From where? There are four possibilities: The actual submission process goes through an automated quality control step, you can submit to ACX AudioLab—which we assume duplicates the submission acceptance process, you can use Audacity ACX-Check, and you can use the long arduous Audacity manual process that ACX-Check replaces.
We expect all of those to produce results within small fractions of each other.
Audacity Audiobook Mastering is designed to guarantee the two physical volume checks, Peak and RMS. You must use all three tools, in order, without messing with the settings. It’s not mix and match. It’s a suite. A harmonious grouping.
Yes, the conversion to MP3 does affect the volume and that slight volume change is built into the tools.
I get the warning from ACX. I have 48 chapters recorded, only 11 of them have issues, all under 1 DB of an issue (with RMS being too low). I paid a sound person to do the settings for me, and they should be within levels of ACX requirements. Like I said, most of the chapters are. Only a few aren’t. I have all the “settings” that are supposed to be there, and have already done the ACX check and it just tells me the same thing. Which sections are too low. However, I dont’ know how to FIX it. I am completely new to audacity and am going on the settings of my sound guy. His recommendations are the same on ACX website as well as this website.
That will also boost your peaks by 1dB (and the noise floor too). If your peaks go out-of-spec you’ll have to run the limiter again. As you know the RMS requirement has a rather wide 5dB range (-18 to -23) so if it’s too low you can raise it more than 1dB, you just have to watch (or re-limit) the peaks.