Broadband and low-frequency sound absorption of compact meta-liner under grazing flow

Design for highly effective acoustic meta-liners with broadband sound attenuation under grazing flow is a challenging topic because their working performance with flow practically is usually discrepant from many theoretical models, and difficult to predict accurately. In this paper, a compact meta-liner is proposed with broadband sound reduction at a low-frequency regime under grazing flow. It comprises a series of designed absorbers with C-shaped enclosures (CSEs) and embedded microperforated panels (MPPs). The theoretical model and numerical model are proposed to evaluate the sound absorption performance of single and parallel configurations of the designed absorbers. After that, the acoustic performance of the meta-liner under grazing flow is studied using finite element method (FEM) and validated by experiments. With a structural thickness of only 30.5 mm, the proposed meta-liner exhibits an average transmission loss of 10.2 dB in the frequency of 400-1400 Hz without flow. More importantly, by utilizing the acoustic impedance model of the MPP that incorporates flow and shallow-cavity effects, the transmission loss (TL) of the meta-liner can be accurately predicted and controlled to maintain high levels at flow speeds ranging from 0 to 30 m/s. There is a good agreement between prediction results and experimental data. With proper design of the perforation property of MPP, the TL peaks and troughs can be maintained or even enhanced with the flow effect. Our results provide new insights into the design of metamaterials for aero-noise control engineering and may promote the development of meta-liners for airflow applications.