The NASA Glenn Rotating Rake can be used to measure inlet and exhaust duct modes on a full-size turbofan engine.  Shown here is the Rotating Rake installed on the TFE 31-60 engine at tested at Honeywell’s static engine test facility in San Tan, Arizona.

To get it right, though, more information about the modal content of the tone is needed. This kind of data can’t be measured with a stationary microphone, which is why NASA Glenn engineers pioneered procedures using rotating microphones in the 1990’s.

Rotating Microphone Rakes have been used by NASA Glenn researchers in a wide variety of tests ranging from low-speed proof of concept tests in the Advanced Noise Control Fan Rig and 9’ x 15’ Low Noise Wind Tunnel, to full-scale turbofan engine tests.