Abstract:

Energy decay curves in room acoustics can exhibit a multiexponential nature. Multiple sloped energy decay curves do not exclusively exist in coupled volume spaces, where they are used to adapt the acoustic behaviour (e.g., in concert halls), but also in reverberation chambers. Once a curved decay is present, the commonly used method of fitting a linear regression to obtain the reverberation time becomes questionable. This work investigates three state-of-the-art algorithms to extract decay times from a given energy decay curve (EDC). The first is a revised version of the variable projection algorithm (VARPRO). The second and third program are both based on the assumption that a decay time distribution can be computed from the given EDC by computing the inverse Laplace transform. The Regularized Inverse Laplace Transform algorithm (RILT) uses a nonlinear least squares fitting algorithm to extract the intensities for a specified decay time grid with a regularisation based on the principle of parsimony. The obtained intensities as a function of the decay times can then be regarded as a decay time distribution. The Maximum Entropy Decay time Distribution program (MEDD) computes a decay time distribution using a quantified maximum entropy method. Measurements from a reverberation chamber are analysed, decay times and decay time distributions are estimated using the proposed methods. Furthermore, the obtained results are used to define the single sloped frequency range in a reverberation chamber, such that rough bounds for the validity of the commonly used linear regression method can be given.

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