Improvement of the sound absorption of flexible micro-perforated panels by local resonances

To further improve the sound absorption enhancement of flexible micro-perforated panels (FMPPs), a new sort of perforated sound absorbers - metamaterial-based micro-perforated panels (MMPPs) - is proposed by combining a micro-perforated host panel and local resonators (LRs) attached on a sub-wavelength scale, targeting the flexural waves. Theoretical and numerical models show that MMPPs are able to further enhance sound absorption in a wide frequency range. The theoretical model is developed based on the effective medium method as the structural wavelength in the host panel is much larger than the distance between the LRs, and the full simulation model, including visco-thermal effects, is conducted by utilizing multi-physical coupling integrated in COMSOL. Besides, a structural finite element unit cell method is used to evaluate the stop band behavior of the MMPP. Good agreement is achieved between the theoretically predicted acoustical properties and the simulation results for both conventional FMPPs and the proposed MMPPs, validating the numerical and theoretical models. Both models reveal that the sound absorption enhancement of the MMPP stems from the resulting acoustic surface impedance improvement, caused by the sub-wavelength attached local resonances. The effect of key properties of the LRs (i.e. mass, damping and multiple resonances) on the sound absorption performance of MMPPs is then analyzed by applying the theoretical model and effective frequency-adjustability of the absorption enhancement performance is found. The proposed MMPP shows great potential for the noise reduction industry.