1. A method comprising: receiving, by a computing device, a sequence of speech features that characterize an unknown speech input, the sequence of speech features including speech portions and non-speech portions; determining, using voice activity detection (VAD), the speech portions of the sequence of speech features; determining C 0 for the sequence of speech features, wherein C 0 comprises an average log-energy of a given speech frame of the sequence of speech features; normalizing selected speech features of the sequence of speech features using a plurality of different feature normalizing functions including a first function used only for the speech portions and a second function used only for the non-speech portions, wherein the first function normalizes the C 0 by subtracting a max C 0 value calculated over the speech portions of the sequence of speech features, wherein the max C 0 value is initially estimated at a start of an utterance including the given speech frame and subsequently updated throughout the utterance; and concatenating the normalized C 0 with the sequence of speech features to produce a sequence of mixed normalized speech features for automatic speech recognition of the speech portions of the sequence of speech features.