The invention provides a method of communicating data imperceptibly in an audio signal. The method comprises, for each sub-band of the audio signal, identifying the tone in that sub-band having the highest amplitude. An audio code comprising the data to be communicated is scaled by a frequency mask profile, the frequency mask profile having maxima at the frequencies of the identified tones. The audio signal and the scaled audio code are aggregated to form a composite audio signal. The composite audio signal is then transmitted.