AU to F4B Converter
Processing entirely on your device — large files may take a moment.
Sun Microsystems Audio and Adobe Flash Audiobook (F4B) exist for different jobs: AU is the µ-law audio format of old Unix systems and Java applets — tiny mono files from another era. Convert F4B audiobooks exactly like M4B — to MP3, M4A or WAV.
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 AU compares to F4B
| AU | F4B | |
|---|---|---|
| Compression | µ-law or PCM | Lossy AAC + chapters |
| Quality | Mono voice-grade (µ-law) | Speech-optimized bitrates |
| Compatibility | Legacy Unix tools | Apple Books, Audible |
| Best for | Old system sounds and recordings | Chaptered audiobooks |
How to convert AU to F4B
- 1
Select your files
Drag & drop your AU 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 F4B". The conversion runs entirely inside your browser using WebAssembly — no upload, no queue, no waiting.
- 3
Download results
Download each converted F4B file individually, or grab everything at once as a ZIP archive.
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
About the F4B 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
Why convert AU to F4B?
Think of it as moving your file to the container that fits the job. µ-law flavor sounds dated and mono F4B works differently: Chapter markers survive on Apple platforms If any of that sounds like your task, run the conversion — it takes seconds and nothing leaves your device.
Frequently asked questions
Is this AU to F4B 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 AU to F4B?
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 AU to F4B reduce audio quality?
If F4B is lossless, the output contains everything the AU file contains — no further loss is added. If F4B 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.
Can I convert a whole folder of AU files?
You can select many files at once and they are processed in a single batch. Output files land in one ZIP download when there is more than one result.