Harvesting low-frequency acoustic energy using quarter-wavelength straight-tube acoustic resonator

Abstract An acoustic energy harvester is introduced that uses a quarter-wavelength straight-tube acoustic resonator with polyvinylidene fluoride (PVDF) piezoelectric cantilever beams placed inside the resonator. When the tube is excited by an incident wave at its first acoustic eigenfrequency, an amplified acoustic resonant standing wave is developed inside the tube. The acoustic pressure gradient of the amplified standing wave then drives the vibration motion of the PVDF piezoelectric beams, generating electricity due to the direct piezoelectric effect. In order to maximize the amount of the harvested energy, each PVDF piezoelectric beam has been designed to have the same structural eigenfrequency as the acoustic eigenfrequency of the tube. With a single PVDF beam placed inside the tube, the harvested voltage and power become the maximum near the tube open inlet where the largest acoustic pressure gradient vibrates the PVDF beam. As the beam is moved to the tube closed end, the voltage and power gradually decrease due to the decreased acoustic pressure gradient. Multiple piezoelectric beams have been placed inside the tube with two different configurations: the aligned and zigzag configurations. With the zigzag configuration which has the more open path for acoustic air particle motions, the significant increases in the harvested voltage and power have been observed. Due to the interruption of acoustic air particle motion caused by the beams, it is found that placing PVDF beams near the closed tube end is not beneficial. The total output voltage of the piezoelectric beams increases linearly as the incident sound pressure increases.