ACX check query.

Dear forum members,
I record for audible/ACX. A 20-minute clip, say, passes the ACX check in audacity as a whole, but soem, not all, individual sections of the clip, usually of passages of speaking without silent pauses in between, do not pass ACX check, mainly on noise floor values. Does ACX itself detect those sections within the chapters, and does it reject recodings as a result? Why is it that the entire clip passes, but sections do not, and what to do to avoid / repair / overcome this issue if it can affect success with Audible / ACX themselves?
Any advice most welcome!

I record for audible/ACX.

You potentially might be recording for ACX?

ACX Check doesn’t detect a thing. If you have no background noise/silence/Room Tone in the clip, ACX Check will pick the quietest thing it can find and measure that. When it does that you can get wildly high and bad noise readings.

You need a minimum of 3/4 second of clean Room Tone to give a good noise measurement. Don’t even think about Generating Silence for those areas. That process gives a crazy, unrealistic noise measurements and rings alarm bells.

ACX Requires Room Tone segments in specific places of your submissions. Read the submission requirements carefully. That means Room Tone segments are going to happen anyway, or you will get bounced.

And yes, that makes you crazy when you make it through all this and get bounced because you put one second of silence in where ACX requires two (as an example).


Which audiobook mastering process are you using?
Description? Links?


Many thanks for your response, Koz! I now better understand how the ACX check works. I have been following the audiobook mastering 4.0 advice, and 4 books were approved on that basis by ACX, but the 5th, in which I did everyhting in the same way, was not. Their comemnt was that all files failed because of too high noise floor. I am attaching one of the files in question.

I think they caught you artificially manipulating your noise levels. ACX Check measures the quietest thing it can find. That would be the area before the first word which measures -84dB. However, your real noise floor is probably at 14 seconds which, depending on exactly where you measure is -59dB. It’s audible. You don’t need a tool to hear background sound at that time. If I had to guess at it, I’d call that your computer fan noise.

ACX Check is a handy collection of other tools folded into one convenient package, but you can use other tools independently. Drag-select the “blank” area just after 14 seconds and Analyze > Contrast > Foreground > Measure Selection. That’s the Z weighing noise level—no restriction, and that noise level will not pass ACX inspection.

The area after the last word is yet a third noise number.

I call the first test that ACX does The Robot because it blindly checks technical specifications much like ACX Check does. But then the work goes to Human Quality Control and that’s where they find bad voices, compression distortion, and fake noise suppression. You can create a chapter which passes ACX Check easily and yet sounds like a bad cellphone and talking into a wine glass. Both of those will fail the second quality control pass.

Your voice is fine, you probably just need conventional Noise Reduction.

I hope you kept high quality WAV copies of your raw reading. If you did, select the fan noise or whatever that sound is and Effect > Noise Reduction > Profile. Then select the whole clip by clicking just right of the up arrow (on the left) and Effect > Noise Reduction > 6, 6, 6 > OK.

That’s not going to work if:

— You did that already. Multiple passes at noise reduction can make your voice sound funny.

— All you have is the “fake” processed work. We can’t take effects, filters or processing out of a mixed, final production.

— All you have is the MP3 you submitted to ACX. You can’t re-edit an MP3 without voice distortion increasing.

So you may be reading that work again. When you get to a chapter end or good stopping point, Export a WAV (Microsoft) 16-bit safety sound file. Then continue with the reading. If you had WAV files of the raw reading, that may have kept you from having to go through the reading again.

When you get to the end of production editing, Export another WAV and that one is your Production Master Archive. Only then do you Export the MP3 for ACX submission.

It’s good computer hygiene to keep your backup and safety WAV files in two separate places. Thumb drives, portable hard drives and even cloud storage in addition to your computer internal hard drive. I have an extreme case where I had two separate internal hard drives and copied valuable files to both. The system drive failed. I pulled the second drive out of the machine and put it into an external cabinet, connected it to a second computer and kept on working.


You may not need the Noise Reduction step if you get rid of the computer fan noise. It is possible to read for ACX with only Mastering 4. I have a good quiet room with no echoes and I can produce a good reading at any time with just Low Rolloff, RMS Normalize and Limiter.

I’m calling that sound computer fan noise. It could be your refrigerator or air conditioner. It’s up to you to find it and make it quieter. Do Not restrict the cooling air flow to a computer.


Thank you so much, Koz, for these very detailed and easy to understand explanations and advice. Very useful as I am new to this!