Mastering Audiobooks for Audible / ACX

by Andrew Hughes – 17th January, 2023

Why the requirements?

In short, Audible and Amazon want their customers to enjoy listening to quality audiobooks. Therefore they have put certain technical standards in place for audiobook submissions. No-one wants to hear an audiobook with lots of clicks, pops and hiss all over the recording.

Imagine how frustrating it would be if each chapter was a different volume, or the last few seconds got cut off. Nobody would want to listen to a book where the volume suddenly changed, making the poor listener jump out of their skin!

Where can I read the requirements?

You can find the latest submission requirements here: https://www.acx.com/help/acx-audio-submission-requirements/

Don’t worry about the jargon. I’ll explain each term in turn.

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Your audiobook must:

• Be consistent in overall sound and formatting.



Nobody wants to listen to a book where the narrator ready slowly in one chapter and speeds up with no reason in the next one. You must make sure your book has a consistent sound.



• Include opening and closing credits.



Opening credits need to include the name of the audiobook, the author and the narrator’s name in the most basic form. Two words are all that are required for closing credits: “The End”. Although most narrators also recap the details above from the opening credits. Obviously, the credits need to match the details on the book’s cover art!



• Be comprised of all mono or all stereo files.



For simplicity we usually provide all-mono files. All-stereo files is a perfectly acceptable option, but your book will be rejected if you mix and match.



• Include a retail audio sample that is between one and five minutes long.



This is your showcase for the book. Think of it as your shop window! You should pick a section between 60 seconds and 5 minutes in length that will grab the listener and make them want to buy the audiobook. Alongside the book’s image on the Amazon website will be an “audible sample” button. Potential listeners can press this before making their purchasing choice.

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Each uploaded audio file must:

• Contain only one chapter/section per file, with the section header read aloud.



Don’t go including three chapters in one file and the split the next chapter over two files (unless the files are very long – see below). The section header part of this instruction means that you should read an appropriate header at the start of each section. This could be “Prologue”, “Chapter 7”, Epilogue etc.



• Have a running time no longer than 120 minutes.



If a chapter is over two hours in length, you’ll need to ignore the previous instructions and split it over two files.



• Have room tone at the beginning and end and be free of extraneous sounds.



Room tone is the natural sound the studio you are recording in, which must be present throughout the recording but not too loud! Basically it’s silence, without being too silent or too loud! Don’t be tempted to strip all silences out on editing software – this may lead to files being rejected.




We generally leave 0.5 to 1 second of room tone at the start of each file and 1 to 5 seconds of room tone at the end of each file. The reason for this requirement is that some people’s audio players take a fraction of a second to spring into life and this additional silence avoids words being clipped at the start and end of chapters.




Extraneous sounds refer to clicks, pops, rustles and belches! Anything that spoils the listening experience.



• Measure between -23dB and -18dB RMS and have -3dB peak values and a maximum -60dB noise floor.



This is where is starts to sound more technical and you need to understand a bit about volume levels. Imagine a room, where the ceiling is labelled zero. You can’t get any higher than zero, because it’s the ceiling. The walls are measured downwards from zero, in minus numbers. There are markings down the walls, at -10, -20, -30. This is a good analogy for audio file volumes. Let’s break the requirements down into sections:



• Measure between -23dB and -18dB RMS



This means that the average volume level must be between two particular levels. Basically “not too loud” and “not too quiet”



• Have -3dB peak values



Peak values are when the narrator at their loudest. You can’t record an audio file louder than zero dB – it’s the ceiling. -3dB is a tiny bit below this ceiling and ACX doesn’t want ANY part of the audio recording to go above this level.



• Have a maximum -60dB noise floor



If you sit incredibly quietly and listen, eventually you’ll hear tiny noises. They could be your central heating, your tummy or perhaps the very faint sound of traffic and we call this the “noise floor”. It’s the faint background noise in any environment. Audible wants to hear a very faint noise floor to make the narration sound natural, so don’t be tempted to use any “strip silence” plugins as your files are likely to be rejected.
In layman’s terms, “maximum -60dB noise floor” is VERY quiet indeed, so you’re probably going to need to consider using a professional studio. You don’t want background noises spoiling the listener’s enjoyment of your audiobook.



• Be a 192kbps or higher MP3, Constant Bit Rate (CBR) at 44.1 kHz



You’ll need to submit your audiobook as MP3 files. These need to be of adequate quality otherwise they won’t be much fun to listen too. The numbers above tell you the minimum quality required (192 kbps) as well as a couple of other quality parameters that you’ll need to stick to.



• Your submitted audiobook must be narrated by a human.



Text-to-speech software is pretty good these days, but it’s not good enough for an audiobook; you need to use a real person to read it!

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How do I make sure I comply with all these requirements?

There’s quite a lot to think about and it’s very easy to fall foul of the submission process; this can cause significant delays to getting an audiobook online and ready to sell. There are a couple of tools you can use to check your files before you submit them:

• 1. ACX Check for Audacity.



Audacity is a free audio editor which you can download onto your PC. Once you’ve done that, you can then download and install Nyquist’s “ACX Check” plugin. This is predominantly a check of the various volume levels. Since these are the most common reasons for a file being rejected, this is a valuable audiobook mastering tool and can flag up the majority of issues.



• 2. 2nd Opinion



If you want to perform a few more checks or if you can’t work out the reason why Audible have rejected a file, then we recommend the 2nd Opinion app from Steven Jay Cohen: https://www.stevenjaycohen.com/2ndopinion




This looks at a few more audiobook mastering parameters, including:


– Silences at start and finish

– The length of the retail sample

– The tech specs of the MP3 files




In addition volume level checks carried out by Audacity’s ACX check

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If it all sounds a bit complicated

You could always get a studio to master your audio for you. You need to have decent quality audio in the first place – a studio isn’t going to be able to remove a significant amount of background traffic noise for example. To give you a rough idea of cost we recently mastered an 11-hour audiobook for Audible in 6 hours at a cost of £540+VAT. We checked through the majority of the edits, ensured a consistent sound and then ran the audio through the checking tools above and the book was submitted successfully first time.

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Find out more about our audiobook mastering service: https://www.radiofacilities.com/audiobook-studio/