In situ absorption measurements using a transfer function technique and MLS

The transfer function (or two-microphone) technique is a tool for measuring materials absorption related parameters as impedance or absorption coefficients. But this method is laboratory bound in its original form due to free field assumption and practical issues. It is shown that after proper modifications the transfer function technique can be used successfully in noisy and reverberating fields and work well at low frequency even in small measurement rooms, becoming therefore a really in situ measurement technique. A wide frequency range implementation is presented, based on a two-channel MLS measurement system, ‘‘energy ratio invariant’’ time windowing, a basic specular reflection propagation model with spherical decoupling, and a particular geometrical configuration. The measurement method is tested on a wide range of absorbers. For impedance measurements using infinite surface propagation models the uniqueness of the inversion procedure is investigated using the conformal mapping technique. In addition, faster inversion and model computation routines are proposed. Measurement results obtained under realistic in situ conditions are provided and compared with reference methods.