Why Clipping is Worse in MP3?

  1. I have an audio file.
  2. I select all. I normalize.
  3. I have View->Show Clipping “On”. There are no clipping instances shown. I’m viewing the entire file. So I assume all is well.
  4. I export as MP3.
  5. I re-open the file that I just exported. There are now three vertical red lines showing instances of clipping. Why does this happen? Encoding as MP3 can result in clipping where there was none in the original? Because MP3 is an approximation? Or what? If I export the file as aiff and read that file back in, there is no clipping indicated. Also, if I use the Analyze → Find Clipping tool with the default thresholds, no clipping is found, despite the presence of the vertical red lines. Do those red lines have a different have a different criteria for clipping? Thanks.

Yes, plus the fact that MP3s can go over 0 dB.

Just three lines is likely to be inaudible. If you zoom in close, I’d expect that each line is just a single sample, so on playback, that sample value will simply be reduced to 0 dB.

If you wish to entirely avoid red clip lines, allow a couple of dB headroom before exporting as MP3. (I generally allow 1 dB headroom for WAV files, and 2 to 3 dB for MP3s).

Yes, plus the fact that MP3s can go over 0 dB.

There is another reason. MP3 is a compressed format and one of its tricks to getting small files is to leave some sounds out.

Complex waves are made up of a lot of different harmonics, beats, and overtones. It’s one of the things that let you tell different violins even if they’re all playing the same note. Harmonics and overtones don’t always add up. Some of them subtract on purpose and when MP3 leaves them out, a note can get louder.

Not by much, but it does happen. As you noticed, if you snug a performance right smack up against the 0dB limit and then make an MP3, these errors and red bars can come out of nowhere. It can get worse with smaller files (higher compression).

Audiobook maximum submitted MP3 volume is -3dB, but the Audacity mastering system shoots for -3.5dB, slightly quieter to help deal with this oddity.