Validation of an absorption coefficient measurement technique using two cardioid microphones in any impedance tube

We have proposed a technique for measuring sound pressure and particle velocity using a combination of cardioid microphones and then applied it to obtain the acoustic absorption coefficient in a tube. So far, we have revealed that either using one cardioid microphone measured twice or using a pair of cardioid microphones measured once are both suitable for the measurement and that the calibration of the cardioid microphones is required. If the technique works correctly, this technique holds promise for measuring the acoustic absorption coefficient of various specimens in any tube and with any cardioid microphones. To validate the technique's effectiveness, we conducted experiments in multiple square-section wooden or round-section steel tubes, with specimens of glass wool and polyester terephthalate (PET) wool. As a result, this technique can provide accurate absorption coefficient measurements in any tube configuration.