Uncertainty evaluation for the measurement of acoustic impedance in a freefield with two microphones

Abstract

The measurement of the acoustic impedance and/or the reflection coefficient of an acoustical system placed at the end of an impedance tube has recently generated great interest. The development of the two‐microphone method has, for one thing, greatly increased the ease and speed with which such acoustical measurements can be made. The use of a small sample size inserted in a tube is, however, still required. Recently [J. F. Allard and B. Sieben, J. Acoust. Soc. Am. 77, 1617–1618 (1985)] suggested measurement of the acoustic impedance in the freefield. The present paper addresses theoretically the accuracy of such a freefield measurement. With the hypothesis of spherical wave propagation, the uncertainties associated with the finite‐approximation error, phase mismatch and the near‐field error are evaluated for the case of various acoustical materials. The importance of the selection of the appropriate type of wave propagation in the calculation of the reduced impedance at the surface of the sample is also ...