Broadband ultra-thin acoustic metasurface absorber with coiled structure

We theoretically and experimentally propose 2 designs of broadband low-frequency acoustic metasurface coiled absorbers for the frequency ranges of 460Hz~972Hz and 232Hz~494Hz (larger than 1octave), with absorption larger than 0.8, and having the ultra-thin thickness of 4.5cm and 9cm respectively (λ/17 for the lowest working frequency and λ/8 for the highest frequency). The designed supercell consists of 16 different unit cells corresponding to 16 eigen frequencies for resonant absorptions. The coupling between multiple resonances leads to broadband absorption effect in the full range of the targeted frequency spectrum. the coiling structure lead to the ultra-thin thickness of the metasurface absorbers.