ACX audiobook mastering
Audiobook Mastering That Meets ACX Requirements
Got an ACX rejection or files that sound uneven from chapter to chapter? We master every file to Audible’s RMS, peak and noise-floor specs and deliver retail-ready MP3s for every platform.
ACX specs
Every File Checked Against ACX Audio Requirements
ACX runs automated and human checks on every audiobook. A single chapter outside spec can delay your release. We measure each file before delivery and send you a compliance report.
These requirements come from ACX’s published audio submission requirements.
| Requirement | ACX standard |
|---|---|
| RMS loudness | −23 dB to −18 dB |
| Peak level | −3 dB maximum |
| Noise floor | −60 dB RMS maximum |
| Format | MP3, 192 kbps+, CBR, 44.1 kHz |
| Room tone | 0.5–1 s head, 1–5 s tail |
| Consistency | Same channel format (mono or stereo) across all files |
| File length | Under 120 minutes per file |
What’s included
What Our Audiobook Mastering Includes
Loudness & peak control
Transparent compression and limiting to hit ACX RMS and peak targets while keeping your natural dynamics.
Noise floor & tone
Gentle noise reduction and EQ for clarity and warmth, with consistent room tone across every chapter.
Files & metadata
Head and tail silence, chapter naming, ID3 tags, opening and closing credits, and a 1–5 minute retail sample.
ACX rejected your files?
We Fix ACX Rejections Fast
The most common ACX rejection reasons are noise floor above −60 dB, inconsistent levels between chapters, missing room tone and clipped peaks. Send us the rejection email and your files — we’ll diagnose the issue on a free sample and re-master the book. See our guide to common ACX rejection reasons.
- Noise floor and hiss problems
- RMS too low or too high
- Peaks over −3 dB
- Missing or uneven room tone
- Wrong file format or bit rate
Pricing
Audiobook Mastering Price
Mastering-only is for files that are already edited. If your audio still has retakes or noises, choose Edit + Master instead.
| Package | Includes | Rate |
|---|---|---|
| ACX Mastering | Mastering to spec, room tone, retail sample, compliance report | $35 / PFH |
| Edit + Master | Full editing plus everything in mastering | $75 / PFH |
FAQ
Audiobook Mastering FAQs
What is audiobook mastering?
Mastering is the final audio processing stage. It sets consistent loudness, controls peaks, lowers the noise floor and formats each file so the audiobook meets platform requirements and sounds even from start to finish.
Is ACX mastering the same for Apple Books, Spotify and Findaway?
Mostly. ACX’s specs are the strictest common standard, so files mastered for ACX are accepted by Apple Books, Spotify for Authors, Google Play Books, Kobo and Voices by INaudio. We adjust format on request.
Can mastering fix a noisy recording?
Mastering can lower moderate background noise, but heavy noise, echo or distortion needs editing and restoration first. We tell you honestly on the free sample what can be fixed.
Do I need mono or stereo files?
Either is fine for ACX as long as every file uses the same format. Most spoken-word audiobooks are delivered in mono.
Related Services
Audiobook Editing
Retakes, clicks, breaths and pacing.
Audiobook Proofing
Word-for-word checks against your script.
Pricing
Per-finished-hour packages and add-ons.
Get Your Audiobook ACX-Ready
Send a chapter or a five-minute sample. We’ll master it to spec and send back a compliance report with your quote.