16. An article of manufacture including a non-transitory computer-readable storage medium, having stored thereon program instructions that, upon execution by one or more processors of a system, cause the system to perform operations comprising: transforming an input audio training signal into a sequence of feature vectors, wherein each feature vector of the sequence bears quantitative measures of acoustic properties of the input audio training signal; processing the sequence of feature vectors with an auto-encoder 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 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.
16. An article of manufacture including a non-transitory computer-readable storage medium, having stored thereon program instructions that, upon execution by one or more processors of a system, cause the system to perform operations comprising: transforming an input audio training signal into a sequence of feature vectors, wherein each feature vector of the sequence bears quantitative measures of acoustic properties of the input audio training signal; processing the sequence of feature vectors with an auto-encoder 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 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. 19. The article of manufacture of claim 16 , wherein the system comprises the normalizer, wherein the supra-phonetic acoustic properties are indicative of particular audio characteristics, the particular audio characteristics being at least one of pitch, tone, prosody, tempo, accent, microphone equalization, or channel distortion, and wherein processing the duplicate copy of the sequence with the normalizer comprises generating a normalized signal in which the particular audio characteristics have been reduced to less than a threshold level.