AU file format: what it is and how to use it

The Au format is Sun Microsystems' 1991 audio container: simple headered PCM and μ-law sound that powered early Unix and Java applets. Learn when to use it and what to choose instead.

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What is a AU file?

An .au file is audio in the format Sun Microsystems defined around 1991: a small header (magic .snd) declaring encoding, sample rate and channels, followed by the samples: typically 8-bit μ-law, but also PCM and compressed variants. It was the sound of early Unix workstations and, famously, the format Java applets could play.

When should you use AU?

  • Reading legacy Unix audio: old mail attachments, system sounds and archives still surface as .au.
  • Studying format history: its tiny header makes it a classic teaching example.
  • μ-law telephony audio: the encoding it popularized still lives in voice systems.

When is another format a better fit?

For uncompressed interchange today, WAV does the same job with universal support. For lossless storage FLAC halves the size; for anything shared or streamed, MP3 or AAC is the right shape. AU is heritage, not a working choice.

The short version: AU is the Unix audio ancestor: convert it to WAV or FLAC and only the history books keep it.

How to open a AU file

On Windows

Double-click the file, or open it with VLC.

On macOS

Audacity opens AU files on macOS.

In your browser

Open AU files right in a browser tab or with the dexrr viewer. No install, no upload.

Also opens in

  • VLC
  • Audacity
  • SoX

View or convert AU files

A quick preview or a new format. No install, no upload.

View AU files Preview and inspect AU files Convert to AU Turn files into AU AU to WAV Waveform Audio File AU to AIFF Audio Interchange File Format AU to FLAC Free Lossless Audio Codec

AU technical details

Full name
Sun/NeXT Au audio
Extensions
.au .snd
MIME type
audio/basic
Magic bytes
2E 73 6E 64 (.snd)
Compression
None (uncompressed)
Developer
Sun Microsystems

AU questions, answered

How do I open an .au file?

VLC, Audacity, FFmpeg and most players read it; stock OS players sometimes need help. dexrr plays and converts au files directly in your browser.

Can I convert AU files online?

Yes, dexrr converts au locally in your browser: AU to WAV for compatibility or AU to FLAC for compact lossless storage.

Is AU better than WAV?

No, they carry the same PCM idea, but WAV won the ecosystem. AU survives mostly in archives and telecom-flavored niches.

Why does my .au file sound muffled or noisy?

Most .au files are 8-bit μ-law, telephone-grade quality by design. Convert it, but expect the source fidelity, not hi-fi.

Is the AU format outdated?

Yes, essentially historical, kept alive by archives, old Unix systems and telephony-descended workflows.