Multi large low frequency band gaps in a periodic hybrid structure
A hybrid structure composed of a local resonance mass and an external oscillator is proposed in this paper for restraining the elastic longitudinal wave propagation. Theoretical model has been established to investigate the dispersion relation and band gaps of the structure. The results show that the hybrid structure can produce multi band gaps wider than the multi resonator acoustic metamaterials. It is much easier for the hybrid structure to yield wide and low band gaps by adjusting the mass and stiffness of the external oscillator. Small series spring constant ratio results in low frequency band gaps, in which the external oscillator acts as a resonator and replaces the original local resonator to hold the band gaps in low frequency range. Compared with the one dimensional phononic crystal (PC) lattice, a new band gap emerges in lower frequency range in the hybrid structure because of the added local resonance, which will be a significant assistance in low frequency vibration and noise reduction. Further, harmonic response analysis using finite element method (FEM) has been performed, and results show that elastic longitudinal waves are efficiently forbidden within the band gaps.