Feasibility of In-Duct Beamforming

*This paper discusses the possibility of source loca tion by phased array beamforming on fan and stator of turbofan engines using an intake wall-mounted microphone array. To demonstrate the feasibility, beamforming techniques were applied to existing measured data of a circular microphone array in the intake of a f an rig. This array is normally used for azimuthal mode detection. From the measured data set a test case was selected with low engine speed, and with an intake liner between the fan and the array. Beamforming methods with both stationary and rotating focus are applied , and the contributions of tonal noise and broadband noise were separated. Thus, tonal and broadband noise sources can be identified on both fan and stator. The free-field Green’s func tion is used for the definition of the steering vectors, thereby ignoring duct wall reflec tions. By a simulation study it is shown that the presence of an intake liner is a necessary condition for obtaining meaningful beamforming results. Further simulations showed that significantly improved beamforming results can be obtained with an in-duct “cage” arra y consisting of a number of parallel microphone rings.