What are ACX standards?

ACX’s audio standards are the rules every file must meet before Audible will sell it: RMS loudness between −23 and −18 dB, peaks no higher than −3 dB, a noise floor no higher than −60 dB, MP3 at 192 kbps or higher constant bit rate and 44.1 kHz, one chapter per file, no file over 120 minutes, all files mono or all stereo, and room tone at the start and end.

ACX (Audiobook Creation Exchange) is Audible’s platform for authors, publishers and narrators. It is the self-serve way to put an audiobook on Audible, Amazon and Apple Books, and a marketplace where rights holders hire narrators.

Can I upload WAV or FLAC files to ACX?

No. ACX asks for MP3 files at 192 kbps or higher, constant bit rate, 44.1 kHz. Keep your WAV masters, but export MP3s for upload.

Should ACX files be mono or stereo?

Either, as long as the whole book is the same. ACX says mixed formats cause errors. A single voice is usually recorded and delivered in mono.

Does ACX accept AI narration?

Not unless Audible has authorised it. ACX’s requirements say the audiobook must be narrated by a human unless otherwise authorised, and that unauthorised text-to-speech, AI or automated recordings are prohibited. Audible says it is working on accepting third-party text-to-speech and will share timing later.

How long does ACX review take?

ACX’s producer checklist says its team reviews files within 10 business days of submission; its publishing page says publishing times are now much shorter and to contact ACX if a title has been in review for more than a few days. Apple Books can take up to two extra business days.

What LUFS does ACX require?

None: ACX states its loudness rule only in RMS, between −23 and −18 dB. LUFS reads higher than RMS on narration (0.5 to 3.6 dB on the recordings we measured), so a file at −20 dB RMS is usually around −19 to −17 LUFS, but there is no fixed conversion. Measure RMS for ACX.

Why was my audiobook rejected by ACX?

Most often for things a meter can’t see: outtakes and bad edits, mouth noise and plosives, levels or tone that change between chapters, a missing spoken chapter header, credits that don’t match the cover and title, or over-processing (heavy noise reduction, a noise gate cutting the room tone in and out). Missing copyright details, section titles, cover art or retail sample stop a title being approved at all.

How much does ACX cost?

ACX doesn’t list any fee to upload or sell. You pay for production, either a fee per finished hour to a producer or a share of royalties, and Audible keeps the rest of the sale price after your royalty.

Monthly, by electronic payment from Audible in USD, GBP, EUR or CAD. If you are owed less than $50 in a period, Audible can carry the payment forward to the next one.

What royalty does ACX pay?

Under Audible’s new royalty model, open to everyone since 26 May 2026, 50% for exclusive distribution and 30% for non-exclusive. Titles still on the legacy model earn 40% or 25% until the end of 2026, when the legacy model ends.

Why does my file fail the noise floor when the room sounds quiet?

The noise floor is measured in the quietest half-second, and boosting the voice to reach −20 dB RMS boosts the room noise by the same amount. A room that measured −62 dB before mastering can end at −55 dB after it. Record quieter, or use gentle noise reduction before you raise the level.

Can I use pure digital silence instead of room tone?

Avoid it. ACX asks for room tone, the sound of your room with nobody speaking, so the gaps sound natural. Pure silence makes the background drop out and come back around every sentence.

How do I take my audiobook off Audible?

You can ask ACX to remove it once it has been on sale for 90 days. If you used royalty share and the 7-year term isn’t over, the narrator or producer has to agree too.

ACX Audio Requirements Explained (and How to Pass)

The short answer

Every ACX file must have RMS loudness between −23 and −18 dB, peaks no higher than −3 dB and a noise floor no higher than −60 dB, as a 192 kbps or higher constant-bit-rate MP3 at 44.1 kHz. One chapter per file, 120 minutes at most, the chapter header spoken, all files mono or all stereo, and 1 to 5 seconds of room tone at the start and end.¹

Loudness (RMS): −23 to −18 dB
Peaks: ≤ −3 dB
Noise floor: ≤ −60 dB
Format: MP3 192k CBR
Longest file: 120 min
Room tone: 1–5 s each end

ACX CheckerDrop your chapter files to see every rule below checked, with the fix for anything that fails. Free · runs in your browser · nothing uploadedCheck my files

Every ACX requirement, in one table

These are the rules from ACX’s own audio submission page, in plain words, with what each one is for and how to fix a file that misses it.¹

