Files pass ACX Check in Audacity - but the mp3 files that I output don't

I have been working really hard to make sure that I am punch copy/pasting room noise to replace breaths and mouth noises… while on the timeline, these files are passing the ACX Check.

Then I export them as mp3 files and bring them back into Audacity, run the ACX Check again, and the mp3 files don’t pass…

Saving as mp3
Bit Rate Mode - constant
Quality - 192 kbps
Channel mode - stereo (although the files are actually mono)

Any idea where I am going wrong?

What’s failing?

MP3 can end-up boosting your peaks so you may need to use a lower limiter setting (the normal recommendation is to set the limiter to -3.5dB).

Or if your RMS levels have some room to play with you can reduce the volume by 1dB or so.

How are you getting there? Are you using Audacity Audiobook Mastering Macro? That was specifically designed to avoid many of these problems.

I know ACX says you can pass with a noise of -60dB, but in real life, you have to make it by -65dB or more.

There is a forum posting with instructions for getting the tools and installing them.

As we go.


You Export your chapters in Perfect Quality WAV format as safety backups, right? Then, after everything settles, apply Mastering and export as 192 Constant Quality MP3.

Something happened. If you have a mono chapter and export it as an MP3, it should come back as a mono sound track.


There is a formal entry for this.

Post the ACX-Check panel and its readings. It’s possible to have submitted noise too quiet.


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Does your Edit Master sound file say “Mono” on the left-hand panel?

Screen Shot 2023-08-09 at 8.10.30 PM


Yes, is says mono and 44100 hz & 32-bit float…

It’s the noise floor… says it’s too low.

When I am doing the ACX Check on the edited file it says it is 76.52 dB but when I bring the mp3 file I created back in… it’s -136.67 (too low - dead silent sounds unnatural).

Here is the original (edited) file:

And this is what it looks like when I bring the same file back in:

It does come back as mono. That wasn’t the issue. I was just saying I left it in STEREO for the export process (which shouldn’t make a difference).

I am going to go through the process again from the start using the Audiobook-Mastering-Macro you suggested.

I’m not expecting that much difference. That looks like a completely different file. That could be just a mistake. I know I never make mistakes like that…[harumph]

Since you’re in Windows, did you set Windows Preferences to show you file name extensions?


Quick, which one is the WAV file? This is what that looks like on my machine.


You can get away with hiding filename extensions and making everything look cool and all if everything goes perfectly according to plan, but the first time you have trouble, it’s good to know exactly what’s there.

We are in the midst of changing the forum around. I’m discovering changes and additions as I go and the last couple of postings have been after the coffee wore off.

Audacity Audiobook-Mastering-Macro is quite a project and it’s due a master class. The goal is one step that takes whatever you have and forces it into ACX compliance. There should be no audible change to the work other than volume. If you produced the voice track in a quiet, echo-free room (and you don’t make the mouth noises I do) you may be done.

As we go.


The conversion between stereo (two channels) and mono can cause serious problems in addition to taking up too much storage space and being difficult to edit. That’s not recommended. Announce and produce the work in one-track mono and carry that through the whole job.

One track mono is recommended by ACX. They say if you insist on doing the work stereo, the whole thing has to be stereo. There is no batting back and forth between stereo and mono.

That immediately brings up the difficulties in recording your voice in mono. That’s not for the easily frightened. My stand-alone voice recorder naturally produces stereo. I let it and mix it down to mono in Audacity before doing anything else.

It’s important to do these tricks before Mastering.


There is something very strange going-on here and I’m pretty sure it’s not related to MP3.

Is your “original file” a mono WAV file?

You can easily peak & RMS by reducing 2dB (the Amplify effect at -2dB). That will make your peaks -4.33dB and RMS -18.99.

But the big issue is the noise floor… I don’t know where that (nearly) dead silence is coming from and lowering the volume by 2dB will only make it 2dB worse…

There’s a trick to ACX-Check. It measures noise by plowing through the whole production and picking the quietest part to measure. In plain Room Tone sense, that’s the one place where you held your breath for a second and whatever is there represents, well, the room (plus microphone noise, computer fan noise, etc. etc. etc).

However, if you Generate > Silence anywhere in the production, that will be seen as the quietest part of the show and measured. That’s unnatural and will totally ring bells and be rejected.

Why would they reject complete and total silence? Because that means you messed with the sound and they hate effects, filters, and corrections. Each effect represents an opportunity to distort the sound and be distracting. They hate distractions. That’s one reason Audiobook Mastering can’t be heard while it’s working.

Everything is hooked to everything else.

One other note. ACX has their own, on-line measurement method called ACX Audiolab.

It measures the same things as ACX-Check (designed by Audacity), but critically, does not measure noise. Noise is hardest to manage and the most likely to cause you to fail.

It’s not necessary to have a blue denim jacket and no hair.


You measured the wrong file.


There’s another branch of this.

Say you actually passed ACX, but with a reeeely close noise reading, -62dB, for example. -60dB is the limit. You may elect to apply Noise Reduction. Drag-select a portion of the show you expect to be well-behaved background noise (not that unnaturally quiet thing).

Effect > Noise Reduction > Profile (closes automatically).

Select the whole chapter and Effect > Noise Reduction > 6, 6, 6 (Noise Reduction of the Beast) > OK

ACX-Check should give new noise of -68dB which is stable and easily passes—and you can’t hear it working or tell what you did.

You can cheat a bit and use 8, 6, 6 or even 9, 6, 6, but any reduction heavier than that is audible. Fix your studio.


Noise is not “anything I don’t like.” Noise Reduction only works on most sounds that don’t change such as computer fan noise, air conditioning sounds, microphone electronic noises, or spring rain in the trees (fffffff). It is not the party next door, the delivery truck going by, the TV game show in the next room, or the jet going over. If you have those, they can make you record the chapter over again.

There is one famous noise that takes special consideration. There is a USB connection defect that can cause Frying Mosquitoes sound in the show.

That has the same characteristics as Baby Screaming on a Jet. It doesn’t conform to the -60dB noise limit and there are special tools to get rid of it.


OK. Fresh coffee and a stab at this process.

I opened up a clear voice performance in 3,3,3 which does not pass ACX-Check.



I resampled it to 44100 and applied Audiobook Mastering.



I exported a 192 Constant Quality MP3, opened it, and analyzed it.



Perfect, right? Note all the measurements are approximate, but within spittin’-distance of the originals, and it passes.

Except for the sample rate which seems to have snapped back to 48000. One would wonder why it did that.


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