The radiation properties of rotating sources radiating in free field and enclosed in a duct are compared. The relevance of making the comparison is in the understanding of acoustic sources produced by thrust devices; a distinction is required between free field rotors such as propeller and helicopter rotors and ducted rotors such as fans and gas turbine compressors. The source mechanisms are similar in each case, only the character is different depending on the rotor geometry and operating conditions. The purpose of this paper is therefore to consider what part the duct plays in the radiation process. A ray model is used to illustrate the differences between free field and ducted rotors for both hard and soft walls and for flow and no-flow conditions.