I’ve put quite some time in researching why some of my mp3’s in iTunes sounded so bad.
First, I suspected bitrate. But then i found some lower bitrate mp"s sounded good.
Then I suspected the software used to convert to mp3. There was no pattern to be found.
So I started experimenting with CD’s and ripping them with iTunes and other encoders. No pattern either. Until I ripped a Metallica CD that was compressed to zero dB and already had some clipping. It was awful, once ripped.
So I took some very good quality productions from Zappa, Chris Rea and others and started amplifying those before ripping. No problem until you go over -3 dB.
Long after that, I found a youtube vid from TC Electronic in Denmark. It was about the new LUFS metering system for broadcast and how that would kill the incentive to over compress. And there was a specialist from TC Electronic confirming what I discovered on my own.
Lemme see if I can dig up the vid. It’s very enlightening on the subject of compression.
NOTE - A proper scientific, blind, > [u]ABX Text[/u] > must be level matched. That means you’d have to reduce the loud MP3 (after encoding) before comparing the sound quality of the loud & quiet versions. And, you’d have to do that with a tool like MP3directCut that allows level changes without de-compressing/re-compressing.
I know about levels and have the software to do blind listening tests. I wouldn’t use a tool like MP3directcut, as you can easily adjust playback level in the DAW when necessary.
When I started all of this, I started with the suspicion that my old monitors needed a rebuild. In fact, I rebuilt some of them, to be quite disappointed. The mp3’s even sounded a bit worse, afterwards…
I also don’t ever do blind listening tests on my own. We do these with at least five listeners, all of them audio engineers or at least people who have listening experience and can compare with live (classical) music. One person rules the button that switches, the four others listen and take notes. Of course, we rotate the person who controls the button…