Elucidation of the mechanism of acoustic impedance technique using two cardioid microphones

Recently, there is growing the need for in-situ impedance measurement of several materials. We have proposed using two cardioid microphones (called C-C) sensor for it. C-C sensor has many advantages against two omnidirectional microphones (called P-P) sensor or one omnidirectional microphone and one sound velocity (called P-U) sensor. At first, this report introduces the mechanism of impedance measurement with C-C sensor theoretically. As results, C-C sensor is able to measure acoustic normal impedance using one plane wave incidence or random wave incidence. In addition, the error factors like the distance of microphones were checked numerically. As results, we can see that the difference of sensitivity of two microphones affects severely the measurement results of acoustic impedance and absorption coefficient.