Mind you, I have no idea what any of this means. I’m just a reasonably successful author trying to DIY those books of mine that the publisher(s) did not turn into audiobooks.
That’s not an unusual situation. If you are a famous author or celebrity, or if you have lots of extra money you can go into a pro studio with a recording engineer and maybe even a producer to help you with pronunciation and enunciation, etc. But a LOT of authors have to become recording engineers if they want an audiobook…
For noise reduction, the book has me use, Effect>Noise Reduction, and then set Noise Reduction (dB) at 30, Sensitivity at 15.00, and Frequency Smoothing (bands) at 4.
That’s why you’re getting a warning and it might be too aggressive. Nobody should be recommending noise reduction settings without knowing what kind of particular noise problems you have (if any). Even then, it often takes some trial-and-error.
The Audacity-recommended procedure has some noise reduction hints (if you fail noise) after everything else.
For amplification is has me set the new peak amplitude to 0.
If you are using compression it’s good to have a “starting point”. Otherwise, this is redundant because RMS Normalization is going to make another volume adjustment. But the Audacity recommendations don’t include compression.
For compression, it directs me to set Threshold to, -30 dB, Noise Floor to -60 dB, Ratio to 3:1, Attack time to 0.10 seconds and Release Time to 1 second.
Dynamic compression (unrelated to file compression like MP3) evens-out the volume by making loud parts quieter and/or by making quiet parts louder.
Limiting, which we do recommend, is a kind-of fast dynamic compression.
It has me set my Target RMS level to -20 dB.
Perfect! Again, RMS is an indication of loudness. At -20dB you’ll have the same loudness as other audiobooks and loud enough so the listener doesn’t have to crank the volume all they way up, etc.
That’s a simple-linear volume adjustment (up or down) for whatever is needed to hit the -20dB target. The change to the RMS, peak, and the noise floor is the same dB amount. Most often, it’s a volume boost which increases the noise level and usually pushes your peaks out-of-spec. That’s why it’s important to do everything in the correct sequence. (If you run RMA normalize a 2nd time with the same settings, nothing happens.
It has me set Limter to, Type: Hard Limit, Input Gain (right and left) to 0 (I record in mono with a Blue Yeti), Limit to (dB): -7.00 db, Hold (ms) to 10.0 and Apply Make up Gain: No
Too much limiting. After RMS normalization your peaks are usually too high. That’s just the nature of natural-unprocessed dialog or music… The difference between the peaks and RMS is often more than 20dB.
Limiting “pushes down” the peaks without messing-up the RMS level. The ACX spec says your peaks should be no-higher than -3dB. (Digital dB levels are usually negative so a smaller negative number is higher and a bigger negative number is lower.) The Audacity recommendation of -3.5dB (and soft limiting) gives you 1/2dB of safety margin so you’re not on the edge of the actual ACX spec.
ACX mostly wants a natural sounding recording with predictable loudness/levels and without excess background noise. It shouldn’t sound “over processed” or “enhanced”. It should (almost) sound like you are there live reading the book to the listener.