Done something similar for a module for uni, so I'll quickly whack it up here.
I ripped a track from a CD (in this case, Fanu - Salam from Daylightless) into a wav, then exported 5 second-sections at different bitrates. On the clip I've done, there is;
5 seconds of Wav 44k/16bit
5 seconds of 320 mp3
5 seconds of 192 mp3
5 seconds of 128mp3
Not in that order. See if you can work out which is which
For the record, I done this for a number of different songs in different genres and eras. My own take is that mixdown/mastering can make a massive difference to the quality of the Encoded MP3. There are, in my opinion, too many variables involved to make any sweeping statements.
File located here.
http://files.filefront.com/FanuTest...;/fileinfo.html
Have fun!
EDIT - 16bit, not 24 bit