3D Hilbert fractal acoustic metamaterials: low-frequency and multi-band sound insulation

In this work, we present a class of three-dimensional (3D) labyrinthine acoustic metamaterials with self-similar fractal technique, which can produce multiple frequency-band sound insulation in deep-subwavelength scale. By simultaneously exploiting the multi-frequency bandgaps and the low-frequency characteristics, the Hilbert cubes are explored to design the 3D Hilbert fractal acoustic metamaterials (HFAMs). The multiple-band features of the HFAMs are examined by the finite element method and the effective medium theory, in which the negative bulk modulus and the mass density are responsible for the formation of the multi-bandgaps. These multi-frequency properties are induced by the Fabry-Perot multi-resonance of 3D HFAMs, which possess an ultra-high refractive index. Hence, the multi-band sound insulations of 3D HFAMs with the negative effective property are achieved below 500 Hz. These properties of the designed 3D HFAMs provide an effective way for acoustic metamaterials to achieve multi-band filtering and noise attenuation in the low-frequency regime.