Crackles in WAV file (but not mp3) after export

[Itoshikis_Despair]

Audacity for Mac 2.4.2


Hi! I'm a complete newbie to Audacity and music editing in general. This is my first project ever so I apologise in advance if any of these questions are really dumb. I'm currently making a mashup, so there are a lot of individual tracks stacked on top of one another. None of the individual tracks go over 0db. I normalised everything to -1db, then I used the envelope tool in various places to arrange the sound and also amplify here and there. I then normalised everything again to -3db to leave some headroom for when everything came together.

When I played it back in Audacity I noticed that some parts of the song would occasionally tip the bar over 0db (presumably due to the stacking). I therefore went through and sorted out the problem parts, using the envelope and amplifier tools (to reduce the sound). I replayed everything and watched the bar meticulously the whole way through. It didn't go over 0db.

When I export to mp3, no problem. However, when I export to WAV, AIFF, etc (I'm using 32 PCM) , there are occasional crackles or pops (I'm guessing this is clipping?). This also happens when I import the *mp3* into the movie programme I'm using to make the accompanying video. When I imported the finished WAV into Audacity to check the waves, I noticed that there were red peaks at the crackles. I went back into the original project, looked at the individual problem areas and reduced the sound more with envelope and amplifier. I replayed and checked the bar again. Looked fine, but it's still happening on export.

Why is this still happening and what can I do to fix it other than just reduce the overall volume of the whole project to leave more headroom? Would the easiest way be to just slide the gain on all tracks over to -1db? Then everything is reduced in the same proportion and I won't lose any of the tweaking I've done with fading in/out, etc.

Many thanks in advance for your help!

[rayan008]

It shouldn't be though, I used music from The Chronic to help me get a standard volume, and I actually had crackling in VLC player on that music, but that wasn't the issue because my music still crackles on Windows Media Player. I'll get back to you after I have another look.

That would really suck though, I'm hoping the music isn't too loud because it matched perfectly with the reference tracks, that would mean my project is going to be quiet as shit, lol.

[steve]

When I played it back in Audacity I noticed that some parts of the song would occasionally tip the bar over 0db (presumably due to the stacking).

Yes it's probably due to "stacking". When tracks are mixed ("stacked"), all overlapping tracks are literally "added" together.

You will notice that in Audacity, the track's vertical scale goes from +1 to -1. That's equivalent to 0 dB. If the waveform goes above +1 or below -1 then it is exceeding 0 dB and will be clipped on playback.

If you have two tracks and one track has a peak at 0.75 (on the track's vertical scale) and at exactly the same time another track has a peak of 0.75, then the exported file, which is a mix of the two tracks will have a peak of 0.75 + 0.75 = 1.5.

It's virtually impossible to predict the exact level when mixing tracks, because we don't know in advance whether peaks in one track will line up with peaks in another. For example, if one track has a peak of 0.75 and at that exact time the other track has a trough at -0.75, then the mix at that moment in time will have a level of 0.0 ("silence").

To work around this uncertainty, we can mix the tracks before exporting ("Select all" then "Tracks menu > Mix > Mix and Render").

Assuming that we are working in 32-bit float (default and highly recommended), even if the sum (mix) of tracks exceeds +1 or -1, there is no actual damage yet, because 32-bit float can safely go over 0 dB (though it will still clip on playback because sound cards can't go over 0 dB). So we can then normalize the track to bring the biggest peaks within the legal 0 dB range before exporting.

In some cases, the mix may have occasional peaks that are significantly higher than the rest of the audio. In this case, normalizing may make the mix sound too quiet. We can combat this by "limiting" the peaks (with Audacity's "Limiter" effect: https://manual.audacityteam.org/man/limiter.html). However, you should be aware that excessive use of the limiter can be harmful to sound quality.

Tip: If you intend to save the project and work on it more at a later date, "Undo" the mix before saving so that the saved project still has separate tracks. If it is saved with the tracks mixed down to a single track, you lose the ability to work on individual tracks - it can't be "un-mixed".