Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Geometrical and Measurement Errors

In the second of a series of papers on the measurement of acoustical impedance of absorbing surfaces in-situ, geometrical and measurement errors for the two microphone transfer function technique are detailed. The effects of finite sample area on the impedance measured are considered, and experimental results show that for edge effects to be negligible, the minimum dimension of a sample should be two wavelengths. The method also becomes inaccurate when the transfer function tends to unity. This is shown to occur at low frequencies, but more importantly, at higher frequencies when an integral number of half wavelengths exists between the path length differences to the microphones. The results of the error analysis show that great care should be taken in determining the precise locations of the microphones. In particular, care should be taken in the alignment of the microphones with respect to the normal to the sample, since errors due to the alignment of the probe combine features of errors not only in the microphone positions, but also in the source location and the angle of sound incidence. In comparison with typical errors in the experimental geometry, errors due to the precision of typical transfer function estimates are very small.