Acoustic impedance measurement of various porous media using a cardioid microphone in an impedance tube

It is widely known that Microflown's PU sensor is among the most advanced pressure-velocity sensors. However, the sensitivity of this series of sensors is so high that their application in various environments, such as outdoor fields, is impractical. Thus, we have proposed a new pressure–velocity sensor, employing cardioid microphones, and designated it as the “c–c” method. This method has been demonstrated to facilitate the acquisition of both the absolute values of sound pressure and particle velocity using any cardioid microphone. It is imperative to calibrate the pressure and particle velocity sensitivities of each cardioid microphone. In this time, several cardioid microphones were prepared for validation examinations. Initially, the calibration of these microphones was conducted within the acoustic impedance tube up to 1.6 kHz. Subsequently, the absorption coefficients of various kind of porous media are measured using these calibrated sensors and the tube. Consequently, it was determined that the absorption coefficients of these materials can be obtained regardless of the type of cardioid microphones was employed. In addition, these results were compared with those obtained by the transfer function method and by using the PU sensor. The findings, derived from the use of three different sensors, exhibited a high degree of agreement.