Exported mp3's are losing/changing some of the processing

Many commercial mp3 tracks these days come with peaks over 0dB, so I process them in Audacity, and limit them to approx -0.3dB, as in theory, this works well for getting consistent, high volume levels in DJ software. I then export the tracks as mp3’s, but even with the highest quality audio settings, the exported track has been changed. If I re-import, the track will have many peaks over 0dB, up to pretty much the same level as before.

After experimenting with both hard and soft versions of clipping and limiting, it seems that whatever changes are made in the process, they are more or less lost/undone in the exporting of the track. This does not happen when exporting as AIFF, so I guess it’s in the mp3 encoding process.

This was not quite so obvious in previous versions of Audacity (the ‘restored’ peaks were not as high as the originals), but with the latest 3.4.0, it’s as if I didn’t limit at all.

What can I do to convert to an mp3 that is identical to the track I have created, or is this an unavoidable part of the encoding to mp3 process?

Working on OS 12.6.9

I hope I have described this well enough for others to understand my issue.

Right! As I’m sure you know MP3 is lossy (it doesn’t match the lossless original). The commercial MP3s were probably made from originals limited to 0dB. (CDs cannot go over 0dB, but MP3 can.)

…MP3 also adds a little silence to the beginning and end.

When you export you are creating a new MP3 and you can’t predict what the peaks will be but 1dB of “headroom” is usually enough.

A couple more things…
MP3 can go over 0dB without clipping. The MP3 is not clipped (at least it’s no-more than the original might have been).

But, you’ll clip your ADC if you play it at “full digital volume”.

I’ve never heard clipping caused by this and I’ve never heard-of it being audible. A LOT of my MP3s (ripped from CDs) go over 0dB and I don’t worry about it. If you hear an MP3 compression artifact it’s probably something else that won’t go-away by lowering the volume.

Peaks don’t correlate well with perceived loudness. Many quiet-sounding tracks have normalized/maximized 0dB peaks.

Usually you’ve got enough analog gain to overcome quiet files. Usually you’ve enough “excess gain” that you can turn-up the volume and overdrive your amp into distortion, even with quiet files.

If you use ReplayGain, MP3Gain, or Apple Sound Check for loudness-matching it makes DJing easier. And these loudness-matching tools end-up lowering the volume of most tracks, so to some extent that will help with your potential clipping problem. Since many quiet-sounding tracks have 0dB or nearly 0dB peaks and they won’t boost them into clipping, they use a lower target-volume and lower the loud tracks more than they boost the quiet tracks.

Thank you, that’s really helpful and informative. I knew mp3 encoding was lossy, but in my mind, that didn’t seem to equate to a gain of peaks and a gain of level !!! These peaks aren’t audible, I know, but they can be annoying, as my DJ software assigns tracks their pre-defined levels based on them, for the auto-gain function, which can end up turning down the level , just for a few tiny peaks. Also, I like to ‘re-master’ old vinyl rips, so that they can be played alongside new, digitally-produced music, without sounding muffled and quiet, and every dB counts! ))

Thanks again!

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