The increasing demand for environmentally friendly aircraft requires the use of more fuel efficient engine technologies, such as the counter-rotating open-rotor (CROR). Despite of their good emission performance, CROR-engines exhibit a significant drawback regarding the high-amplitude low-frequency noise, generated by the propellers. The challenge of dealing with that problem implies measures, which attenuate structure- and air-borne low-frequency sound, caused by the engines. In this paper, two major approaches for such measures are discussed. The first approach aims at the reduction of structure-borne sound transmission between two adjacent fuselage sections. In this context, a modification of the traditional fuselage section interconnection has been investigated numerically and compared with the conventional design. Models with increasing complexity were created in order to observe the structure-borne sound transmission performance of the newly proposed design: A beam model for analytical pre-evaluation, succeeded by plate and fuselage models for the numerical investigations. The second approach is concerned with the shielding of exterior noise. For this purpose, a noise shield mounted at the fuselage has been considered. Membrane type metamaterials were used as a middle layer of a double panel structure to improve low-frequency sound attenuation. Based on that concept, panels with different geometries, involving a framed single membrane structure as well as layered and multi-celled arrays have been investigated and the influence of the arrangement on the sound transmission behavior observed. The related numerical and analytical results are discussed and validated via experiments.
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