Micro-perforated tower sonic black holes for compact broadband sound absorption

Low-frequency broadband sound absorption remains difficult for compact passive absorbers because reducing structural length generally weakens low-frequency response, whereas adding internal resonators can degrade high-frequency performance. We propose a stepped power-law sonic black hole coupled with multilayer micro-perforated plate (T-SBH-MPP), in which the axial impedance gradient slows the incident wave and the distributed micro-perforated interfaces provide thermoviscous dissipation. Transfer-matrix and finite-element models identify an optimized 20-layer, 180 mm configuration that first reaches an absorption coefficient of 0.8 at 267 Hz and maintains α ≥ 0.8 up to 2000 Hz. To isolate the geometric contribution, the T-SBH-MPP is compared with a conventional SBH-MPP having the same length, inlet radius, micro-perforated-plate parameters, and 20-layer count. The conventional structure reaches α = 0.8 at 318 Hz; therefore, the stepped geometry lowers the effective absorption frequency by 51 Hz. A 10-layer conventional SBH-MPP reaches the same 267 Hz threshold but exhibits substantially weaker middle- and high-frequency absorption. Relative impedance and complex-frequency analyses show that the broadband response results from a resistance close to the characteristic impedance of air, successive reactance compensation, and overlapping resonances of the coupled cavities. Impedance-tube measurements from 50 to 1600 Hz reproduce the principal theoretical and numerical trends. The results demonstrate a compact method for combining low-frequency onset with broadband dissipation without attributing the improvement solely to an increased number of layers.