What to apply to meet ACX Check

I have an audio file that comes close to meeting the ACX Check
The Peak Level is -2.3 -this is my issue
The RMS is -22.6 (ok)
The NoiseFloor is -93.3 (ok)

My question is, what should I do to get the peak level to at least -3 without making the RMS an unacceptable number?

Thank you

I’m not an ACX expert but try the Limiter set to hard limit at -3dB (without make-up gain). MP3 compression can slightly-boost the peak levels so you might have to “play with” that setting.

Or since the RMS level is on the low-side, you can use a bit more limiting and then Amplify (or Normalize) to bring the peaks back-up to -3dB, while also boosting the overall RMS level.

…Your noise floor is suspiciously low. Again, I’m not an expert but “obvious” noise reduction or noise-gating might get it rejected.

As DVDDoug above, you have two problems. The combination of RMS (loudness) and Peak is out of range with each other and the Noise is too low. The only way you can get noise that quiet is to mess with it. ACX hates messing with it.

Start with the raw reading (no processing at all) and apply Audiobook Mastering 4.


It’s only three tools, but you have to download one of them. You already have ACX Check.

There’s a note about ACX Check. You can fake it out. You have to have at least 3/4 second of pure background sound (Room Tone) for it to work right. This is a little posting from a separate page.


If you don’t have access to the original raw reading, that’s mistake number three. It’s a New User mistake to patch, adjust and filter the original reading on the way to a completed audiobook posting. Instead, at the end of a raw reading, File > Export a WAV (Microsoft) 16-bit sound file. Save that somewhere safe. You should never have to read something again because of a machine or Audacity crash.

If you haven’t closed Audacity yet, you can Edit > UNDO to get back to the original reading. If you have closed Audacity, that’s the end of the world because we can’t take effects, filters or processing out later.

ACX requires all your chapters to match, so you can’t custom patch each reading.

Let us know where you get stuck.


I went back to the original file and applied the three Mastering 4 steps.

Attached is a screenshot of the new ACX Check. Everything passed.

I do have a very quiet recording room. I tested it with a Quiet Whisper app and the avg is around 38.6

Thank you very much for the help!

Quiet Whisper app and the avg is around 38.6

Which, since it’s a positive number, is probably dBSPL. I don’t speak dBSPL (Sound Pressure Level) very well, but I assume it’s really quiet.

So you win.

This can still go into the dirt. All Mastering 4 does is make you pass ACX Technical Conformance. After you pass that, you have to pass Human Quality Control. That’s where you die if you can’t read out loud or have other theatrical problems.

There is a way to submit a test to ACX.

Let us know.


I’ve found Audacity’s ACX-tool gives a bogus low-measure for noise-floor if the selected audio contains any true flat-line silence.
One second of flat-line silence added to one minute of audio lowers its noise-floor by 100dB ! …

ACX tool gives false low reading for noise floor if there is true silence.gif

ACX Check depends on good actors and a reasonable production style. That’s one reason the process of producing a forum test clip is written as it is.


You can get artificially low numbers by adding Generate Silence anywhere in the piece since all ACX Check does is look for half-second of the quietest work. That error is pretty easy to find since it produces irrational numbers. You can be even sneakier by making a chunk of rational Room Tone and dropping it into the work.

You can also get artificially noisy numbers by not having that half-second of background noise or Room Tone. ACX Check, again, will look for the quietest half-second which could be you breathing.


We’re spoiled rotten. We should remember what it looks like if you don’t use ACX Check.


That is also, as it says in that panel, a way to check how rational the ACX Check numbers really are.


If that’s the algorithm, it would be better to have it ignore any sections which contain true flat-line silence.
Some sort of average of all the quiet moments in the selected audio would be a more representative value.

If that’s the algorithm, it would be better to have it ignore any sections which contain true flat-line silence.

And where might that be? If you do that inside Audacity, the cutoff would be somewhere in the -120dB or so because of the 32-floating internal format. You can’t use -96dB (16-bit), either because Audacity adds dithering to its exports—sometimes.

Some sort of average of all the quiet moments in the selected audio would be a more representative value.

You totally can’t do that. That would wipe out 98% of the home readers. What do you use as an acceptance window? There’s a reason the measurement is bottom-biased—simple. It’s dead easy to derive (most times), common to all performers and most important, we believe it’s the way ACX measures it. It doesn’t matter how fancy-pants we measure it, the goal is to pass ACX Conformance.

ACX Check is not an ACX tool. Flynwill wrote it by integrating a number of existing Audacity tools in one place.


Koz, I’ve downloaded the De-click and de-esser links you posted. Now how do I get them into the Audacity “Add plug-ins”?

Thanks for your help!


This is the publication for Windows.


downloaded the De-click and de-esser links you posted.

Probably not me. I’m not a fan of those two because of the right-brain gymnastics you need to do to use them.

As always, Export your original performance as a WAV safety backup before you work on it. Please also note that sometimes stiff noise reduction can give boosted harshness, ticking and essing.

Trying to fix a ratty performance in post production is an adventure.