I’m using windows 10 and just converted to Audacity 3.2.3. I’m doing all my usual noise reduction and limiting, and producing a final file with peak level of -3.2 levels, about 84.5 noise floor and rms of approx -20.21.
When I export it as an MP3 file, it blows it up to max levels and fails the test in every parameter. I fixed it by normalizing the MP3 file, but there has to be a better way to do it.
It’s not unusual for the (highest) peaks to increase a little with MP3. Some peaks get higher and some get lower. MP3 is lossy compression and it changes the wave shape.
The ACX spec is -3dB maximum and the [u]Recommended Procedure[/u] is for -3.5dB limiting. That usually works but if not you can go lower (more negative).
I fixed it by normalizing the MP3 file, but there has to be a better way to do it.
It’s best if you go back to the uncompressed file and try a different limiter setting. When you open an MP3 in Audacity (or any regular audio editor) it gets decompressed. If you then re-export as MP3 you are going through another generation of lossy compression and some “damage” accumulates.