Novel fractal acoustic metamaterials (FAMs) for multiple narrow-band near-perfect absorption

In this work, we introduce fractal acoustic metamaterials (FAMs), in thicknesses ranging from 5 (λ/69) to 25 mm (λ/18), which are observed to provide multiple narrow-band low-frequency absorptions of acoustic signals. The fractal structures used in this work are carefully designed and fabricated using a side branch Helmholtz resonator design, making these structures easily tunable to multiple frequencies. Using different sizes of the side branches distributed in a fractally oriented configuration onto a plane rigid baseplate, the propagation velocity of acoustic waves is slowed down considerably. There is also a shifting resonating response of the structures toward lower frequencies (