An uncertainty investigation for liner impedance eduction methods

Impedance eduction methods have been developed for decades to meet the increasing need for high-quality impedance data in the design and optimization of acoustic liners. To this end, it is important to fully investigate the uncertainty problem, to which only limited attention has been devoted so far. This paper considers the possibility of acoustically-induced structural vibration as a nonnegligible uncertainty or error source in impedance eduction experiments. As the frequency moves away from the resonant frequency, with the increase in the value of cavity reactance, the acoustic particle velocity inside liner orifices possibly decreases to the extent comparable to the vibration velocity of liner facing sheet. Thus, the acoustically-induced vibration, although generally being weak except at the inherent structural frequencies, may considerably affect the impedance eduction results near the anti-resonant frequency where the liner has poor absorption. To demonstrate the effect of structural vibration, the vibration velocity of liner facing sheet is estimated from the experimentally educed admittance of the liner samples whose orifices are sealed with tape. Further, a three-dimensional numerical model is set up, in which normal particle velocity is introduced over the solid portion of liner facing sheet to imitate structural vibration, rather than directly solving the acoustic-structural coupling problem. As shown by the results, the vibration of liner facing sheet, whose velocity is as small as estimated by the experiment, can result in anomalous deviation of the educed impedance from the impedance model near the anti-resonant frequency. The trend that the anomalous deviation varies with frequency is numerically captured.