A COHERENCE-BASED TECHNIQUE TO SEPARATE EJECTOR INTERNAL MIXING NOISE FROM FARFIELD MEASUREMENTS

Different coherence-based analysis techniques were examined and derived for the purpose of separating correlated internal mixing noise from correlated and uncorrelated extraneous noise such as external background noise and external jet noise at farfield microphone locations. Limitations and assumptions associated with these techniques were also examined. These analysis techniques were applied to experimental measurements acquired with simulated upstream noise source. The assumptions for these analysis techniques were verified with experimental data. Coherence-based techniques were applied to educe the correlated internal mixing noise simulated by an upstream acoustic driver. A method derived from conditional spectral analysis using five microphones was applied to the experimental data to separate correlated extraneous noise from farfield measurements. Subsequently, farfield and nearfield acoustic measurements were acquired in model tests of a mixer nozzle exhausting into an ejector duct along with acoustic data acquired from microphones flush- mounted on the side wall of the ejector. Selected data were analyzed and the correlated noise educed. Nomenclature