Free online AU to M4B converter
Convert AU audio to M4B audiobook format with FFmpeg and metadata support.
Convert AU to M4B online - free
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How it works
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Upload your .au file
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How to convert au to m4b file
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3mos
AU is an audio file format commonly associated with Sun/NeXT systems and early Unix environments. It can store uncompressed or lightly compressed audio, and it is often used for simple sound clips. M4B is an MPEG-4 audiobook format that usually contains AAC audio and supports chapter markers, bookmarks, and metadata such as cover art and track position.
Why convert au to m4b
Converting AU to M4B is useful when you want to turn a legacy audio file into an audiobook-friendly format. M4B is widely supported by audiobook players and Apple devices, and it is better suited for long-form listening because it can remember playback position and organize content with chapters.
Best software for au to m4b conversion
FFmpeg is the best overall tool for this conversion because it is free, powerful, and supports both formats. It can convert AU audio to AAC-based M4B efficiently, and it also allows you to add metadata and chapters when needed. For a simpler graphical option, VLC can convert audio files, but it is less ideal for creating properly tagged M4B audiobook files.
If you want the most reliable result, use FFmpeg. If you prefer a user interface, try Audacity for editing first, then export with FFmpeg or another converter that supports M4B output.
How to convert au to m4b
With FFmpeg, the basic conversion can be done from the command line. A typical example is:
ffmpeg -i input.au -c:a aac -b:a 128k output.m4b
This converts the AU file to AAC audio inside an M4B container. You can adjust the bitrate depending on quality needs. For audiobook use, 96k to 128k is usually a good balance between size and clarity.
If your AU file contains multiple segments or you want chapter support, prepare chapter metadata separately and include it during conversion. That makes the resulting M4B much more useful for audiobook playback.
Conversion tips
- Check the source audio quality before converting, because M4B will not improve poor AU recordings.
- Use AAC encoding for broad compatibility with audiobook players.
- Add metadata such as title, author, and cover art for a polished audiobook file.
- Keep chapters if the AU source is part of a longer recording.
Recommended workflow
- Open the AU file in FFmpeg or another converter.
- Choose AAC as the audio codec.
- Set the output container to M4B.
- Add audiobook metadata and chapters if available.
- Save and test the file in an audiobook player.
Compatibility notes
M4B works especially well with Apple Books, iTunes-compatible players, and many audiobook apps. If your player does not recognize the file, check whether the audio is encoded as AAC and whether the file extension is correctly set to .m4b.
Summary
AU to M4B conversion is best handled with FFmpeg because it offers the most control and the best output quality for audiobook use. Convert the audio to AAC, keep the bitrate reasonable, and add metadata for a clean, portable audiobook file.
Note: This au to m4b conversion record is incomplete, must be verified, and may contain inaccuracies. Please vote below whether you found this information helpful or not.
AU vs M4B: format comparison
How the AU and M4B formats compare on the properties that matter most for this conversion.
| Property | .AU Sun/NeXT audio file | .M4B MPEG-4 Audiobook File |
|---|---|---|
| Compression | Uncompressed | Lossy |
| Audio quality | — | High |
| Audio channels | — | Up to stereo |
| Metadata support | Basic | Extensive |
| Streaming | Yes | Partial |
| DRM protection | No DRM | Optional |
| Opens in a web browser | Partial | Partial |
| Typical file size | Large | Small |
| Open standard | Yes | Yes |
| Best used for | Speech and voice | Music |
| Introduced | 1988 | 2001 |
| Developer | Sun Microsystems / NeXT | ISO/IEC |
| MIME type | audio/basic | audio/mp4 |
Frequently asked questions
Is the AU to M4B converter free?
Yes, converting AU to M4B on 101convert.com is completely free. No registration, email address or installation is required. If you need a higher file size limit or want to convert more files at once, see our plans.
How large can my AU file be?
You can upload AU files up to 1000 MB and convert up to 100 files at once in a single batch.
What happens to my uploaded files?
Files are processed automatically and deleted from our servers within a few minutes after conversion. We never view or share your files.
Do I need to install any software to convert AU to M4B?
No. The AU to M4B conversion runs entirely online in your browser and works on Windows, Mac, Linux, Android and iPhone.
Did the conversion fail?
In most cases, the cause is an incorrect file format. Please upload a valid AU file for this AU to M4B conversion. Occasionally, the issue may be on our side. We analyze recurring conversion failures and disable or fix the converter if we detect a defect.