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This paper deals with the comparison between open rotor and ducted fan noise. Similar theoretical formulations for the free-field and in-duct noise problems are derived from the original equation for noise generation given by Goldstein. 1 The form of the final equations enables to separate the terms related to noise propagation from those representing the acoustic source. Among the similarities shared by the free-field and in-duct cases, the possibility to define a common cut-off ratio for the spinning modes provides some insight in the relation between the position of the sources and the excited modes in a way more general than that already proposed by the authors. 2 With help of this formalism we hope to reach a better understanding of the differences in noise levels to be expected on contrarotating open rotor (CROR) and contra-rotating turbofan (CRTF) concepts.

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