Enhanced multi-band acoustic energy harvesting using double defect modes of Helmholtz resonant metamaterial

Acoustic metamaterials (AMs) based on phononic crystals have been widely employed for acoustic energy harvesting, for their capacity to amplify incident sound waves and transfer them to piezoelectric devices. By substituting a resonator unit with a piezoelectric material having distinct characteristics, the periodicity of the AM is locally disrupted, resulting in the generation of defect bands within the band gap. At the frequencies corresponding to these defect bands, the AM exhibits the phenomenon of local resonance, which concentrates the incident acoustic energy at the defect sites and significantly enhances the output power of the piezoelectric devices. Conventional AMs primarily consist of elastic resonators, which can be regarded as spring-mass systems. The elastic resonances of these resonators lead to local resonance in the AM and are utilized for single-band acoustoelectric conversion. In contrast, Helmholtz resonators (HRs), in addition to demonstrating mechanical resonance, generate acoustic resonance at specific frequencies. By combining AM with HRs, the resulting Helmholtz AM (HAM) achieves energy localization effects within two defect bands, thereby increasing the output power and broadening the operational frequency range of the AM. This study aims to investigate the energy localization in HAM with multiple point defects within the two defect bands through numerical simulations and experimental analysis. Multiple HRs are intentionally removed from the HAM to introduce these multi-point defects. The interaction of elastic waves localized within these defects further enhances the energy harvesting efficiency of the HAM. Comparing the voltage frequency response functions, it is observed that, in both the first and second band gaps, the output voltage of the three double-defect HAM structures surpasses that of the single-defect HAM. As the distance between the two defects decreases, the energy harvesting at the defect modes intensifies due to a stronger coupling effect.