Experimental investigation of two-microphone method for in situ measurement of the acoustic impedance of a duct liner

The two-microphone method is investigated for the objective of in situ measuring the impedance of an acoustic liner In a duct. In order to avoid the influence caused by directly inserting the comparatively large microphones into a practical acoustic liner typically consisted of small honeycomb cells, the probe tubes as small as 1. 5 mm in diameter are used to conduct the sound pressure at the liner surface and that at the back plate to the microphones. A calibration method is designed to ensure high measurement accuracy of the thus-formed probe microphones. By the use of the two-microphone method, the acoustic impedance of a honeycomb-structure acoustic liner is measured when exposed to the sound waves at oblique incidences inside a duct. The measurement result is compared with the data obtained on a standard impedance tube and the prediction of an existing impedance model. The reliability of the present two-microphone method is validated by the good agreement between the different results.