ACX Check: RMS vs Peak

System: Windows 10
Audacity 2.4.2

Hi -
Hope no one minds another ‘mastering my audiobook for ACX’ question.

I followed the recommendations here:

and I found my RMS level failed the ACX check. I used RMS Normalize and my peak level was too high. I’ve been using Amplify to lower the Peak, but then my RMS is too low.

Just wondering if there is any advice? Do you tweak it again and again until it passes?

I did do things a bit out of order - I tweaked the bass and treble to make my voice a little stronger. I have some musical clips as well. Then I followed the steps at
Is it possible to make a bit of audio that can never be ready for ACX?

Latest ACX check result after using RMS Normalize:
Peak level: -2.15 dB Fail
RMS level: -22.4 Pass
Noise floor: -71.02 Pass

I followed the recommendations here:

You need to follow the recommendations a little more carefully. :wink:

and I found my RMS level failed the ACX check. I used RMS Normalize and my peak level was too high.

That’s to be expected. The next step is using the limiter to “push down” the peaks. The limiter has almost no effect on the RMS level. (If your peaks are not too high, the limiter does nothing.)

I’ve been using Amplify to lower the Peak, but then my RMS is too low.

Amplify is a linear effect. It changes RMS & peak measurements by the same dB amount.

Audiobook Mastering is a “suite.” A harmonious grouping. You need to take all the tools in order, don’t add any, and don’t leave any out. As you’re finding, they do the work and then clean up after each other.

If you apply Mastering to any reading, the RMS (Loudness) and Peak will pass. If you recorded in a quiet, echo-free room, noise will probably pass and you’re done. The tools are chosen so you still sound like you at the end with little or no distortion.

Home performers have the most trouble with noise. Noise is a whole chapter in the mastering instructions.

Do you use Noise Reduction, Noise Gate, or any other processing? That may not be the best idea. ACX doesn’t like that very much.

ACX has a couple of odd restrictions from the last time I did it. Can I buy your book right now on Amazon? That’s sometimes full stop for new New Users.


I have some musical clips as well.

I just noticed that. ACX doesn’t like “Radio Theater.” No organ music, orchestra background, bands, or other effects, or theatrical sounds. I have heard brief introduction music at each chapter, but that’s about it. If you’re goal is to make the presentation as desirable as possible first time out, then no music is good.

They’re not just being hard-nosed about it. They’re a business. If you do include music, you need to also provide copyright, identity, release, and performance restriction documents along with your voice. You have all that, right?

This is the ACX panel on the type of books they like and don’t. We have had posters intent on reading cookbooks and yoga relaxation chants. There’s zero chance either of them made it. We do know what the rules are and we have a very good record of getting people published.

Scroll down.


That “rule” does not appear to be cast in stone (and there are many other audiobook publishers - ACX / Amazon are just the biggest), but as you say, music complicates copyright and licensing. I wouldn’t be surprised if ACX rejected a submission for having music, (unless they expected it to become a best seller).

Thanks for all the responses.

Just to try, I tweaked it a bit more and the ACX check said…

Peak Level -3.45 dB Pass
RMS Level -23.00 Fail (too quiet - RMS must in range of -23 to -18)
Noise floor -71.62 dB Pass

Why is RMS -23.00 bad if -23 is good?

Is there some hidden decimal like -23.001 ?

Is there some hidden decimal like -23.001 ?

That’s a reasonable guess…

But again it looks like you’re not following the instructions and setting the RMS to -20dB before limiting. …Limiting can drop the RMS a little but I don’t think it will ever drop by 3dB.

As long as your noise floor is OK (and yours is) the instructions are foolproof and it works every time! (You can have other issues but peak & RMS levels will pass.)

The display value is rounded to 2 decimal places. Thus -23.001 (a little less than -23.00) will display as -23.00.
Better to aim for around -20 dB so that you are well within the specified range.

I tweaked it a bit more

What does that mean? The mastering suite was designed to make any reading pass, tweaked or not.

Announce a ten second sound test and post it on the forum.

Read down the blue links. They’re very short. Do not apply any corrections. Read it, cut it to length, export WAV, and post it.


I ran RMS Normalize on it, and set it to -20.

After that, ACX Check said the peak was too high. So I ran the limiter and…

ACX check said everything passed.

I had 7 other files I had treated in the same way. 5 of the files passed ACX Check the first time I ran it.

The other two had a peak that was too high, but I ran the limiter on them, and they passed the ACX Check.

I uploaded them to ACX successfully. Now waiting for their QA check.

I had watched a bunch of Youtube tutorials on mastering audiobooks - they said to use Noise Reduction and Normalize. Sorry, I discovered the Mastering steps a bit late in the process.

I ran RMS Normalize on it, and set it to -20.

That’s not what the instructions say. Did you have any trouble getting the Low Rolloff for Speech correction?

Screen Shot 2020-11-14 at 09.35.03.png
That’s not a theatrical correction. Many home microphones and recording systems have sub-sonic rumble, earthquake, and thunder sounds whether or not you can hear them. They throw off RMS Normalize.

So after you get rid of the rumble, then you apply RMS Normalize or the built-in effect Loudness Normalization. That almost always produces one or two spikes or hits louder than -3dB, the ACX limit. Soft Limiter set to -3.5dB corrects those and also adds a little boost so you don’t run into trouble when you convert to MP3 for the audiobook submission. MP3 conversions can cause volume changes.

So it may seem like three simple tools, you can’t leave any out, add any, or take them out of order.

You’re not going to burn that ten second sound test, are you?