Audio
What is an AU file?
Updated Jul 2026
AU (Sun Audio) is an audio format that Sun Microsystems built for Unix workstations in the early 1990s. It stores sound with a very simple header, classically using a basic form of compression, which made it easy for early software to read and write. Its main limitation today is age: most modern apps and devices favor newer formats, so an AU file often needs converting before it plays anywhere outside old Unix or Java tools.
- Extension
- .au
- Type
- Audio
- Typically
- Unix / legacy audio
Why AU exists
Sun Microsystems created the AU format for its Unix workstations, and it became one of the first audio formats widely used on the early internet because of how simple its header is. A program only needs to read a handful of fields to know the sample rate and encoding, which made it trivial to support.
Under the hood, an AU file stores its audio right after that small header. The classic version uses a simple mu-law encoding that trades a little quality for a smaller size, though later versions can also hold plain uncompressed audio. Either way, the format stays easy for old software to read.
You're most likely to run into an AU file today through Java, which still uses it for built in system sounds, or by digging up old Unix era audio archives. Modern players, phones, and editing software rarely default to reading it, so people convert it to WAV or MP3 to actually use the recording somewhere current.
The trade-offs
Strengths
- Compact for its era thanks to simple encoding
- Very simple header, easy for old and new software alike to parse
- Small, predictable structure that has stayed readable for decades
Watch-outs
- Rarely the default format on modern phones, players, or editors
- Uncompressed audio means larger files than formats like MP3
- Mostly encountered in legacy Unix systems or Java resources now
A note on privacy
AU files don't typically carry the kind of personal metadata that photos do, but the audio itself, a voice recording, a meeting, a system log, can still be sensitive. Uploading a file to an online converter puts that audio on someone else's server, even briefly. Converting it on your own machine means the recording never has to leave your computer to become a format you can actually use.
Questions
How do I open an AU file?
Some media players and audio editors still read AU files directly, but support is inconsistent on modern systems. Converting it to WAV or MP3 first is usually the more reliable way to open and play it anywhere.
Is AU better than WAV?
They fill similar roles for simple audio, but WAV stores uncompressed sound and is far more widely supported today, so unless you specifically need AU for a Unix or Java tool, WAV is the more practical choice.
Why do I have an AU file?
Usually it comes from old Unix era software, an archived recording, or a Java application, since Java still bundles some of its default system sounds in this format.
Can I convert an AU file without uploading it?
Yes. A desktop app like Morphjet converts AU files on your own computer, so the audio never has to travel to an external server just to change format.
Morphjet opens and converts AU and 1,800+ other formats, all on your own computer. Launching this July.