## 1 Introduction
Figure 1: (**a**) Conventional acoustic liner with hexagonal honeycomb structure consisting of perforated face sheet, **1**, hexagonal honeycomb core, **2**, and, a back plate, **3**; (**b**) novel liner concept with partially flexible walls, **4**. Common core structures have a cell size of 1/8\" (3.175 mm) to 1\({}^{\prime}\) (25.4 mm) [18; 19]. Especially in low temperature regions, such as the bypass area, epoxy-based FRP top layers and core structure are used [20]. Typically a unit cell of the core structure has a prismatic cellular structure with a triangular, square or hexagonal shape. For a long time, liners were manufactured in sections and installed inside the nacelle [21]. This resulted in liner splices between the acoustic liner segments, which causes acoustic scattering due to discontinuities in the acoustic impedance around the circumference of the duct [21]. Therefore, latest developments include spliceless liner barrels, which are far more expensive and difficult to handle for assembly and maintenance. Depending on the raw material and the desired geometry different manufacturing methods are available to realise the segmented acoustic liner [22; 23]. [PERSON] [22] described, inter alia, an expansion manufacturing process, a corrugation manufacturing process and a strip slotting method. Most honeycombs are fabricated by the expansion process. Thereby thin metal or paper sheets are laser welded or adhesive bonded to Hobe blocks, respectively [22; 24]. These blocks are cut and afterwards stretched to create hexagonal structures or over expanded to form rectangular structures. To enable sheet stretching, a moderately high inter sheet bond strength is required. Thus, this manufacturing process is only suitable for thin-walled semi-finished products. For honeycombs made of stainless steel sheets, the corrugation process is used. The sheets are corrugated by rolls or a press and stacked [22]. The layers are welded or adhesively bonded together. The strip slotting method is used for square and triangular honeycombs of non-ductile materials such as ceramics [22; 25]. The strips are bonded by means of adhesives or by brazing. The installation effort increases significantly with increasing number of connections between strips which makes the process primarily suitable for small quantities of honeycombs. However, the strip slotting method enables the manual production of arbitrary honeycomb wall structures and is used in this study to fabricate the core structure with partially flexible walls. In this study a new design of a HR liner with partially flexible walls is investigated using different materials as flexible walls. An approach for the manufacturing of this novel acoustic liner and the determination of acoustical and structural properties is described. The acoustic damping characteristics of all manufactured liner samples and the deformation behaviour of the square honeycomb core structure is investigated. Furthermore, the influence of the applied fabrication technology and the production-related manufacturing tolerances is analysed. A comprehensive assessment of the properties and implications of this novel acoustic liner is made with respect to designated future aircraft application.