ACX audio specs explained for 2025 and 2026

You finish a 100,000-word book, open ACX to upload the audiobook, and run straight into RMS, peak amplitude, noise floor and constant bit rate. None of that was part of writing the book, but every file has to pass these checks before Audible will sell it.

This guide explains what the ACX specifications really mean, why files fail them, and how to check every chapter before you submit.

To check a finished file against these numbers, run it through the free ACX compliance checker . It reports RMS, peak level, noise floor, bitrate and sample rate.

TL;DR

• ACX wants MP3 files at 192 kbps or higher, constant bit rate, 44.1 kHz.
• RMS must sit between -23 dB and -18 dB, peaks no higher than -3 dB, and the noise floor below -60 dB.
• Each chapter goes in its own file, no longer than 120 minutes, with room tone at the start and end.
• Most rejections come from room noise, uneven levels and over-processing, not from the performance.
• ACX requires human narration unless otherwise authorized. AI-narrated audiobooks go to other stores.

The ACX audio specs, one by one

1. File format

• MP3 only, at 192 kbps or higher
• Constant bit rate (CBR), not variable
• 44.1 kHz sample rate
• Mono or stereo, but the same for every file. Mono is the usual choice for narration.

Exporting a 128 kbps file or a variable bit rate MP3 is one of the quickest ways to get a file sent back.

2. RMS level (average loudness)

RMS measures the average loudness of a file. ACX wants every file between -23 dB and -18 dB RMS. Too quiet and listeners keep turning the volume up. Too loud and the voice sounds squashed. Keeping every chapter in the same range also stops volume jumps between chapters.

3. Peak level

The loudest moment in a file must not go above -3 dB. This leaves headroom and prevents clipping, the harsh distortion you hear when audio goes past 0 dB.

4. Noise floor

The noise floor is the level of the background sound when nobody is speaking: hiss, hum, fans and traffic. ACX wants it below -60 dB. This is the spec home recordings fail most often.

5. Room tone

Each file needs a short stretch of room tone, the natural quiet of your recording space, at the start and the end. ACX allows no more than 5 seconds. Use real room tone rather than digital silence, so the file doesn't sound like it cuts in and out.

6. One chapter per file

Each chapter or section goes in its own file, and no file can run longer than 120 minutes. Split very long chapters at a natural break.

7. Opening and closing credits

Opening and closing credits are uploaded as their own files. The opening credits name the title, the author and the narrator. The closing credits mark the end of the book.

8. Retail sample

The retail sample is the preview listeners hear on the store page. It can be up to five minutes long and must not contain explicit content.

Why these specs matter

The numbers protect the listener. A book that jumps in volume between chapters, hisses under every pause or distorts on loud lines is tiring to listen to, and listeners leave reviews about it. ACX checks the numbers so every title on Audible meets the same baseline.

Why files fail ACX checks

• Room noise: air conditioning, fridges and computer fans push the noise floor above -60 dB.
• Uneven levels: chapters recorded on different days, or at different mic distances, land at different RMS levels.
• Clipping: peaks above -3 dB, often from a loud line or a laugh.
• Over-processing: heavy noise reduction or compression used to hit the numbers can make the voice sound thin.
• Wrong export settings: variable bit rate, 128 kbps, or 48 kHz files.
• Missing pieces: no room tone, no credits, or a chapter split across files.

Our guide to why ACX rejects audiobook files covers each rejection message and how to fix it.

How to check your files before you upload

1. Master every chapter with the same settings. Our audiobook mastering guide explains a simple chain for RMS, peak and noise floor.
2. Export with the right settings: MP3, 192 kbps CBR, 44.1 kHz.
3. Run every file through the ACX compliance checker , not just the first chapter.
4. Listen on more than one device. Headphones reveal noise and clicks. A phone speaker shows whether the voice is clear at low volume.
5. Check the pieces: credits files, room tone at both ends, and a retail sample under five minutes.

If you're recording at home, how to pass ACX audio requirements without a studio walks through a setup that meets these numbers.

Where AI narration fits

ACX's rules say audiobooks must be narrated by a human unless otherwise authorized, and they prohibit unauthorized text-to-speech, AI and automated recordings. Meeting the audio specs doesn't change that: an AI-narrated file that hits every number still can't go through a standard ACX upload.

Audible does sell some AI-narrated titles, but only through Amazon's own programs: KDP Virtual Voice, an invite-only beta where Amazon generates the audio, and ACX Voice Replica, a limited beta for human narrators.

Other stores accept AI narration you produce yourself. Spotify for Authors, Google Play Books and Kobo Writing Life all take it, with a disclosure label. Each store has its own file specs. Our AI audiobook distribution map lists them side by side, and this guide to AI narration on Audible, ACX and Findaway explains each route.

If you're making an AI-narrated edition for those stores, Narration Box's Audiobook Creator turns an EPUB, PDF or Word manuscript into chapters you can narrate, edit and export one at a time.

Quick tips

• Record or produce one test chapter and check it before you do the whole book.
• Keep chapter openings short: the chapter title, a beat of room tone, then the text.
• Choose a retail sample that starts with the story, not the front matter.
• Re-check files after any edit. Small changes can push peaks or RMS out of range.

FAQs

What are the ACX audio requirements?

MP3 at 192 kbps or higher, constant bit rate, 44.1 kHz. RMS between -23 dB and -18 dB, peaks no higher than -3 dB, and a noise floor below -60 dB. One chapter per file, no longer than 120 minutes, with room tone at the start and end.

Can I use AI to make an audiobook for ACX?

Not through a standard ACX upload. ACX requires human narration unless otherwise authorized. You can publish an AI-narrated audiobook on Spotify for Authors, Google Play Books or Kobo Writing Life with a disclosure.

Can I narrate my own audiobook on ACX?

Yes, if you record it yourself. A clone of your own voice made with a third-party tool counts as AI narration, which ACX doesn't accept unless authorized.

How do I become a narrator for Audible books?

Create a narrator profile on ACX and audition for titles that rights holders post there.

How can I check if my files meet ACX specs?

Upload each file to the free ACX compliance checker . It reports RMS, peak level, noise floor, bitrate and sample rate.