M4B Audiobook Generator — industry-standard audiobook files
M4B is the file format every audiobook player and distributor expects. Audibloom packages your generated audio as a proper chaptered M4B — embedded chapter markers, cover art, ACX-compliant LUFS, and silence padding — so you upload one file and listeners see chapters on Audible, Apple Books, Spotify, and Audiobookshelf.
What M4B-ready actually means
Most TTS tools give you raw MP3. Audibloom gives you a finished audiobook.
Building a real M4B is more than renaming a file extension. Distributors will reject your upload if any of these are wrong.
Embedded chapter markers
Each chapter from your manuscript becomes a real M4B chapter marker, so listeners can skip directly to "Chapter 7" from Audible, Apple Books, Audiobookshelf, or VLC.
Embedded cover art
Cover image is embedded in the M4B at proper dimensions and color profile. No need to upload it separately to your distributor.
ACX-compliant LUFS levels
Audio is loudness-normalized to –20 LUFS (within ACX's –23 to –18 dB target range), peak-limited to –3 dBTP, with noise floor below –60 dB.
Silence padding per chapter
Each chapter starts with 0.5–1s of silence and ends with up to 5s — meeting ACX's "room tone" requirement and preventing abrupt cuts in player apps.
Full metadata: title, author, narrator
Title, author, narrator, year, language, and book description are written to the M4B's tag table so they show up correctly in every audiobook player.
Validated before download
Every M4B is checked against the ACX, Spotify, and Apple Books spec sheets before you can download it. If something's out of range, we tell you exactly what's wrong.
Inside the M4B
Every M4B Audibloom produces meets these specs
| Spec | Audibloom default | Why it matters |
|---|---|---|
| Container | MPEG-4 (.m4b) | Required by Audible, Apple Books, Audiobookshelf |
| Audio codec | AAC-LC | Universal audiobook codec; supported by every player |
| Sample rate | 44.1 kHz | ACX requirement; matches CD-quality narration delivery |
| Bitrate | 128 kbps mono / 192 kbps stereo | Above ACX 192 kbps MP3 floor when transcoded |
| Loudness | –20 LUFS integrated | Within ACX –23 to –18 dB RMS range |
| True peak | ≤ –3 dBTP | Avoids platform clipping after re-encoding |
| Noise floor | ≤ –60 dB RMS | ACX requirement; clean reading without hiss |
| Chapter markers | One per chapter | In-app chapter skip on every major platform |
| Cover art | Embedded JPEG, ≥ 2400×2400 | ACX cover spec; Apple Books minimum |
Pricing
M4B export starts on Author Pro
Free and Creator plans output MP3. M4B export with embedded chapter markers and ACX mastering is included on Author Pro, Studio, and the Per-book Pack.
Free
See how your book sounds in two minutes.
5 minutes / month
Try free- 5 minutes of generated audio
- Upload PDF / EPUB / TXT
- 6 starter voices
- MP3 download (watermarked)
- M4B export
- Multi-character narration
Creator
Short stories, blogs, and serial chapters.
2 hours / month
Start with Creator- 2 hours of generated audio
- MP3 export, no watermark
- 20+ standard voices
- Auto chapter detection
- M4B export
- Multi-character narration
Author Pro
For self-publishing a full novel on Spotify, Apple Books, Kobo.
10 hours / month
Start with Author Pro- 10 hours of generated audio
- ACX-ready M4B export LUFS-normalized, chapter markers, cover art
- Multi-character narration auto character detection + per-character voice casting
- Pronunciation dictionary IPA + per-book name overrides
- Full chapter re-generation
- MP3 + WAV per chapter
- Voice cloning
Studio
For prolific authors and small presses publishing across multiple titles.
30 hours / month
Talk to Studio team- 30 hours of generated audio
- Everything in Author Pro
- Voice cloning 3 cloned voices, 5–60s sample
- Batch projects Run 5 books in parallel
- Commercial distribution license
- Priority render queue
Per-book Pack
One-and-done. Pay only when you have a book to ship.
- Up to 12 hours of audio per book
- ACX-ready M4B export
- Multi-character narration
- Pronunciation dictionary
- No subscription, no monthly cap
FAQ
Questions, answered
Can I upload an Audibloom M4B directly to ACX?
Yes — that's the whole point. Audibloom's M4B output meets ACX's technical specs (LUFS range, true peak, noise floor, chapter markers, cover art). ACX accepts AI-narrated audiobooks as long as you disclose AI narration in your title metadata, which we make easy.
What if my distributor only accepts per-chapter MP3?
Audibloom always exports both: a chaptered M4B and per-chapter MP3 files (and WAV on Author Pro+). Use the M4B for ACX, Spotify Audiobooks, Apple Books, Kobo. Use the per-chapter MP3 set for Findaway Voices or Google Play Books, both of which prefer per-chapter uploads.
Will my M4B play in Audiobookshelf and other open-source players?
Yes. We test every M4B against Audiobookshelf, VLC, Apple Books, BookFusion, Smart AudioBook Player, Bound, and Plex. Some Linux-only players (Cozy) have limited M4B support, in which case the per-chapter MP3 set is the fallback.
What's the largest book size you support?
M4B has no practical size cap up to ~250 hours per file. We've tested 12+ hour audiobooks routinely. If you need to ship multi-volume sets, you can also export each volume as its own M4B.
Get early access
Get an ACX-ready M4B in beta
Send us a finished manuscript and we'll generate a fully-mastered M4B for you during early access — to test on your favorite audiobook platform.
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