Investigation of the systematic phase mismatch in microphone-array analysis

Abstract

The paper investigates phase deviations as they occur in aeroacoustic array measurements in open jet wind tunnels. The accuracy of an approximative zero-thickness shear layer model, which is used in practice to predict the phase shift between source and microphones, is investigated numerically. The exact wave propagation through a generic shear layer is calculated by solving the full linearized Euler equations using a finite element method and by means of geometrical acoustics. The results are taken to determine the phase deviations between the simplified model and the exact case. It is demonstrated that the position of the zero-thickness shear layer is important for the accuracy at high frequencies. Additionally measurements are made to analyze the phase fluctuations which are generated by the scattering at turbulent structures in the open-jet shear layer. A procedure is proposed to compensate these phase fluctuations and to improve the array efficiency.