Sound absorption in Hilbert Fractal and Coiled Acoustic Metamaterials

TL;DR

This paper introduces fractal Hilbert and coiled acoustic metamaterials with tunable sound absorption properties, validated through experiments and models, enabling enhanced noise mitigation over broad frequencies.

Contribution

It presents a novel class of fractal and coiled acoustic metamaterials with predictable absorption based on cavity resonance and fluid velocity differences, advancing noise control design.

Findings

Acoustic absorption correlates with cavity resonance in tortuous paths.

Maximizing fluid velocity differences enhances absorption at specific frequencies.

Experimental results align with finite element models across multiple fractal orders.

Abstract

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 to the tortuous paths. For a given fractal/geometry order, the acoustic absorption at specific frequencies is also enhanced by maximising 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.