In a previous post, I discussed the physical interpretation of digital music (and all digital audio) as both a time-varying signal and a frequency-varying signal. This, of course, makes intuitive sense - as we spend three or four minutes listening to a song, we can hear the guitars and horns coming in and fading out, a singer intone the chorus of a song, etc. At the same time, we appreciate the frequencies changing as well — the singer hitting different notes, the instruments forming different chords. Understanding the time and frequency behavior of auditory information is fairly intuitive - in fact, one could even argue that visualizing the frequency-based content is more informative than visualizing the time-based content.
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