Fig. 4: Comparison of the absorption efficiency of the higher HLs with a single-hole structure of the same void area.
From: Sierpiński carpet-inspired hierarchical patterning of porous materials for sound absorption
Absorption coefficient as a function of frequency for a 2nd-order and b 3rd-order hierarchical levels (HLs). The results for the hierarchical structures are shown as continuous lines, while the dashed lines represent single-hole configurations (analogous to the 1st HL) with an equivalent total air-filled area.
