Audiobook ACX Check & Master
Get audiobook chapters ready for ACX: levels, room tone and MP3 format.
ACX specs: RMS, peak, noise floor, room tone · 192 kbps MP3
What it does
Give it your chapter files and get back ACX-ready MP3s, one per chapter, each measured against ACX's published audio specs. Every chapter comes with a before-and-after report showing which specs it failed, what was changed and that the finished file passes. If a recording can't be fixed automatically, such as one with a noisy room or electrical hum, it tells you so instead of passing it. It fixes levels, room tone and MP3 format so you don't have to.
What you get
- An MP3 for every chapter at 192 kbps CBR, 44.1 kHz, with the same channel format across the whole book, as ACX requires.
- Volume set to ACX's range (RMS −23 to −18 dB) and peaks held below −3 dB, with no clipping added.
- Room tone at the start and end of each file trimmed or topped up to 1–5 seconds using your own room's sound, never digital silence.
- Gentle noise reduction when the background is close to the limit, kept only if the measured noise actually drops.
- A pass/fail report for every chapter on all 7 checks, with the numbers before and after and a plain reason for anything that still fails.
Example: one chapter before and after
| Check | ACX spec | Before | After |
|---|---|---|---|
| Loudness (RMS) | −23 to −18 dB | −24.7 dB ✗ | −20.7 dB ✓ |
| Peak | at most −3 dB | +0.5 dB ✗ | −3.8 dB ✓ |
| Noise floor | at most −60 dB | −59.5 dB ✗ | −66.9 dB ✓ |
| Room tone, start | 1–5 s | 0 s ✗ | 2.5 s ✓ |
| Room tone, end | 1–5 s | 3.7 s ✓ | 2.5 s ✓ |
| Length | at most 120 min | 13.4 min ✓ | 13.4 min ✓ |
| Format | MP3, 192 kbps CBR, 44.1 kHz | 64 kbps, 24 kHz ✗ | 192 kbps, 44.1 kHz ✓ |
Best for
- Narrated books headed to ACX or Audible, recorded reasonably cleanly, that need their levels, room tone and file format sorted out.
- Checking a whole book before upload, so a rejection doesn't cost you a week.
Not for
- Fixing a bad recording. It won't remove hum, echo, mouth clicks, plosives or mistakes, and it can't undo distortion. It tells you when the numbers still fail.
- Guaranteeing approval. ACX also listens to every book for performance and consistency; a pass here covers the published technical specs only.
Cost and how to start
- Open it on Apify Store and sign in, or create a free Apify account.
- Give it direct download links to your chapter files (up to 1 GB and 120 minutes each) and start the run.
- Download the mastered MP3s and the report for each chapter.
US$0.02 per audio minute mastered. Minutes are rounded up to whole minutes. Charged for audio measured and mastered; files that cannot be loaded or measured are not charged. Each run also has a start charge of US$0.00005 per GB of run memory, minimum one GB. There is no separate platform usage charge.
Questions
- Will ACX accept my book if every chapter passes here?
- Not guaranteed. ACX checks the same technical specs, then a person listens for things no meter measures, such as mouth noise, edits and consistent performance. Passing here covers the audio specs ACX publishes. ACX's own measurements may differ slightly near a limit.
- Will it change how my voice sounds?
- It changes loudness and peak level and trims or tops up room tone. Noise reduction runs only when the background is near the limit, and only if it measurably helps. No voice processing or AI is applied to the audio.
- How long does a book take?
- A 13-minute chapter takes a few minutes on Apify's default settings. For a long book, raise the run timeout or split the chapters across runs.
How we tested it
- It doesn't pass what it can't fix. Test recordings with a noisy room or loud mains hum come back marked FAIL with a plain reason, instead of being passed. A recording made 11 dB too loud is brought back into range.