AU file format (.au)

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 a AU file

Unix audio tools, VLC, and Java applications.

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.