The two-microphone transfer function method for measuring absorption coefficients and impedance: Some limitations and precautions

Abstract

The transfer function (TF) method described by Chung and Blaser [J. Acoust. Soc. Am. 68, 907–921 (1980)] is an attractive alternative to the conventional, and tedious, standing wave ratio technique. Many laboratories have already implemented the new method and are obtaining useful results. However, several problems are being encountered that have not been satisfactorily addressed in the literature. This paper discusses some of these problems, points out the limitations of the TF method as it is currently implemented, and suggests some necessary precautions. In addition, the importance of calibration is stressed and demonstrated through some examples (here, unlike two-microphone intensity measurements, the relative phase match between the microphone channels is just as critical at the higher frequencies as at lower frequencies). The results of measurements with the TF method, using two impedance tubes (100- and 30-mm diameters) to cover a frequency range of 90–6300 Hz, will be discussed.