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I know how to use Audacity to increase the volume of one MP3 at a time. What I have is a folder of audio files and I'd like to increase the volume of all of them without the need to open each one separately in Audacity.

Any good solutions out there?

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3  
What do you want to do? Peak Normalize the files? Replay-Gain the files? Amplify them by an arbitrary level you choose? – slhck Aug 16 '11 at 14:19
    
hmmm - my ignorance shines through - I'm not sure. They are quiet and I can't hear them in my car. I just want them louder. What would that be? – andersonbd1 Aug 16 '11 at 14:23
up vote 6 down vote accepted

There are different types of gain control:

  • Peak normalization will search for the loudest part of the file, amplify it to the loudest level possible and then amplify the rest accordingly. It's a pretty "stupid" technology in the sense of not making use of any psychoacoustics.
  • ReplayGain is an algorithm that analyzes the files on a psychoacoustic basis and amplify them accordingly, so they all have the same perceived loudness.

Batch-Normalization with Audacity

If you want to simply batch-normalize using the Audacity feature, you have to do this, according to a blog post:

  • Open Audacity
  • File > Edit Chains
  • Add "Normalize"
  • Insert "Normalization"
  • Ok
  • File > Normalize > Apply to Files

ReplayGain

I assume you are on Windows, but this works for OS X as well:

What does it do?

MP3Gain analyzes and adjusts mp3 files so that they have the same volume.

MP3Gain does not just do peak normalization, as many normalizers do. Instead, it does some statistical analysis to determine how loud the file actually sounds to the human ear.

It will analyze your files and make them as loud as possible, thereby equalizing all of them in terms of loudness. So normally, it should be enough to let this run on your library. MP3Gain includes a batch option for your convenience. They will not be re-encoded during this process and therefore not lose quality.

However, your car audio would need to support it. Try it out and let me know if it's not working.

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So the answer is that there's no way to do with with Audacity? – Mooing Duck May 4 '15 at 23:47
    
It seems to be quite easy; see my updated post. – slhck May 5 '15 at 7:18
    
Good, but there seems to be no way to save the files in other than a limited set of formats. I need to save in .m4a and seems to be no way to do that. – Chris Prince Sep 12 '15 at 1:11
    
@ChrisPrince What part are you talking about exactly? The one using Audacity? It should be able to do that: manual.audacityteam.org/index.php?title=AAC_Export_Options – slhck Sep 12 '15 at 21:15

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