## 1 Introduction  
Acoustic liners are used for dissipation of aero-engine noise, for instance in the intake and bypass ducts of aircraft engines. Basically, conventional aircraft liners convert acoustic energy to turbulent fluid motion and thermal energy due to the induced periodic in and outflow as well as viscous effects. Helmholtz resonators (HR) efficiently reduce propagating noise at their resonance frequencies and depict a simple construction requiring only small assembly space [1]. Single-Degree-of-Freedom (SDOF) liners consist of a perforated face sheet covering a series of small cavities [1] and a rigid back layer (Figure 1a). By varying the porosity, the thickness of the face sheet, or the cavity dimensions the sound attenuation properties can be tuned [1]. Several studies exist addressing damping of acoustic liners [2]. Conventional acoustic liners have the main disadvantage of fixed, rather narrow absorption peaks due to their geometry. However during different flight phases--corresponding to varying engine operating conditions, such as take-off, cut-back, and approach- the required frequency of maximum absorption (e.g., the first and second blade passing frequency) will change. There are several concepts to improve sound absorption and obtain more broadband noise absorption. One promising approach is to use multilayer HR liner, so-called double- or multiple-degree-of-freedom (DDOF/MDOF) liners [3]. This kind of resonators usually require more installation space that is not available for flow-optimised engine nacelles, as well as more challenging fabrication technology, due to their multi-layer construction. For SDOF liners different concepts for tuneable HR-structures can be found in literature [4]. One approach is the adjustment of the cross-sectional area \(A_{L}\) of the perforation [5; 6; 7], another the change of the cavity's volume [8; 9; 10], a combination of both or the change of the neck length \(l_{H}\), All active geometry adjustment approaches are technologically difficult to implement due to size, weight, and reliability issues [4]. In addition to these geometric adjustments, the usage of active sound absorbing structures, such as HRs with a compliant piezoelectric composite back-plate [11; 12] or HRs with movable walls [13] comprise the possibility to shift the resonant frequencies and tune the impedance. Another promising approach is the combination of conventional HR liners with other noise absorbing structures. Studies by [PERSON] et al. [14] and [PERSON] et al. [15] show that the partial replacement of rigid resonator cell walls by flexible walls with intrinsic damping has a positive effect on sound absorption of acoustic liners. The resulting structure is sketched in Figure 1b. Due to the acoustic-structural interaction inside the cells, the overall damping performance is enhanced. In this realization, one or more walls of the honeycomb structure have rectangular cut-outs which are covered by highly-flexible films made of thermoplastic polymers. It is assumed that the mechanism responsible for the improved sound absorption by the flexible cavity walls is similar to the working principle of panel sound absorbers [16; 17]. The liners are called FlexiS (flexible structure) liner. For the validation of the FlexiS liner concept, samples have been build that are investigated in a flow duct with plane acoustic wave excitation. Thus the flexible walls were perpendicular to the main flow direction and the cell walls in mean flow direction were kept rigid to ensure structural strength. For the manufacturing of the honeycomb-like core structure used in conventional aero engine liners, different materials and technologies are used. Depending on the service temperature, honeycombs are made of aluminium, stainless steel or FRP [18]. Components of FRP can be aramid fibre, glass fibre or cellulose fibre as reinforcement and phenol resin or polyimide as matrix material.