Investigation of aero-engine core-noise using a phased microphone array

Abstract

The sound radiation from the nozzle, i.e. the core noise, of a BR700 aero-engine is studied with the aid of a phased line array of microphones. The engine was mounted on a noise-test pylon, and the microphone array consisted of 64 microphones laid out on the ground along a line parallel with the jet. The microphone signals and the pressure fluctuations in the combustion chamber of the engine measured with a rumble probe were recorded simultaneously. The sound emitted from the nozzle is studied for emission angles ranging from 60 to 135 degrees relative to the forward direction with a sub-array of 25 microphones. The focused, time shifted signal of the sub-array is used to calculate the coherence with the signal of the rumble probe. The coherence between the microphone signal focused on the nozzle and the rumble probe was found to be surprisingly high for low engine power settings at frequencies below 300Hz. For higher power settings, coherence is considerably reduced. The coherent output power is compared with farfield microphone spectra in order to assess the contribution of the core noise to the total noise emitted by the engine. It may be concluded that at low power settings, where other sound sources are relatively weak, noise originating from the combustion chamber is a major contributor to the noise emitted from the nozzle. The combined array and correlation techniques are shown to be useful tools for core noise investigations.