This is a spectograph of "Under African Skies" by Paul Simon. This graph shows frequency magnitude data on the X and time on the Y coordinates. This file was converted from 16 bit WAV to Apple's AAC 320 kbps and then back to WAV.
This second spectrograph is of the same file, converted from WAV to 320 kbps MP3 and then back to WAV. It seems like at this bitrate the encoders both discard exactly the same frequency data. I can't really tell the difference between the two at this bitrate. When I did a subjective listening test, the MP3 seemed to have a more strident upper mid range point (perhaps around 2.5 Khz), and the AAC seemed a little looser sounding in the lower mid (250 hz).
I think the next experiment will be to render the audio at a lower bit rate in each format and do another listening test / electronic measurement.
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