Broadband fractal acoustic metamaterials for low-frequency sound attenuation

We fabricate and experimentally characterize a broadband fractal acoustic metamaterial that can serve to attenuate the low-frequency sounds at selective frequencies ranging from 225 to 1175 Hz. The proposed metamaterials are constructed by the periodic Hilbert fractal elements made of photosensitive resin via 3D printing. In analogy to electromagnetic fractal structures, it is shown that multiple resonances can also be excited in the acoustic counterpart due to their self-similar properties, which help to attenuate the acoustic energy in a wide spectrum. The confinement of sound waves in such subwavelength element is evidenced by both numerical and experimental results. The proposed metamaterial may provide possible alternative for various applications such as the noise attenuation and the anechoic materials.