1. A method comprising: transforming an input audio training signal, using one or more processors of a system, into a sequence of feature vectors, each feature vector of the sequence bearing quantitative measures of acoustic properties of the input audio training signal; processing the sequence of feature vectors with an auto-encoder implemented by the one or more processors to generate (i) an encoded form of the quantitative measures, and (ii) a recovered form of the quantitative measures based on an inverse operation by the auto-encoder on the encoded form of the quantitative measures; processing a duplicate copy of the sequence of feature vectors with a normalizer implemented by the one or more processors to generate a normalized form of the quantitative measures in which supra-phonetic acoustic properties of the input audio training signal are reduced in comparison with phonetic acoustic properties of the input audio training signal; determining an error signal based on a difference between the normalized form of the quantitative measures and the recovered form of the quantitative measures; providing the error signal to the auto-encoder; and by adjusting parameters of the auto-encoder to reduce the magnitude of the error signal, training the auto-encoder to compensate for supra-phonetic acoustic properties of input audio signals.