AU File Format (.au)
An AU file holds audio in Sun Microsystems' Unix workstation format, a plain header followed by raw sample data that was the standard system-alert and early web audio format before WAV and MP3 took over.
AU at a glance#
| Full name | Sun AU |
|---|---|
| File extension | .au |
| Category | Audio Files |
| Developer | Sun Microsystems |
| MIME types | audio/basic |
| Other extensions | .snd |
| Compression | Varies from file to file |
| FileFlip support | FileFlip reads and writes this format |
What a .au file can hold#
These are the things a AU file either supports or does not, and they are what decides whether a conversion keeps everything or quietly drops something.
| Embedded metadata | Yes |
|---|
Strengths and limitations of AU#
What AU does well
- Header is only 24 bytes before the audio data starts, which is why Unix systems used it for short alert sounds
- Carries an annotation block for free-text notes alongside the audio, predating modern tagging systems
- Supports plain uncompressed PCM as well as compressed encodings like mu-law, so a decoder does not have to guess
Where AU falls short
- Predates ID3 and similar tag standards, so its metadata is a single freeform text block rather than structured artist and title fields
- Most consumer software today only opens it through a converter or an editor like Audacity, not as a native format
- The original headerless 8-bit mu-law variant sounds noticeably worse than modern codecs at similar bit rates
AU still shows up as the format Java's javax.sound and some Unix system tools default to for short sound effects, where its simplicity is the point.
Convert AU to WAV or MP3 when the audio needs to play in modern media players, browsers, or editing software that no longer ship AU support by default.
How to open a .au file#
A .au file opens in the programs below, grouped by the platform you are on. If you would rather not install anything, FileFlip converts AU into a format your machine already opens, and it does it in the browser tab rather than on a server.
| Platform | Programs that open it |
|---|---|
| Windows | VLC, Audacity |
| macOS | QuickTime Player, VLC, Audacity |
| Linux | Audacity, SoX, VLC |
| Web | FileFlip |
AU questions people ask#
What creates an AU file?
AU files come from Unix and Sun workstation software, from Java's built-in sound APIs, and from older system sound effects, since AU was the default audio format on SunOS and NeXT machines. Websites in the 1990s also used it for embedded sound clips before WAV and MP3 became standard.
What still opens AU files?
Audacity and VLC open AU files on any platform, and command-line tools like SoX handle them on Linux and macOS. Native support has otherwise faded from most operating systems and media players.
What should I convert an AU file to?
Convert AU to WAV to keep full quality for editing, or to MP3 for a smaller file that plays anywhere. Either target is more broadly supported than AU, which almost no current software treats as a default format.
Converting AU files#
Convert AU to another format
Convert another format to AU
Every conversion runs inside your browser. The file is read from your disk, the work happens on your own machine, and nothing is uploaded. How FileFlip works names the engine behind each family and how to verify that for yourself.