Rule	ACX’s requirement	If your file misses it
Loudness	RMS between −23 and −18 dB	Raise or lower the gain; aim for −20 dB
Peaks	No higher than −3 dB	A limiter with its ceiling at −3.5 dB
Noise floor	No higher than −60 dB RMS	A quieter room, or gentle noise reduction
Format	MP3, 192 kbps or higher, constant bit rate (CBR), 44.1 kHz	Re-export with those settings
Channels	All files mono, or all stereo	Re-export the odd ones out
Room tone	1 to 5 s at the start and end; no gap over 5 s	Pad with your room’s own tone, or trim
One chapter per file	Each file one chapter or section; credits as separate files	Split the file at the chapter break
Section header	Each file starts with its header spoken (“Chapter 1”)	Record a pickup and edit it in
Length	No file longer than 120 minutes	Split it and add a spoken secondary header
File names	Standard US letters and numbers only	Rename without accents or symbols, e.g. 03_Book_Ch01.mp3
Retail sample	5 minutes or less, from the book, nothing explicit	Cut the first five minutes
Clean audio	No plosives, pops, clicks, mouth noise or outtakes	Edit out takes; clean up clicks, pops and breaths; listen through
Narrator	A human, unless Audible has authorised otherwise	No unauthorised AI or text-to-speech

The three level rules, on one scale

Loudness, peaks and noise are all measured in decibels below full scale, where 0 dB is the loudest a digital file can go and every other value is negative. Seeing them on one scale shows how they fit together: the voice sits in a narrow window around −20 dB, its loudest moments stay below −3 dB, and the room underneath has to be more than 40 dB quieter than the voice.

Where ACX wants your audio to sit: peaks, average loudness and the room underneath.Thresholds from ACX’s audio submission requirements. The dots are an example of a file that passes.

Loudness: RMS between −23 and −18 dB

RMS (root mean square) is the average level of a file, close to how loud it sounds. ACX asks for every file to land between −23 and −18 dB so listeners don’t have to change the volume between chapters.¹ Aim for the middle, around −20 dB: that leaves room either side for small differences between chapters.

A recording made with healthy headroom often measures below −23 dB. That is the right way to record: the level is raised afterwards, in mastering, with a limiter so that raising it doesn’t push the peaks over −3 dB.

LUFS or RMS? What ACX measures

ACX states its loudness rule in RMS only: between −23 and −18 dB. None of its help pages mentions LUFS.¹ LUFS, the loudness unit of EBU R 128 used by Spotify, Apple Podcasts and broadcasters, measures differently: it weights the frequencies the ear hears best and leaves out silence, so pauses don’t drag the figure down.¹⁶

On the LibriVox narrations we measured, LUFS read 0.5 to 3.6 dB higher than RMS: a narrator who leaves long pauses gets the biggest gap. So a file at ACX’s −20 dB RMS usually reads about −19 to −17 LUFS, but there is no fixed conversion. If a guide tells you to hit “−20 LUFS for ACX”, it is mixing up the two; meter RMS, as the ACX checker does. For listening outside the stores, the volume booster works in LUFS.

Peaks: no higher than −3 dB

A peak is the single loudest moment in a file: a plosive “p”, a laugh, a raised voice. ACX’s limit of −3 dB leaves headroom so nothing distorts and so its own encoding of your files works cleanly.¹ A limiter set to −3.5 dB holds every peak under the line with a small safety margin, because converting to MP3 can push a peak up by a fraction of a decibel.

Noise floor: −60 dB or lower

The noise floor is the level of everything that isn’t your voice: computer fans, traffic, the fridge, the hum of the room. ACX measures it as an RMS level and wants it at −60 dB or lower.¹ Checkers measure it in the quietest half-second of the file, which is usually the room tone at the start or end.

Noise Floor MeterMeasure your room with your microphone, live, before you record a word. Free · runs in your browser · nothing uploadedMeasure my room

Room tone: 1 to 5 seconds at the start and end

Room tone is the sound of your recording space with nobody speaking. ACX recommends 1 to 5 seconds of it at the beginning and end of every file, and says room tone spacing must not exceed 5 seconds.¹ It makes each chapter start and end naturally instead of cutting in on the first word.

One chapter file: room tone, then the narration, then room tone.Older ACX guidance, still repeated on many sites, said 0.5 to 1 second at the start. ACX’s current page says 1 to 5 seconds at both ends.

Use real room tone, recorded in the same room with the same settings, not digital silence: silence makes the background vanish and return around every gap. Our room tone guide covers recording and using it.

Format: 192 kbps CBR MP3 at 44.1 kHz

Every file must be an MP3 at 192 kbps or higher, constant bit rate (CBR), with a 44.1 kHz sample rate. ACX says you can upload 256 or 320 kbps but listeners won’t hear the difference, and that it needs CBR because it re-encodes your files into the formats Audible sells.¹ All files have to be mono, or all stereo, never a mix.

Check your export settings: a variable-bit-rate (VBR) MP3 or a 48 kHz sample rate fails even when everything else about the file is perfect.

Files, headers, credits and the sample

One chapter or section per file. Upload opening and closing credits as their own files.¹
Say the header. Each file must include its section header, such as “Prologue” or “Chapter 1”. Missing headers mean revisions and a later release.¹
120 minutes at most. Split a longer chapter and say a secondary header (“Chapter 12, part two”) at the split.¹
Opening credits state the title, author(s) and narrator(s), matching the cover and the book’s details. Closing credits must sound final; ACX suggests “You have been listening to…”, the title, author and narrator, then “The End”. Music is fine if it doesn’t cover the voice.¹
Leave out the table of contents. ACX doesn’t recommend reading it.⁶
The retail sample is 5 minutes or less, taken from the audiobook, ideally its first five minutes, with nothing explicit. Apple Books uses the first five minutes of the book whatever you choose.¹

The credits generator writes ACX-style opening and closing credits, and the sample maker cuts a sample with clean fades and checks it.

