Study on Acoustic Metamaterial Unit Cells: Acoustic Absorption Characteristics of Novel Tortuously Perforated Helmholtz Resonator with Consideration of Elongated Acoustic Propagation Paths

The paper addresses a practical problem: conventional sound-absorbing materials struggle to reduce low-frequency noise in small spaces such as automobile air-conditioning duct mufflers. It focuses on acoustic metamaterials, especially Helmholtz resonators, and proposes a new unit-cell muffler structure that combines a perforated panel with a multi-channel tortuous cavity. The tortuous design changes the effective channel length within limited space to extend the sound propagation path and increase energy dissipation. To evaluate performance, the authors use acoustic theoretical modeling, finite element simulation, and parametric optimization to study how key structural parameters affect sound transmission loss (STL). They report that, compared with a traditional folded-channel metamaterial, the resonance frequency differs by 38 Hz, transmission loss by 1.157 dB, and effective bandwidth by 1 Hz.