A compact and low-frequency acoustic energy harvester using layered acoustic metamaterials

As an environmental energy, air-borne sound can be harvested for the long-term local energy supply of wireless sensors and portable devices. A compact acoustic energy harvesting (AEH) system is proposed, which consists of a beam-based PZT transducer and a dual-layer of acoustic metamaterial denoted as the layered acoustic metamaterial (LAM). The pressure amplification effect inside LAM can substantially increase motions of the PZT transducer. Therefore, in order to effectively improve the electromechanical conversion efficiency, the first bending mode of the PZT transducer is designed to generate the resonating vibration at the frequency related to the pressure amplification effect. An impedance-based theoretical model is developed to investigate the conditions for maximizing the pressure amplification effect. Experimental results show that the LAM sample in the proposed AEH system enhances the sound pressure level by 18 dB at 318 Hz. Under the condition of the incident sound pressure level of 100 dB, the maximum open-circuit output voltage of the proposed system reaches 72.6 mV, which is 4.2 times greater than that of the same PZT transducer without LAM.