Broadband low-frequency acoustic absorption using a compact tapered helical duct

We propose a tapered helical duct absorber with power-law geometric gradients to control low-frequency acoustic waves in a fluid medium. The structure functions as a non-uniform acoustic waveguide, achieving both graded impedance matching and an extended propagation path for enhanced wave energy dissipation. The absorber is modeled using the transfer matrix method and validated through finite element simulations and experiments on two samples with different geometric parameters. The numerical and experimental results agree well with theoretical predictions. Theoretical and numerical results demonstrate that a concurrent power-law decrease in pitch and increase in diameter synergistically improves low-frequency attenuation and broadens the effective bandwidth, through enhanced impedance matching and a prolonged acoustic path. This work provides a fluid-dynamic perspective on the design of compact waveguide structures for efficient acoustic energy control in confined environments.