A Comparison of Ensemble Averaging Methods Using Dean's Method for In-Situ Impedance Measurements

Abstract

*Dean’s two-microphone method is a well known technique to determine the in-situ impedance of a locally reacting acoustic liner using local measurements of the dynamic pressure at the face sheet and at the back sheet. The method was originally developed during the era of analog data recording and processing systems. The advent of digital data recording and processing systems has expanded the capabilities of the method. This paper explores several different ways to compute the impedance using alternative ensemble averaging methods. The impact of signal noise is investigated by applying the different averaging methods to in-situ measurements of an acoustic liner acquired during a full scale engine test. The differences between the computed impedance are compared to gain insight into the characteristics of the signal noise and to explore an apparent difference in the liner impedance between tonal and broadband excitation. Nomenclature