Using Audacity to limit dynamic range of Garage Band file

I am recording and producing an audiobook of a book I have written for ACX, which is a division of I recorded a sample chapter using Garage Band and submitted it to ACX. The critique I received said that audio files must have a dynamic range between -23 and -18dB RMS with audio peaks at -3dB. My file was somewhat outside of that range. Garage Band apparently is not capable of re-processing my file to achieve those limits. Can I do this in Audacity? If so, what type of file will Audacity accept from Garage Band? I have not downloaded the program yet. My OS X version is 10.8.4.

Somebody else posted almost a word-for-word message not that long ago. I saw your life flashing before my eyes.

My file was somewhat outside of that range.

Did they say which way off you were? Can you find out? That’s not inconsequential. It would be good if we don’t have to tell you how to fix The World, but instead how to fix your particular problem.


Save an AIFF file in Garage Band for Audacity.

Your critique did not say I am sure that the dynamic range must be -23 dB to -18 dB RMS (that would be meaningless). What they mean is that the average level (RMS) should be lie somewhere between -23 dB and -18 dB.

If your RMS value was too low (a more negative figure than -23 dB, such as -30 dB) then you can follow the suggestions here to use some compression. To measure if the RMS is then within their range, you can use Contrast Tool or the stats plug-in mentioned in the Forum post.

If your RMS value was less negative than -18 dB, that is more difficult, unless your audio is simply too loud. If it is too loud, try Effect > Amplify and set “New Peak Amplitude” to -3 dB.

If your audio is lacking difference between loud and soft and you cannot get the RMS level low enough, ask again.


Thanks for the help. According to ACX, my audio file measured -24dB RMS with peaks at -0.1dB, so I guess that means it was slightly louder and slightly softer at various points than what they require. Sounds like Audacity might be able to help. I will search this forum to see if this issue has been addressed previously.

Try downloading Chris’s Compressor then set the maximum amplitude to 0.7 and apply it a couple of times.


apply it a couple of times.

While I agree with the tool, I don’t know that I’d apply it more than once. IMHPO it does a very respectable job in one pass. See what happens

With that one value change (0.5 to 0.77), Chris approximates the broadcast compression in one of the local FM stations. Chris designed it so he could listen to opera without constantly messing with the volume control in the car.

It will be interesting to see what happens when you take a breath or pause. It’s not supposed to suck the room noise up like a regular compressor can do.


I wouldn’t apply it more than once either. It’s better to undo and slowly tweak the parameters.
Have you installed the wave-stats plug-in mentioned in the linked post?
Do the results match the ones they gave you?
Even if they are slightly off, they will indicate if you’ve hit the right parameters in the end.

I rarely use that tool and in fact had v1.1 which seems to need two passes to get the maximum amplitude down to the requested amount (at least with the audio I was using, which had a peak at -0.1 dB).

v1.2.6 which I linked to seems much better in that respect. One pass should be the better advice, if those who use this tool say so.


It is rather a pure subjective aversion against multiple applications of an effect ingeneral, knowing how much quantization errors do accumulate during the process.
Furthermore, I’ve got the impression that Chris’ Compressor over-emphases the first part of the audio a little bit and repeatedly using this effect could make this very prominent.

I have downloaded Chris’s dynamic compressor and see it in my applications folder. However, I cannot drop it into the plug-ins folder in Audacity. It refuses to drop into that folder. If I double click the folder containing Chris’s dynamic compressor, I’m told I need to find an application to open it with. I am using Mac 10.8.5

You download it as a zip file. Have you extracted the zip? You can try right-click > Open with > Archive Utility or download The Unarchiver: .

Note that you need an administrator account to modify the /Applications folder.