ACX Audio Requirements: The Complete Checklist for Authors and Narrators

ACX (the Audiobook Creation Exchange, Audible’s production and distribution platform) checks every audiobook against a set of technical audio requirements before it goes on sale. If even one chapter misses a requirement, the whole book can be sent back. This checklist explains each ACX requirement in plain English, how to check it, and how to fix it.

ACX audio requirements at a glance

Requirement	ACX standard	What it means
RMS loudness	Between −23 dB and −18 dB RMS	The average loudness of each file
Peak level	No higher than −3 dB	The loudest single moment in each file
Noise floor	No higher than −60 dB RMS	Background noise when nobody is speaking
File format	MP3, 192 kbps or higher, CBR, 44.1 kHz	The export settings for every file
Room tone	0.5–1 s at the head, 1–5 s at the tail	Short silence at the start and end of each file
Channel format	All mono or all stereo	Every file must match
File length	Under 120 minutes per file	Split long chapters into parts
File content	One chapter or section per file	Plus opening and closing credits as separate files
Retail sample	1–5 minutes, no explicit content	The preview listeners hear on the store

Source: ACX’s published audio submission requirements. ACX also does a human listen for consistency and quality, so a file can pass the numbers and still be flagged.

1. RMS: −23 dB to −18 dB

RMS is a measure of average loudness. ACX wants every file in a narrow window so listeners don’t have to adjust the volume between chapters. Most home recordings come out too quiet (around −26 dB). Simply turning the volume up also raises the background noise, which is why RMS and noise floor have to be handled together.

How to fix it: use gentle compression and a final normalization or loudness step aimed at about −20 dB RMS. That leaves safe room on both sides of the window.

2. Peaks: no higher than −3 dB

Peaks are the loudest instants, such as plosives (“p” and “b” sounds) or a sudden laugh. A limiter set to a ceiling of −3.5 dB keeps every file under the limit without audible distortion.

3. Noise floor: −60 dB RMS or lower

The noise floor is the hiss, hum and room sound between sentences. It is the most common reason ACX rejects self-recorded audiobooks. Computer fans, air conditioning, fridges and traffic all add up.

• Record in the quietest room you have, with soft furnishings around you.
• Turn off fans, AC and notifications while recording.
• Keep a good distance between the microphone and your computer.
• Use light, careful noise reduction — heavy settings make voices sound underwater.

4. Room tone at the head and tail

Each file needs 0.5 to 1 second of room tone (natural silence, not digital silence) at the start, and 1 to 5 seconds at the end. Record 30 seconds of room tone at the start of every session so your editor has clean material to use.

5. File format, length and naming

• Export as MP3, 192 kbps or higher, constant bit rate (CBR), 44.1 kHz.
• Keep each file under 120 minutes.
• One chapter or section per file, with opening and closing credits as their own files.
• Use the same mono or stereo setting for every file.

6. The human quality check

Beyond the numbers, ACX listens for consistent sound between chapters, missing or repeated words, background noises, and edits that cut off breaths or words. This is where careful audiobook editing and a separate proof listen pay off.

Common reasons ACX rejects audiobooks

• Noise floor above −60 dB
• RMS outside −23 to −18 dB
• Peaks clipping above −3 dB
• Missing or uneven room tone
• Inconsistent levels or tone between chapters
• Wrong file format or bit rate
• Opening or closing credits missing

How to check your files

Free tools like the ACX Check plugin in Audacity measure peak, RMS and noise floor for each file. Run every chapter through it before upload, not just the first one — levels often drift between recording sessions.