Mechanical Systems and Signal Processing issue:accepted
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 congurations and the educed impedance is compared with reference results obtained with a dierent approach at another test rig, and shows good agreement. A sensitivity analysis on the in frequence of the input parameters is carried out showing the influence of the rigid-/impedance wall transition effects, flow Mach number and microphone positioning accuracy. Some specific methodological issues are discussed and different methods for overcoming these diculties are presented.