I’m looking to amplify the volume of about 100 mp3 files. Does anyone know of a freeware or cheap shareware program that can bulk amplify digital audio?
I’d like to be able to choose a load of files, set the amplification amount, and go to work. There’s usually something out there that will do this sort of thing. (I even got .rma files to bulk convert to mp3.)
I think this may do what you want. let me know. 
At this time, there is no technology by which you can do any changing of any kind to an mp3 without reconverting it to .wav format and re-encoding it back to mp3, which results in an audio file with up to 96% of its constituent parts removed.
Audio editors work in .wav format. With the appropriate codec, they will open compressed audio files, but they will be converted to .wav upon opening, and it is physically impossible to return them to the mp3 state without further compression. In all cases, the results are guaranteed to sound worse.
That’s a decent point about mp3 editing, but this is only voice, so I’m sure I don’t care much.
I did have Audacity, but I had v1.0, which didn’t have the amplify command. Hmmm… I see it does now, but it won’t do bulk processing. Since I have so many files, I don’t know if I’ll actually bother. My new car stereo goes higher than the last one, so I can just crank up that amp until I can hear decently.
Thanks for the help, anyways.
Why does amplifying this very quiet signal make distortion, when so many other signals are so much louder? What’s happening with the amplifying that makes it so buzzy?
You can’t do batch processing with Audacity.
SoX is a command line utility that will do the trick. (You’ll have to download an additional library to handle .mp3s.)
Scroll down to “Release Information” for the download link.
Well, I can’t hear your sound, but I’d say that the distortion was probably there all along, at a level so low it was unnoticeable. Now you’ve amplified the file and all its warts are showing. The process of amplification adds no distortion on its own.
Never mind about SoX, it’s a pain in the ass to build a version that reads .mp3. Sorry.
Let me amend my statement about amplification. If you amplify an audio file to less than 0 dB digital, it just gets louder. If the highest level in the audio before amplification was -3 dB and you added 6 dB of gain, all the peaks would then be over 0 dB. In the digital realm, 0 is the highest number. After that, there is only clipping, where the triangular peaks of the waveform are chopped off flat at 65,535 samples. That is digital distortion, and it sounds nasty.
This is not true; you can non-destructively (i.e. losslessly) adjust the block normalization (essentially the volume) of an MP3; MP3Gain is a piece of freeware that will do this in batch.
One more vote for mp3gain.
It comes out of the box with Windows binaries that sit on top of a command-line client.
I downloaded the source and built it on Linux with absolutely no tweaks (“make mp3gain” was sufficient).
I am using command-line switches that tell it to treat all passed-in files as one album (adjust all of the files by the same amount) after testing them to find the appropriate level. It worked great on some of the really noisy or quiet albums, making it so I don’t have to mess with the volume control when playing them in sequence.
I’m now working on the Perl script that will run as a cron job, running mp3gain on my library behind the scenes as new mp3s appear. Since I’m a Perl newbie, it’s taking awhile.
…and in case any of that sounds scary to Windows users, installing it on Windows was a breeze and using it is very simple - no more complicated than, say, WinZip.
I stand corrected about there not being a piece of software that will amplify mp3s without reconverting them to .wav first. I could not have known; we in the industry of people who work with wav files every day do not seek out the latest freeware, nor use mp3s. There is still, to the best of my knowledge, not an audio editor that will let you do this, though.
You’re right about that; if you want to alter the output waveform (including amplifying it) of an MP3 file, there’s no way to do this without converting it to an uncompressed form; since Cardinal appears to just be dealing with some files that are normalized a little on the quiet side, this can be corrected without converting. I don’t know much about the details of how MP3 files are encoded, but I assume that this made possible by the normalizing data being encoded in a separate stream within the file.
Here’s a bit of a description of the subject:
it tweaks “global gain fields”