Indirect acoustic impedance eduction in presence of flow based on an analytical two-port formulation

Abstract

In order to assess the performance of advanced acoustic absorbing materials the acoustic impedance should be determined with high accuracy, especially, in presence of a grazing flow. This paper presents an analytical methodology for the grazing flow acoustic impedance eduction, based on a two-port representation of an acoustic system. This procedure is dedicated to the frequency range below the cut-on frequency of the transversal duct modes and its major appeal refers to a reduced number of microphones and the experimental rig simplicity. The methodology is verified by comparison with acoustic finite element simulations. Experiments are performed on two liner configurations and the educed impedance is compared with reference results obtained with a different approach at another test rig, and shows good agreement. A sensitivity analysis on the influence of the input parameters is carried out showing the influence of the rigid-/impedance wall transition effects, flow Mach number, number of microphones and microphone positioning accuracy.

An improved algorithm for liner impedance eduction in low signal-to-noise ratio flow duct

“…Four different combinations of parameter A and B, that is (A, B) = (5, 0.5), (10, 1), (10, 2), (10,5), in equation ( 5) are used, and the corresponding weighting functions are plotted in Figure 13. The impedance educed by the proposed algorithm is shown in Figure 14 and Figure 15, respectively.…”

“…The inverse method, 5,6 also known as the impedance eduction technique, is usually adopted to iteratively find the acoustic impedance. Over the last two decades, continuous efforts [5][6][7][8][9][10][11][12][13][14] have been made in the development of impedance eduction techniques. As shown in Figure 1, these techniques generally follow a similar procedure: a duct acoustics model, including initial impedance boundary conditions, is developed first.…”