Optimal passive metamaterial selection for electric-Vertical-Take-Off-and-Landing aircraft: A filtering concepta).

Electric-Vertical-Take-Off-and-Landing (eVTOL) aircraft represent a cutting-edge innovation in modern aviation, potentially transforming future mobility. Among the key propulsion technologies for eVTOLs, ducted fans offer efficiency and compact design. However, controlling the noise from these fans remains a significant challenge due to space and weight constraints. To address this limitation, researchers develop advanced acoustic metamaterial liners to minimize noise emissions, gain public acceptance, and meet certification standards. Nevertheless, looking at the numerous metamaterials already designed, a critical question arises: which metamaterial is the best suited for a specific application, and how can these metamaterials be filtered for a particular case scenario? This article presents an overview of acoustic passive metamaterials and their efficiency in noise reduction. A database has been established to analyze and classify metamaterials from the literature into specific subgroups. For each subgroup, various metamaterials are designed and fabricated, and the acoustic characteristics are assessed using both experimental and numerical investigations. The properties obtained are subsequently incorporated into the filtering matrix. A universally applicable filtering concept is introduced, generating a score distribution for evaluating the applicability of different metamaterials. This is illustrated through the example of ducted fans and the interior cabin in an eVTOL aircraft.