The article describes an approach to assessing the intelligibility of speech in the process of speech rehabilitation by finding the measure of the similarity of the standard and distorted pronunciation of phonemes. The approach is based on the calculation of the correlation coefficient between the transformed signal envelopes. The envelope of the signal is constructed on the basis of the calculation of the short-term energy of signal. The selection of the short-term energy parameter (window size) is also described. The parameter selection is based on comparing the differences between the correlation coefficients for pairs with normal pronunciation and pairs with distorted pronunciation, calculated for different window sizes. The window sizes for each problem phoneme are selected.