Metamaterial and Helmholtz coupled resonator for high-density acoustic energy harvesting

High-density acoustic energy harvesting is one of the power solutions for wireless sensor network nodes in the Internet of Things. In this paper, we present a novel metamaterial and Helmholtz coupled resonator (MHCR) to enhance the sound energy density by energy focusing and pressure amplification. Metamaterial refers to a type of structural composite material, usually periodic. The local modification of the material by introducing a defect can make the wave at the defect band frequency be confined to the defect area to achieve acoustic energy focusing. The Helmholtz resonator is added to the defect of the metamaterial to amplify the focused sound waves. The variation in channel pressure causes the plug of the air in the neck to oscillate in and out, producing adiabatic compression and expansion of the air in the cavity to amplify sound pressure. The mathematical models of band structure, resonant frequency, vibration amplitude with vibroacoustic coupling and output voltage with electromechanical coupling are developed to design MHCR. The maximum voltage of the coupled energy harvester was about 3.5 times that of the maximum voltage of the metamaterial energy harvester. Field tests illustrated the effectiveness of the proposed MHCR with the maximum transmission ratio of 30.83 mV/Pa in mechanical noise environment, which was 48 times the maximum transmission ratio of the metamaterial energy harvester in the chirping of cicadas.