ACX is the platform authors use to get an audiobook onto Audible, and searches for it have climbed sharply this year. Before any book goes on sale, ACX checks every file against a short list of technical rules. Most of them are easy to meet with a decent microphone and some patience. One, the noise floor, is where home recordings often come unstuck, and it is the best reason there is to record in a treated room.
What ACX actually checks
These are from ACX's submission requirements, last updated on 15 April 2026.
- Loudness: every file between -23 dB and -18 dB RMS.
- Peaks: below -3 dB.
- Noise floor: below -60 dB RMS, which is measured in the quiet between words.
- Format: MP3 at 192 kbps or higher, constant bitrate, 44.1 kHz. All files mono or all files stereo, never a mix.
- Structure: one chapter or section per file, each opening with a spoken header such as "Chapter One", and no file longer than 120 minutes.
- Room tone: ACX recommends one to five seconds at the head and tail of every file. Older guides say half a second to a second; that is no longer the wording.
- Credits: an opening credit giving the title, author and narrator, and a closing credit that signals the end, ideally "You have been listening to…" followed by the title, author, narrator and "The End".
- Retail sample: up to five minutes, preferably from near the start, with nothing explicit.
- Cleanliness: no plosives, mouth noise, mouse clicks or outtakes, and sound, level, tone and spacing that match from the first file to the last.
Two more rules sit alongside the audio. ACX says a book "must be narrated by a human unless otherwise authorized". And UK authors are welcome: accounts are open to residents of the US, UK, Canada and Ireland with a local mailing address, tax ID and bank account.
From word count to hours of work
ACX's own figure is about 9,300 words per finished hour. Divide your word count by 9,300 and you have the running time. Our finished-hour maths guide plans on a rounder 9,000, which errs on the long side, and the first session tells you which is closer for your voice.
Then comes the work. ACX estimates about 6.2 hours of effort for every finished hour: roughly two to record, three to edit and 1.2 to review. It also says new narrators often take five to seven hours just to record one finished hour.
Say your book is 75,000 words.
- 75,000 divided by 9,300 is just over eight finished hours.
- At 6.2 hours each, that is about fifty hours of work before anything is uploaded.
- Around sixteen of those hours are at the microphone. The other thirty-four are editing and listening back.
The third line is the one that surprises people. Recording is the visible part. The edit and the proof-listen, where someone follows the manuscript word by word and marks every misread, are most of the job.
Why the noise floor is the hard one at home
-60 dB RMS is a demanding number. With the voice averaging around -20 dB, the gaps between words have to sit about 40 dB below it, and most rooms people think of as silent are not that quiet. A fridge two doors away, a boiler, a laptop fan, tyres on a wet road: you stop hearing them within minutes, and the microphone never does.
A noisy recording can be pulled under the line with noise reduction, and plenty of people try. The cost is what it does to the voice. Heavy processing leaves a thin, swirling edge on every word, and it has to be applied identically across thirty or forty files recorded over several weeks, because ACX wants tone and noise to match all the way through. A spare room with the heating on in January does not sound like the same spare room in April.
So the cheapest place to fix the noise floor is at source, in a room quiet enough that the gaps between words are already under the line before anyone touches them.
What the room and the engineer change
Our audiobook studio is the treated room in the barn: 4.5 metres square, double-insulated and foam-tiled throughout, with Shure broadcast microphones running through a Dante-networked digital desk into the DAW.
An engineer runs every session, and for ACX that matters more than the microphone. Several things on ACX's list are ones a narrator cannot hear while reading: a popped P, a click of the tongue, a chair creak, a level that has crept up over three hours. The engineer stops the take and you go again from the start of the sentence, while the moment is still warm, instead of finding the problem in the edit a fortnight later.
Record the opening and closing credits in the same sessions as the book, so they sound like the same voice in the same room. After the sessions come the edit, a proof-listen against the manuscript, pickups for anything misread, and a master to ACX's numbers, split one file per chapter and ready to upload.
Whole books are quoted as one price from the word count, with the engineer, edit and master included, so a slow afternoon never costs you more. If a co-author or producer wants to listen in from elsewhere, sessions run over Source-Connect Pro, or over Cleanfeed, Riverside or Zoom, with your voice recorded locally at full quality.
One date for titles already on ACX
Audible's new royalty model, open to everyone since 26 May 2026, pays 50% on exclusive titles and 30% on non-exclusive ones, and the legacy model ends on 31 December 2026. What to check title by title is in our note on the Audible royalty change. And if you are still deciding who should read the book at all, author or professional narrator works through that question.
If you have a word count, send it over and we will turn it into a schedule and a single price.
Sources
- ACX, What are the ACX Audio Submission Requirements?, updated 15 April 2026
- ACX, Create an ACX profile, 28 October 2025
- ACX, Narrate an audiobook, 19 August 2025
- ACX, Audible's new royalty model, 28 April 2026, updated 26 May 2026
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The picture at the top of this article is an illustration generated to show the set-up. It is not a photograph of a client or of a real job.
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Questions we get asked
What are ACX's audio requirements?
Each file must measure between -23 dB and -18 dB RMS, with peaks below -3 dB and a noise floor below -60 dB RMS, delivered as a 192 kbps or higher constant bitrate 44.1 kHz MP3, one chapter per file and no more than 120 minutes long.
Can UK authors publish audiobooks through ACX?
Yes. ACX accounts are open to residents of the US, UK, Canada and Ireland who have a local mailing address, tax ID and bank account, and that applies to narrators and studios as well as authors.
How long does it take to produce an audiobook for ACX?
ACX works on about 9,300 words per finished hour and about 6.2 hours of work per finished hour. A 75,000-word book is roughly eight finished hours and fifty hours of recording, editing and review.
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