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.Acalibration 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.