In this work we consider the problem of spectral envelope estimation using spectra with perceptually warped frequency axis. The goal of this work is the reduction of the order of the spectral envelope model which will facilitate the use of these envelopes for training of voice conversion systems. We adapt the true-envelope estimator to Mel-frequency representations and adapt a recently proposed cepstral model order selection criterion taking into account the distortion of the frequency axis. We evaluate the modified order selection procedure using a perceptual framework for the evaluation of envelope estimation errors. The experimental evaluation carried out with real speech confirms our modifications. The results demonstrate that the Mel frequency based true envelope estimator achieves superior envelope estimation with significantly reduced model order.