Auxetic star-shape cored hybrid acoustic metastructure with fractal characteristics for low frequency broadband sound absorption

To address the challenge of efficient sound absorption at subwavelength with broad frequency range meanwhile preserving lightweight and compact size limitation, this study presents an innovative acoustic metastructure consisting of a periodic array of auxetic metamaterial and micro-perforated panel (MPP), which is integrated together leveraging the fractal technique to introduce multi-scale resonant cavities, thereby achieving continuous overlapping of multiple absorption peaks and capturing the hybrid mechanism of negative Poisson’s ratio coupled with coherent resonances. Analytical modelling, numerical simulation, and experimental validation demonstrate the prominent sound absorption performance, in which the proposed hybrid fractal metastructure (HFMS) can achieve an average sound absorption coefficient (SAC) above 0.8 from 285 Hz to 900 Hz with the thickness corresponding to 1/17 of the resonance wavelength. The parametric analysis also manifests the tunable absorption characteristics of the HFMS, facilitating the subsequent optimization projects for enhancing broadband acoustic performance using genetic algorithm. Consequently, the optimized HMFS is able to possess an efficient absorption band ranging from 325 Hz to 867 Hz where the SAC exceeds 0.8 throughout the entire band, and the average SAC is larger than 0.9 across 320–880 Hz. The findings in this work undoubtedly provide technical support for the low-frequency, broadband and compact sound absorption applications.