Noise Floor passes in Audacity, fails in ACX

I am new to this. I am helping my 83 year old mom get her memoir up in Audible. I have 26 .mp3 files recorded by her. All 26 files pass ACX check. I am using Audacity 3.4.2.

But, when I uploaded her files to ACX, I received the following message from ACX:

Issue: The files submitted for this title contain excessive background/ambient noise and do not meet our noise floor requirement.

Requirement: a maximum -60dB noise floor

I contacted Customer Support on how to find out which of my 26 files has an incorrect noise floor. Amber replied, telling me to use ACX’s Audio Lab. But Audio Lab doesn’t test for noise floor. It say that right there at the top of Audio Lab in the final sentence: “Audio Lab does not detect issues such as noise floor or editing errors, so remember to conduct your own thorough review of your files before submitting finished audiobooks to ACX.”

I went ahead and doublechecked all 26 files and they passed the analysis in Audio Lab, as expected.

But, I am back to square one. I have no idea which of my 26 files has a bad noise floor. When I review the noise floor in my recording software, Audacity, all of the files pass the noise floor.

I don’t know what to do.

Does ACX Check show “close” to -60dB?

Okay, I have gone through each of my 26 files.

The range is -60.62 db to -74.66 db.


You’ve got a few that are only slightly better than -60dB and the algorithms may be different. (I don’t think anybody knows the actual algorithm.) I’d shoot for -63 or -65dB.

Depending on your RMS levels, you may have some wiggle room and you may be able to pass the noise spec by simply lowering your levels. As you may know, a linear adjustment (like the Amplify effect) will change everything by the same dB amount, so lowering the level by -1dB will lower the peak, RMS, and noise floor by 1dB, etc. (But I wouldn’t push the RMS levels to the edge either, and I wouldn’t go below -22dB)

Great. I will tweak the ones between -60 and -65db then.

Do you have Edit Master WAV copies of all the chapters? You can’t make changes to the submitted MP3 files without causing MP3 quality loss. That’s the 192 Constant Quality MP3 thing in the standards. If you open up a MP3 chapter, correct it, and then make a new one, it’s not 192 any more. It drops in quality. MP3 is really entertaining that way. It just gets worse and worse.

Export each chapter as a WAV (Microsoft) file and that becomes your Edit Master. Submit them anyway and hope nobody notices.

How did you get the two other standards to meet—Peak and RMS (Loudness)? Did you use Audacity Mastering? You can do that again, and then apply Noise Reduction of the Beast. 6, 6, 6.

The real noise limit is -65dB (quieter). You are what happens when you try to hit it too close.


One other design note. Both Audacity Audiobook Mastering Macro and Noise Reduction of the Beast can’t be detected in normal operation.

That’s how you can get away with Mastering twice.


I do have WAV files of all my chapters, and then converted to MP3 when I uploaded to ACX.

If updating my noise floor to approximately -65db doesn’t work on 14 of my chapters, I will start over with the WAV files. I submitted yesterday afternoon, so I am now in wait mode with ACX.

To correct my RMS Level, in Audacity, first I select all (ctrl/a) and then I clicked on “Effect/Volume & Compression/Loudness Normalization”, and normalize “RMS” to “-20”.

To correct my PEAK, in Audacity, first I select all (ctrl/a) and then I clicked on “Effect/Volume & Compression/Limiter”, and limit to (dB) to “-3.0”.

I appreciate your reply.

It worked. Changing my noise floor to -65dB from -60dB worked.

I checked ACX today and it now shows:

Screenshot 2024-01-03 171257 review progress

Fingers crossed I don’t have any issues with the audio review. Otherwise, I will be back here!


The only issue was noise floor. My mom’s book is now on Audible/Amazon/iTunes. I am so excited for her.

I really appreciate the community both here and on YouTube, with all the tips to bring her book to others.

My 83 year old mother is now inspired to write another memoir about her 30+ years of being a public school teacher.

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