Comparison with Acoustic Impedance Measurement Results of Cardioid Microphones and Other Probes

For acoustic impedance or absorption coefficient measurements, P-P (two omnidirectional microphones) or P-U (one omnidirectional microphone and one velocity sensor) probe has often been used. It is well known that, however, both probes have some problems. If P-P probe is chosen, we have to be close it to boundary of a material. Therefore, the resolution of low frequency is less than that of high frequency. Moreover, it can not be obtained the impedance values of quarter wavelength shorter than the distance of two microphones. If P-U probe is chosen, it requires careful treatment and the calibration at short intervals. The authors have developed sound pressure and velocity estimating method using two cardioid microphones (called as C-C method). In this report, the possibilities of using cardioid microphones technique for acoustic impedance measurement are discussed with comparison of the results of P-P probe or P-U probe. The results show us that P-P probe can measure up to 4 kHz. On the other hand, C-C probe can measure over 10 kHz. Additionally, we could obtain the nearly equal to the results of using C-C probe and that of P-U probe.