into the front and rear regions of the acoustic GRIN PC lens. A systematic design method based on topology optimization (TO) is developed to inversely design the achromatic coating components. The topology-optimized achromatic coating components are fabricated using 3D printing and coupled with the acoustic GRIN PC lens for acoustic characterization. Both numerical simulation and experimental characterization demonstrate the achromatic focusing capabilities of the GRIN PC lens with the designed achromatic coating layers in a wide range of frequencies (2.5 kHz–5.5 kHz). The proposed concept of applying achromatic coating layers along with the TO-based design method is expected to provide remarkable versatility to design GRIN PC lens-based applications such as energy harvesting, acoustic imaging, and acoustic wireless power transfer in broadband operation.