In future aero-engines, one of the concepts with the largest potential for carbon emission reduction is predicted for the Ultra High Bypass Ratio (UHBR) engine. The acoustic signature of this future UHBR engine is expected to have an increased low-frequency content driven by a reduced fan blade count and lower fan RPMs. The need for a broadband liner covering both low and high frequencies provides a significant challenge for conventional acoustic liners. Therefore, it is necessary to design innovative facing sheet and cavity geometries that allow efficient attenuation at both low and high frequencies to reduce both community noise and cabin noise, all within a limited space envelope. This research, therefore, aims to develop novel acoustic liner concepts that enable increased attenuation over a relatively large frequency bandwidth, across different operating conditions, for application in future UHBR engines. Detailed numerical and experimental investigations have been carried out to predict and measure the liner’s acoustic properties (resistance and reactance) at medium and high sound pressure levels. The aim is to provide improved broadband sound attenuation, and to study the impact of variations in the liner design parameters. Three novel liner concepts are presented, which demonstrate improvements in broadband sound absorption compared to conventional designs. These are the slanted septum core with part perforation, the slanted septum core with varying percentage open area, and multiple folded cavity acoustic liner concepts. In addition, the percentage open areas of novel liners are optimised to provide maximum insertion loss. The slanted septum core with part perforation provides a relatively narrow low-frequency absorption bandwidth, 400-1000 Hz, providing a minimum absorption coefficient of at least 0.8 when the percentage open area of the septum is sufficiently large compared to the face sheet. The slanted septum core with varying percentage open area, with a broadband optimum solution is shown to provide improved sound absorption over a much broader frequency range compared to the part-perforated slanted septum concept. Finally, the multiple folded cavity acoustic liner, referred to as the Multi FOCAL Liner concept, provided the most improved broadband attenuation when each folded segment was tuned for targeted narrow frequency bandwidth.