I think the psychoacoustic masking would certainly degrade the spectrum... time for a little experiment! Step 1) Generate 1 second of white noise in audacity,

I think the psychoacoustic masking would certainly degrade the spectrum... time for a little experiment! Step 1) Generate 1 second of white noise in audacity, look at spectrum Step 2) export to mp3 Step 3) Import back, look at spectrum Result: http://i.imgur.com/vZtZ6xE.png http://i.imgur.com/vZtZ6xE.png (left is original, right is mp3 roundtripped) The mp3 (at least with this encoder) kills some of the high frequencies, so the noise is no longer white.