Broadband noise attenuation in the flow duct using metamaterial-based acoustic liners

In this work, a metamaterial-based acoustic liner design, called meta-liner, is proposed for broadband noise reduction in ducts with flows. It is constructed by eight periodically arranged varying-depth units filled with porous materials, forming a linear reflected phase-shifting within 0 to 2π in a broad target frequency range. Under normal incident waves, sound absorption performance is first examined by numerical simulation, leading to broadband sound absorption. Then, the meta-liner is installed in a flow tube to assess its capability of reducing broadband noise in a duct with aerodynamic flows. The numerical investigations are performed in the finite element method. The effects of flow speeds with the Mach number up to 0.2 and the sound source position (at upstream and downstream sides of meta-liner) are investigated. The results show that a flat transmission loss curve is achieved in the target frequency range in various conditions, suggesting that the designed meta-liner can attenuate sound in a wide frequency range.