Sound absorption in Hilbert fractal and coiled acoustic metamaterials

We describe here a class of acoustic metamaterials with fractal Hilbert space-filling and coiled geometry with equal tortuosity for noise mitigation. Experiments are performed using a four-microphone impedance tube and benchmarked against non-viscous and viscothermal finite element models related to configurations spanning up to five fractal/geometry orders. We show that the acoustic absorption can be predicted by the resonance of the cavities associated with the tortuous paths. For a given fractal/geometry order, the acoustic absorption at specific frequencies is also enhanced by maximizing the difference between the minimum and maximum fluid particle velocity of the air inside the patterns. These principles can be used to design high-performance acoustic metamaterials for sound absorption over broad frequency ranges. (C) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Automated summary

Acoustic metamaterials with fractal Hilbert space-filling and coiled geometry offer noise mitigation through equal tortuosity. Experimental results suggest that acoustic absorption can be predicted by resonance of cavities and can be enhanced by maximizing the difference in fluid particle velocity. These principles enable the design of high-performance sound-absorbing materials across a wide frequency range.