ACX Mastering Macro is a series of tightly integrated tools. The first one, Filter Curve is the Rumble Filter. It gets rid of sub-sonic, barely audible thump, rumble, earthquake, and thunder sounds. It also gets rid of very low pitch errors that many home microphones have because (1) rumble is expensive to suppress—home microphone buyers are acutely sensitive to price, and (2) most people can’t hear it anyway.
It pre-conditions the voice for Loudness Normalization which, as it says on the tin, boosts the overall volume of the work to satisfy the requirement of ACX RMS (loudness). Rumble in the voice can screw this up. Rumble filter is a cousin to the sound filter that many field, movie, and outside broadcast engineers use to suppress wind noise.
It’s not unusual for a loudness boost to cause the tips and peaks of the work to overload or exceed 0dB, so the process is followed by soft, gentle peak limiting. You would think peak limiting would affect RMS (Loudness) and it does, but not enough to throw the work out of specification. You might also think that any work that exceeds 0dB would be permanently damaged, but no. That’s why Audacity works in 32-bit Floating format. While you’re editing, Audacity doesn’t produce overload damage.
Peak Limiter has a goal of -3.5 dB instead of -3dB to make up for possible minor errors in converting the voice to MP3 for submission.
Not bad for three simple tools or one quick Macro.
If you do a reasonable job of studio construction and performance recording, it’s possible to read a chapter, apply the Macro, and you’re done. Export a WAV Edit Master, and only then the MP3 Submission File and go on to the next chapter.
Maybe in some conditions you may need Noise Reduction of the Beast, but that’s it.
Oh, one more. If it’s not needed, Mastering Macro doesn’t do anything. It has no affect if you apply it twice. You might notice a slightly stiffer Low Rolloff, but nothing else happens.
If you have theatrical and sound quality changes, you should probably do them all before the final Mastering Macro.