Boundary-impedance-engineered loss-channel redistribution in Helmholtz acoustic metastructures for low-frequency absorption bandwidth enhancement

Low-frequency broadband sound absorption in finite-thickness passive structures is strongly constrained by the coupling among resonant energy storage, internal dissipation, and external leakage. Conventional Helmholtz resonators usually rely on neck viscous boundary-layer dissipation and therefore exhibit narrowband absorption. In this study, a boundary-impedance-engineered Helmholtz acoustic metastructure is proposed to investigate boundary-induced loss-channel redistribution and its role in enhancing the low-frequency absorption bandwidth. A unified Helmholtz resonant skeleton is adopted, while different passive boundary modules are introduced without changing the main resonant framework. Their boundary properties are specified through geometrical and material parameters at the design stage and remain fixed after fabrication. Four configurations are investigated: a rigid Helmholtz resonator, a porous-lined Helmholtz resonator with cavity-wall porous lining, an MPP-loaded Helmholtz resonator with a localized microperforated-panel interface at the neck entrance, and a hybrid Helmholtz resonator combining the localized MPP interface and cavity-wall porous lining. The localized MPP interface is confined to the neck entrance, whereas the remaining front surface is kept rigid to avoid an additional parallel acoustic inlet and associated parasitic front-cavity effects. An equivalent acoustic model is developed to describe the effects of neck viscous boundary-layer loss, porous bulk loss, and MPP-induced interfacial loss. The total dissipated acoustic power is decomposed into three loss channels, and the corresponding fractions are introduced to quantify their relative contributions. The fixed-skeleton comparison shows a progression from neck-controlled dissipation in S 0 , through coupled neck and porous dissipation in S 1 , to MPP-interface-dominated dissipation with auxiliary neck and/or porous contributions in S 2 and S 3 . Within the investigated S 3 parameter space, the interfacial channel remains dominant, while the relative contributions of the three channels vary continuously. The enhanced low-frequency absorption bandwidth is interpreted by an equivalent multi-channel critical-coupling condition, where the combined internal loss rate is matched to the external leakage rate over an extended frequency range. For the nominal S 3 configuration, B 0.8 = 44.88 Hz and B 0.9 = 28.945 Hz at f p = 222.414 Hz, corresponding to relative bandwidths of 20.057% and 12.981%, respectively. Under the common 85-mm thickness constraint, these bandwidths are 3.23 and 3.13 times those of the frequency-matched optimized conventional-HR baseline. FEM simulations are used to validate the absorption coefficients predicted by the equivalent acoustic model, while the pole-zero analysis represents the same impedance-matching behavior through the locations of the reflection zeros relative to the real-frequency axis. Finally, impedance-tube experiments using a 100-mm-inner-diameter tube over 50–1600 Hz are conducted to validate the proposed mechanism. This study provides a physically interpretable route for designing finite-thickness Helmholtz acoustic metastructures with design-stage-tailored dissipation pathways and improved low-frequency absorption bandwidth.