receiving an audio input;

generating a relaxed frequency-normalized version of the audio input comprises:

calculating one or more statistical measures for one or more hidden features in each of a plurality of feature dimensions in the received audio input; and

generating an instance-frequency-normalized version of the received audio input based on the calculated one or more statistical measures;

generating a classification of the received audio input using the relaxed frequency-normalized version of the audio input and a neural network trained to classify audio into one of a plurality of categories; and

taking one or more actions based on the classification of the received audio input.