Abstract: Disclosed herein is a method of determining a plurality of inter-track auditory masking measurements between a plurality of audio tracks, the method comprising: dividing each audio track into a first set of frequency bands; and determining, for each frequency band of each audio track, an inter-track auditory masking measurement that is a measure of the masking between the frequency band of the audio track and a combination of the corresponding frequency bands of all of the other audio tracks; wherein each inter-track auditory masking measurement is determined in dependence on a psychoacoustic model.