Low-frequency multi-band sound absorption in triangular fractal acoustic metamaterials

Attaining broadband, low-frequency sound absorption within a sub-wavelength footprint remains an outstanding challenge. We introduce a triangular fractal acoustic metamaterial that couples perforated facesheets with coiled, cavity-backed channels. By means of thermo-viscous acoustic theory and finite-element simulations, we quantify how the fractal order, number of layers, channel width, channel thickness, and inlet area govern the absorption spectrum. Spatial maps of the sound pressure field and the resulting thermo-viscous loss reveal that the dominant loss mechanism is the synergistic interference of multiple slow-wave modes confined to the fractal network. Optimizing the fractal order alone shifts the fundamental absorption peak continuously from 63 to 80 Hz without any increase in overall thickness. Ultimately, the metamaterial prototype was manufactured by means of additive manufacturing (3D printing). Subsequent experimental characterization of its acoustic performance revealed that the as-fabricated architecture yields multiple distinct absorption peaks at frequencies below 500 Hz, while maintaining an overall thickness of merely λ/125.