https://forum.audacityteam.org/t/measure-between-23db-and-18db-rms/32770/16
ACX’s goal is a pleasant reading at good volume with no distractions. The RMS (loudness) value makes sure all your chapters match loudness and they match every other audiobook. The peak value keeps you out of distortion from getting too loud. That one is important because overload distortion produces harsh noises and is permanent. High Background Noise is counted as distraction.
Audiobook Mastering will only get you past the technical specifications. You have to pass theatrical qualities, too. When you submit to ACX, they send you through The Robot which works similarly to ACX Check, part of the mastering process. But then it goes on to Human Quality Control where you die if you can’t read out loud, have a terrible voice, stutter or have any other theatrical quality problem.
Everybody Knows you need to take your breathing sounds out of the performance. I’m not so sure. I don’t think ACX has ever bounced anybody for normal human sounds. The metaphor is listing to somebody tell you a fascinating story over cups of tea. They’re probably breathing as they speak. There was one forum poster who sounded clinically asthmatic. That would probably not work, it was uncomfortable to listen to, but they never posted back, so we don’t know.
Koz