The measurement of the absorption coefficient in reverberation chambers according to the international standard ISO-354 shows a poor reproducibility between laboratories. One reason is the non-uniform distribution of absorption due to the placement of the test object. This causes shorter decay times for modes with oblique and perpendicular incidence on the absorber. Modes propagating parallel to the floor are less dampened by the absorber and will lead to longer decay times. Only the shorter decay times of this multi-exponential decay allow precise conclusions about the material.

This study investigates the sound field decay of a simplified measurement setup in a reverberation chamber. The computations rely on the analytic model by Nolan and Davy using the principle of modal superposition to obtain room impulse responses for multiple receiver positions. The decay times of all modes contributing to the respective frequency range are derived as a reference from the analytic model. Three different approaches on decay time estimation are evaluated in respect to their initial decay time approximation and their sensitivity to noise. The linear approximation according to ISO-3382 and two fitting algorithms for a multi-exponential model are chosen. One multi-exponential fitting method is based on the principle of least squares whereas the other uses Bayesian statistics to estimate the decay times. The results indicate that the estimation based on Bayesian statistics reliably determines the order of the multi-exponential decay and its reverberation times.

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