Export mp3

With Windows 10 and Audacity 3.0.2
Each time I save an mp3 file, its volume (measured with MP3Gain) is decreased by 0.5 dB. Can someone tell me why? Can this inconvenience be avoided?
Thank you

You are opening and then re-exporting an MP3 without any editing or anything?

MP3 is lossy compression so there are (hopefully insignificant) changes. When you open an MP3 in Audacity (or any “regular” audio editor) it gets decompressed. If you re-export as MP you are going through another generation of lossy compression.

It’s not unusual for some peaks to increase (and other to decrease) so if you start with a 0dB normalized WAV, the MP3 will often peak slightly-over 0dB (and MP3 can go over 0dB without clipping). But overall, the level/loudness does drop.

Higher bit-rate MP3s usually match the original waveform more closely than low bit-rate MP3.

I don’t normally use MP3Gain, however I was curious about this. On Windows 10 with Audacity 3.0.2, I took an existing 3-minutes MP3 file and made 3 additional MP3 generations of it, each standard quality (170-210kbps).

Here is what MP3Gain showed as the volume for each file:
Original File: 88.6
Generation2: 88.6
Generation3: 88.7
Generation4: 88.7

[u]Nine different audio encoders 100-pass recompression test[/u]:

Some encoders did not only degrade sound quality, but also had some other quirks. For example, the LAME encoder lowers the volume with every encoding pass. The 100th pass was virtually inaudible. I had to normalize the audio to hear anything at all.

I’ve seen the peaks increase (one-pass WAV to MP3) but I never noticed (or tested for) volume loss. But, I was aware of the 100-pass test.

Not currently. You’re better off with a converter like Max. So you can do batch conversion.

Audacity can do batch conversion: Macros - Audacity Manual