An experimental measurement system was developed and implemented at the NASA Glenn Research Center in the 1990s to measure fan duct acoustic modes. The system is a radial array of microphones inserted into the duct that continuously rotates about the duct centerline. This Rotating Rake provides a complete map of the acoustic duct modes at fan harmonics in a ducted fan. It has been used on a variety of test articles: from a low-speed, concept test fan rig, to a full-scale production turbofan engine. The Rotating Rake has been critical in developing and evaluating a number of noise reduction concepts as well as providing experimental databases for verification of several aero-acoustic codes.
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