M4B to AU Converter
Processing entirely on your device — large files may take a moment.
M4B is the audiobook flavour of M4A — AAC audio with chapter markers, used by Audible and Apple Books. Converting AU to WAV preserves the samples in a format every modern tool reads.
Because the whole conversion happens on your device, it is instant, unlimited and genuinely private — no accounts, no watermarks, no files on anyone's server. Multiple files are converted in one go.
How M4B compares to AU
| M4B | AU | |
|---|---|---|
| Compression | Lossy AAC + chapters | µ-law or PCM |
| Quality | Speech-optimized bitrates | Mono voice-grade (µ-law) |
| Compatibility | Apple Books, Audible | Legacy Unix tools |
| Best for | Chaptered audiobooks | Old system sounds and recordings |
How to convert M4B to AU
- 1
Select your files
Drag & drop your M4B files onto the area above, or click to browse. You can add multiple files at once for batch conversion.
- 2
Convert instantly
Click "Convert to AU". The conversion runs entirely inside your browser using WebAssembly — no upload, no queue, no waiting.
- 3
Download results
Download each converted AU file individually, or grab everything at once as a ZIP archive.
About the M4B format
M4B is M4A wearing an audiobook badge: the same AAC audio in an MP4 container, plus chapter markers that Apple Books and Audible apps render as a scrubbing timeline. Buy an audiobook outside Audible and it likely arrives as M4B.
The format is deliberately chained to Apple's book ecosystem. Converting M4B to MP3 sets the book free: car stereos, gym players, old iPods and every app without chapter support will play it — though the chapter marks themselves are flattened away.
+ Strengths
- ✓Chapter markers survive on Apple platforms
- ✓Efficient AAC audio
! Limitations
- −Chapters vanish in MP3 conversion
- −Non-Apple support is patchy
About the AU format
AU is Sun Microsystems' audio format from the Unix workstation age: a small header followed by µ-law (or PCM) samples. Java's applet era shipped it everywhere, and old Unix sound tooling still emits it.
Modern players increasingly skip it. Converting AU to WAV preserves the samples bit-for-bit while moving the file into universal territory — the standard rescue path for system sounds and old recordings.
+ Strengths
- ✓Simple, lossless sample storage
- ✓Native to old Unix/Java tooling
! Limitations
- −µ-law flavor sounds dated and mono
- −Dropped by modern players
Why convert M4B to AU?
Think of it as moving your file to the container that fits the job. Chapters vanish in MP3 conversion AU works differently: Simple, lossless sample storage If any of that sounds like your task, run the conversion — it takes seconds and nothing leaves your device.
Frequently asked questions
Is this M4B to AU converter really free?
Yes — completely free, with no sign-up, no watermark and no daily conversion limits. The site is supported by advertising.
Are my files uploaded to a server?
No. The conversion runs entirely inside your browser using WebAssembly, so your files never leave your device — that is what makes it instant and private.
Does it work on Windows, Mac, iPhone and Android?
Yes. Anything with a modern browser works: Windows, macOS, Linux, Android and iOS. No software installation is required.
Is there a file size limit for M4B to AU?
The only limit is your device's memory, since nothing is uploaded. Files up to a few hundred megabytes work fine on a modern desktop browser; very large files simply take a little longer.
Will converting M4B to AU reduce audio quality?
If AU is lossless, the output contains everything the M4B file contains — no further loss is added. If AU is lossy, one recompression pass is unavoidable; the quality slider controls how much. Loss that already happened in the source file can never be undone by converting.
Are tags like title and artist preserved?
Tag handling varies between target formats and this conversion focuses on the audio itself. For archival copies where tags matter, keep the original file; the converted copy is for playback and sharing.