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I have an mp3 file where is a dialog with two voices and one of the voices is very quiet.

Is there a command or application that is able the detect the quiet passages and amplify them? The two voices are alternating.

I already tried to do it manually with Audacity but this is very time-consuming job.

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I do not believe there is a way to do this in a single command out-of-the-box. However, you might be able to design a filter in Audacity so that you only need to go through the manual process once, and then the heavy lifting could be done for you. – HalosGhost Jul 5 '14 at 17:04
up vote 10 down vote accepted

The best way to do this is a Dynamic Range Compressor. Audacity has one built-in, under "Filters->Compressor."

This is, essentially, a program that removes the range between quiet sounds and loud sounds. It's surprisingly easy to use.

Set the "Threshold" value to something very low. Set the "Noise floor" to around -30DB. Set the ratio to a very high fraction -- Most of the time, I max it out at 10:1. Then check the "make-up gain for 0db" box.

Set both the attack and decay time to the lowest possible values. These are useful when applying compressors to music, but for speech, they just make it sound weird.

Here's an example. I made a quick audio file, which contains myself speaking both very softly, and very loudly (using the Windows version because I happened to be booted into Windows while answering this, but the compressor interface is almost exactly the same on all platforms): An audio file with a wide dynamic range

Now, I apply the "Compressor" filter to it:

Me applying the filter.

And, viola, the quiet parts and the loud parts are roughly the same volume. File post-compression

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This sounds perfect. Thanks a lot. – xralf Jul 5 '14 at 21:59

In version 2.1.2, compiled from source, it's "Effect->Compressor".

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