Manufacturing, modelling and testing of acoustic liners extended with flexible walls

Abstract This study investigates a novel acoustic liner concept from an acoustical and manufacturing perspective. The proposed damping system combines a conventional Helmholtz resonator with additional structural resonances due to flexible walls. Promising flexible wall materials were characterised in terms of their mechanical properties by dynamic mechanical analysis and their resistance to environmental influences. Based on these results, flat liner samples with a flexible intermediate wall were designed, manufactured and experimentally tested. A process chain with a pre-tensioning device for preserving the stress state during the joining process was developed for technical production. Compared with a conventional liner, the new type of liner exhibits additional low-frequency and, in particular, broadband damping. Numerical simulations of the liner showed good agreement with the experimental results, highlighting the role of the flexible wall. In addition, broadband dissipation was observed for the same installation space compared with the reference. This type of liner was then converted into an engine-like curved structure to investigate the influence of higher acoustic modes. Therefore, the design, production, engine integration and acoustic characterisation of a new liner concept with flexible walls for low-frequency and broadband dissipation of engine noise were successfully demonstrated.