Novel Noise-Source-Identification Technique Combining Acoustic Modal Analysis and a Coherence-Based Noise-Source-Identification Method

Abstract

A novel technique is presented, which combines the fields of coherence-based noise-source identification with acoustic modal analysis. Coherence-based noise-source-identification techniques apply the coherence function, estimated between pairs of dynamic pressure sensors, to identify the relative contributions of one or several noise sources to the total noise measured at a location of interest. Acoustic-modal-analysis techniques use arrays of microphones to decompose the pressure field at an axial location of a duct into constituent modal amplitudes. Conditional Spectral Analysis: Modal is the primary novel technique presented herein, and uses conditional spectral analysis to allow the beneficial capabilities of each of these well-regarded fields to be combined into one method. The technique allows the total modal pressure field measured in a duct to be decomposed into parts associated with contributing uncorrelated noise sources. The techniques’ efficacy is validated with a two noise-source-region experimental rig. A second novel technique is also presented: Signal Enhancement: Modal, which is in essence a modal version and enhancement of the now classic signal-enhancement technique, which allows the influence of extraneous noise to be removed from acoustic modal measurements. The modal signal enhancement was developed as part of the modal conditional spectral analysis, but can in fact be used independently in conditions in which only one correlated noise source is measured by the sensors in the duct. The modal-conditional-spectral-analysis technique represents a significant advancement in noise-source identification, and may be applied to a full-scale aeroengine to identify the specific contributions of core-noise sources within the engine to the amplitudes of the acoustic modes propagating through the engine exhaust. This added acoustic modal information can be used to predict the relative contribution of these noise sources to the total noise radiated from the engine and into the far field. Knowledge of the contributions of each core-noise-source contribution to the modal content propagating through the engine exhaust also allows for better mitigation through optimized liner designs and location.