Small "pop" and gap between appended tracks

Audacity 2.3.2
macOS Catalina 10.15.2

Hello! I would appreciate some help with an issue I’m having:

I have two or more music files, and I want to append one to the other so as to make a single file. It’s live music from the same performance. So imagine a band is playing live… they play song A and then song B, so I have “A.mp3” and “B.mp3” and I want to combine them into a single, seamless file.

The files start out as MP3 or MP4, but first I use iTunes to convert them to AIFF. Then I open the the AIFF files in Audacity.
I either cut audio and paste it to the end of another track, or I use “File → Import → Audio” followed by “Tracks → Align Tracks → Align End To End.”

Here is the issue: when listening to the result, not all the time, but sometimes I notice a small pop at what would be the border between tracks. I’d like to avoid this if possible. It seems odd to me, as it’s contiguous audio from the same performance. Maybe someone with more knowledge and experience can guess what’s going on. Perhaps it’s just a flaw in the audio itself?

I don’t know if this is relevant, but I’ve observed that if I zoom in very close at the border between tracks, I see a gap there. For example, here is a screenshot after I used “Align Tracks → Align End To End”:
Screen Shot 2020-03-21 at 15.08.15.png
This might not be relevant, thought, because I can see this gap between tracks even in cases where I don’t hear a pop.

If anyone can guess what’s going on and perhaps offer a workaround, I’d really appreciate it. Thank you!

That’s almost certainly due to the MP3 structure. MP3 doesn’t have pure, clean stop and start points. That and every time you export MP3 masters from MP3 songs, the sound quality gets worse. That’s why we recommend never do production in MP3.

You should be able to zoom into the gaps and edit them out in Audacity. There’s no way to fix the MP3 sound quality damage.


Missed a step. You can use the Time Shift Tool (two sideways black arrows) to push the bottom track to the left and make it meet the top track.


Select a region say 5 samples either side of the pop, & repair.

Thank you Koz and Trebor, this is very helpful. Here’s an update (maybe someone with a similar issue will find this useful):

I discovered that if I actually merge tracks together (by clicking on the boundary between two tracks), the small gap I mentioned seems to disappear.

If I don’t zoom in absurdly far, I can generally see where the pop is.
For example, here you can see a spike in the bottom waveform:
Screen Shot 2020-03-22 at 12.48.10.png
Here’s what it looks like if I zoom in even further. Here I’m about to use the repair effect, so the selection is about 10 samples wide as recommended by Trebor.
Screen Shot 2020-03-22 at 12.59.30.png
And here’s what it looks like after using repair:
Screen Shot 2020-03-22 at 12.59.48.png
And sure enough, the pop is gone!

This is great, thanks again for your help!