AU file (Sun audio)
The Sun/NeXT audio format, still seen in old archives.
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Convert AU to MP3, free and without uploading it.
What is an AU file?
In short: An AU file (.au) is an audio file in the AU format. AU is about as simple as an audio file gets: a short header, then samples. It can be converted to MP3, WAV or FLAC for free in a web browser with iHateFiles, without uploading it.
The Sun/NeXT audio format, from Sun Microsystems in the late 1980s.
AU is about as simple as an audio file gets: a short header, then samples. It was the sound format of Unix workstations and of Java, which is why it still turns up in old software and academic archives.
What AU is good at
- Extremely simple to read
- Lossless in its common forms
- Still handled by Unix audio tools
Where AU falls short
- Essentially obsolete
- No metadata to speak of
- Often uses telephone-era companding
When to use AU
Convert AU to WAV or MP3.
How to open an AU file
Unix audio tools, VLC, and Java applications.
| Windows | VLC or Audacity. |
|---|---|
| macOS | VLC or QuickTime. |
| Linux | Native in most audio tools — AU came from Sun and is deeply embedded in Unix history. |
| Phone | VLC. |
| In a browser | Safari plays some AU files. Chrome and Firefox generally will not. |
The thing that usually goes wrong
Many AU files use 8-bit µ-law companding, which sounds like a telephone. That is how they were recorded, not a conversion fault.
Inside a AU file
The AU format came from Sun Microsystems in the late 1980s and is one of the oldest audio formats still occasionally encountered. Its header is 24 bytes, big-endian, and its default encoding is 8-bit µ-law at 8 kHz — telephone quality, because telephony is what it was for.
It is woven deeply into Unix history: /dev/audio on a Sun workstation accepted AU data directly, and Java shipped with AU support in its earliest audio APIs, which is why a surprising number of 1990s applets carry .au files inside them.
| Full name | Sun/NeXT audio |
|---|---|
| Magic bytes | '.snd' (2E 73 6E 64) |
| Byte order | Big-endian |
| Default encoding | 8-bit µ-law at 8 kHz |
| Also supports | Linear PCM up to 32-bit, A-law, ADPCM |
| Header | 24 bytes, fixed layout |
Not sure the file really is an AU? Identify it by its contents, whatever its extension says.
Questions
What is an AU file?
AU stands for Sun audio. The Sun/NeXT audio format, still seen in old archives. Files use the .au extension.
What does AU stand for?
AU stands for Sun audio. AU is short for audio; it is the sound format of Sun and NeXT computers.
How do I open an AU file?
On Windows: VLC or Audacity. On a Mac: VLC or QuickTime. On a phone: VLC. Or convert an AU here to a format that opens everywhere, free and without uploading it.
Can I convert AU for free?
Yes. Every conversion here is free and unlimited, because your own device does the work rather than a server we have to pay for.
Is AU lossless?
Yes. AU stores every sample exactly, at the cost of a larger file.