Enhancing array techniques for measuring jet engine combustion noise using a modal isolation method

Significant progress has been made over the last few decades in the reduction of turbofan jet engine noise sources such as jet noise and fan noise. This has resulted in combustion noise becoming a more prominent source of noise. To assess the contribution of combustion noise to the overall level of jet engine noise, the combustion noise that propagates to the far field needs to be measured. Coherence-based measurement techniques are used to isolate combustion noise from other noise sources in the far field. A technique called the 3S Array method is currently used to do this but it suffers from frequency range limitations caused by the assumption of single mode propagation in the engine duct. The assumption of singe mode propagation is needed to ensure maximum coherence between in-duct sensors. This paper presents a method which can be used to isolate individual modes propagating inside the duct, thus satisfying the single mode assumption, thereby extending the frequency range over which the 3S Array method operates. The modal isolation method was implemented in no-flow tests carried out in the Institute of Sound and Vibration Research’s (ISVR) Small Anechoic Chamber. The results show a significant improvement in coherence between two in-duct measurement locations when two circular arrays are used to implement the modal isolation method in a cylindrical duct. Combining the 3S Array method with the modal isolation method provides a significant increase in the frequency range over which the 3S Array method can be used. Additionally, the new method makes it possible to identify the contributions of combustion noise that propagates along the duct in specific modes to the far field level.