What Are ACX Audio Requirements?
Quick Answer: ACX requires MP3 files at 192 kbps constant bit rate, 44.1 kHz sample rate, with RMS loudness between -23 and -18 dBFS, peak levels at or below -3 dBFS, and a noise floor at or below -60 dBFS. Each chapter must be a separate file. Files that fail any of these specs are automatically rejected.
These five specifications are the technical gate every audiobook must pass before a human reviewer at ACX listens to the narration. Meeting them is non-negotiable.
The Specifications
| Spec | Requirement | What it means |
|---|---|---|
| Format | MP3, constant bit rate (CBR) | Standard compressed audio format. Must be CBR, not variable bit rate. |
| Bit rate | 192 kbps or higher | Controls audio quality. 192 kbps is the minimum for acceptable fidelity. |
| Sample rate | 44.1 kHz | CD-quality audio standard. |
| RMS loudness | -23 to -18 dBFS | Average volume level. Outside this range and the file is rejected. |
| Peak level | At or below -3 dBFS | Loudest point in the file. Prevents distortion. |
| Noise floor | At or below -60 dBFS | Background noise level. Must be essentially inaudible. |
File Structure
Each chapter is a separate MP3 file. Opening credits (title, author, narrator) and closing credits are separate files. Each file should begin and end with 0.5 to 1 second of room tone. A retail audio sample (typically the first 1 to 5 minutes of narration) is submitted separately.
How to Check Your Files
ACX Check plug-in for Audacity (free). Analyzes your audio file and reports whether it meets each ACX specification. Available through Audacity's plug-in manager.
Auphonic (online tool). Automatically masters audio to meet ACX specs. The free tier processes 2 hours per month; paid plans start at $11 per month.
Manual check in any DAW. Any digital audio workstation (Audacity, Adobe Audition, GarageBand) can display RMS, peak, and noise floor measurements.
Bottom Line
Meet these five specs and structure your files correctly, and your audiobook will pass ACX's automated quality check. The most common rejection reason is noise floor (background noise too high), which is a recording environment problem. The second most common is inconsistent RMS across chapters, which is a mastering problem. Both are fixable before submission with the right tools. For the full technical breakdown, see Audiobook Formatting and ACX Standards.
Q: What happens if your audiobook fails ACX review? A: ACX sends a notification identifying the specific technical failure. You fix the affected files and resubmit. The review process starts over, adding one to two weeks to your timeline. Common failures are noise floor, RMS levels, and incorrect file format.
Q: Do ACX requirements apply to AI-narrated audiobooks? A: Yes. The same technical specifications apply regardless of narration method. As of 2026, AI-narrated audiobooks must also be submitted through a separate process (contact ACX support directly) rather than the standard upload flow.