How to pass ACX the first time

The levels are the part a machine can check, and it’s where most first submissions fail. This order catches problems while they are still cheap to fix.

Check the room before you record
Measure your room’s noise with the microphone you will use. Under −60 dB with nobody speaking is the line; under −66 dB leaves room for the gain mastering adds. Noise floor meter.
Record each chapter as its own file
Say the chapter header first (“Chapter One”), leave a second or two of silence at the start and end, and keep the microphone, distance and settings the same for the whole book.
Edit out mistakes and noises
Remove retakes, mouth clicks, plosives and long gaps. Keep pauses between sentences natural and never longer than 5 seconds.
Master to ACX levels
Raise the level so RMS lands around −20 dB, limit peaks to about −3.5 dB, and pad or trim the room tone. ACX mastering does all three and exports 192 kbps CBR MP3.
Check every file
Run the whole book through an ACX checker and fix anything marked fail or review before you upload. ACX checker.
Listen to the start, a middle and the end of each file
Machines can’t hear a mispronounced name, a page turn or a chapter that sounds different from the one before it. ACX’s reviewers can.

ACX MasteringBring finished chapters to ACX levels in your browser: gain, peak limiting, gentle noise control and room tone, exported as 192 kbps CBR MP3. Free · runs in your browser · nothing uploadedMaster my files

AI narration on ACX

ACX requires a human narrator unless Audible has authorised otherwise, and prohibits unauthorised text-to-speech, AI or automated recordings. It also tells rights holders not to pay a producer they suspect of using text-to-speech, and says Audible is working to accept third-party text-to-speech from publishers and creators.¹ Other stores have their own rules: see AI audiobooks.

Why ACX sends files back

Every title goes through a quality check against the submission requirements; one that fails returns to production for fixes.⁸ ACX’s producer checklist says its team reviews files within 10 business days and returns any that miss a requirement.⁹ ACX’s own Audio Lab checks RMS, peaks, bitrate, sample rate, channels and spacing, but says editing, extraneous noise and dynamics can only be caught by its reviewers.¹⁰ What they listen for, from ACX’s help pages and its own “Why did my title fail QA?” series:

Wrong file grouping. One chapter or section per file; in 2013 ACX called this the number one cause of rejections.^1,11
Extra sounds: plosives, microphone pops, mouth noise, clicks and outtakes left in.¹
Inconsistency: levels, tone, noise, spacing or pronunciation that change through the book.¹
Over-processing. Heavy noise reduction leaves “an overly processed and poor sounding audiobook”; a badly set noise gate makes the room tone cut in and out and clips words. “Your audio should never drop to absolute zero.”^12,13
Missing or mismatched words: no spoken section header, or opening credits that don’t match the title, author and narrator on the cover and listing.¹
Long silences in the middle of a file, and files over 120 minutes.^1,14

Narration Clean-upFind mouth clicks, plosive pops, harsh “s” sounds, loud breaths and hum, and fix only where they are, with a before-and-after preview. Free · runs in your browser · nothing uploadedClean up my narration

The submission checklist

Before you press submit, check the things a meter can’t:

Copyright owner, years and name entered; cover art uploaded; retail sample chosen. Without them ACX won’t approve the title.⁸
One file per chapter, in order, none duplicated or missing; opening and closing credits as their own files.¹
File names in plain US letters and numbers, numbered so they sort in order: ACX’s example is 01_Tom Sawyer_Opening Credits.mp3.^1,14
Each file starts with its spoken header; opening credits match the cover; closing credits end with “The End”.¹
Every file passes the levels and format: RMS, peaks, noise floor, 192 kbps CBR, 44.1 kHz, all mono or all stereo. Run them through the ACX checker.
You have listened to the start, middle and end of every file.

Working in Audacity? ACX Check, a free analysis plugin listed on Audacity’s plugin site, shows the same three level figures inside the editor.¹⁵ Our Audacity guide shows how to install it.

After you submit

To sell through ACX, your book must already be published as a Kindle ebook. You claim the title, produce or commission the audio, approve it and submit it for quality review.² ACX’s reviewers check files by ear as well as by measurement, and titles that pass go on sale on Audible, Amazon and Apple Books.^2,3 ACX no longer quotes a number of days for review; it says publishing times are much shorter now and to contact it if a title has been in review for more than a few days. Apple Books can take up to two extra business days.^2,3

If a file is sent back, fix it and resubmit; the title goes back into the review queue.^3,8 Royalties are 50% for exclusive distribution and 30% for non-exclusive under Audible’s new model; titles on the legacy model earn 40% or 25% until it ends at the end of 2026.^4,5 The publishing guide compares ACX with every other route.