Definition of "Model fit" value in RT60 Decay

[Tdep11]

Hi,

I've been using REW for acoustic measurements, particularly the RT60 Decay graph. I am trying to understand precisely how the Model fit value is calculated.

I understand that it quantifies how well the fitted exponential decay matches the measured Schroeder integral — the lower the value, the better the fit. However, I need the exact mathematical definition in order to use it as a basis for uncertainty propagation in a research context (specifically, propagating the fit quality into an uncertainty on the decay rate).

If there is any documentation that describes this calculation or if anyone knows how it's done, I'd be bery interested to know.
Thanks !

 

[John Mulcahy]

It's the mean squared error between the measured response (the slices of the waterfall) and the model response after the model response has been convolved with the left window to account for its effect on the measurement. The error is the difference in the dB values, with uniform weighting from the fit start (green dot) to fit end (red dot) for points on the curve that are within 30 dB of the fit start level and weighted by pow(2, (level - (fitStartLevel - 30))/10) below that to attenuate the contribution of parts of the response that are more than 30 dB below the fit start.

The model is based on frequency domain data, the Schroeder integral is not used at all.