What is the AU file format?
.au is a simple headered audio container developed by Sun Microsystems and NeXT Computer in the late 1980s. It was the native sound format on SunOS/Solaris and NeXTSTEP workstations, and later became the default audio type for the Java platform, served under the MIME type audio/basic.
Files begin with the four-byte magic number .snd at offset 0, followed by a compact 24-byte big-endian header that stores the data offset, data size, encoding code, sample rate, and channel count. An optional text annotation field can occupy the space between the header and the audio samples. The big-endian byte order reflects the NeXT/Sun architecture heritage and distinguishes .au from the little-endian WAV format dominant on Windows.
The most classic encoding is 8-bit µ-law PCM at 8000 Hz - the same standard used in telephone audio - making .au files compact enough to serve as system sounds. The format also supports linear PCM at 8, 16, 24, and 32 bits, A-law, 32- and 64-bit IEEE floating-point samples, and G.72x ADPCM variants, all selected by the encoding code in the header.
For new audio work, .au has been largely superseded by AIFF on macOS, WAV on Windows, and FLAC or MP3 for distribution. It remains broadly readable by modern players such as VLC and Audacity.
Security & safety
RISK: LOWAU is a simple, passive audio format and very low risk; it carries no scripting or macros. The only practical caution is sourcing a player: avoid sketchy 'AU player'/'AU converter' download sites that bundle adware, and use the open-source VLC or Audacity instead. Malformed AU headers have historically tripped some old decoders, but modern players handle them safely.
Format details
in a nutshellPrograms that open AU files
Windows2 apps
VLC media player
Open-source Open the .au directly in VLC to play it; works for µ-law and PCM AU files. 
macOS2 apps
VLC media player
Open-source Open the .au in VLC to play it (replaces the long-discontinued QuickTime AU support). 
Linux1 app
VLC / sox / aplay
Open-source Play with VLC, or use 'play file.au' (sox) / aplay; AU is the native Unix sound format. 
Android1 app
Technical details
deep spec| Magic bytes | `.snd` - hex `2E 73 6E 64` at offset 0 (big-endian file signature shared by AU and the NeXT `.snd` variant) |
| Byte order | Big-endian throughout the header and all multi-byte sample fields (NeXT/Sun architecture heritage) |
| Minimum header size | 24 bytes - six consecutive 32-bit big-endian fields: magic, data offset, data size, encoding code, sample rate, channel count |
| Optional annotation field | Variable-length ASCII text block placed between byte 24 and the start of audio data; may be zero-length or padded with null bytes |
| Primary MIME type | `audio/basic` |
| Alternative MIME types | `audio/x-au`, `audio/au` |
| Classic encoding | 8-bit µ-law (mu-law) at 8000 Hz - the historic Unix and Java system-sound standard, matching telephone-quality audio |
| Supported encodings | 8/16/24/32-bit linear PCM, 8-bit µ-law, 8-bit A-law, IEEE 32-bit and 64-bit floating-point, G.721 and G.723 ADPCM (selected by encoding code field) |
| Supported sample rates | 8000 Hz (classic telephony), 22050, 44100, 48000 Hz; the header field accepts any arbitrary integer rate |
| Compression | µ-law and A-law apply logarithmic companding, achieving roughly 2:1 bit reduction compared to 16-bit linear PCM; all PCM modes are uncompressed raw samples |
| Typical file size | Tens of KB to a few MB; 1 second of 8 kHz mono µ-law audio occupies approximately 8 KB |
| Container layout | Flat single-block structure - fixed header immediately followed by a contiguous raw audio sample stream; no chunk or sub-block framing |
| Platform origin | SunOS/Solaris and NeXTSTEP workstations; also the historic default audio format for the Java `javax.sound` API and early Java applets |
| Alternate extension | `.snd` (used on NeXT systems; identical binary format, same magic bytes and header layout) |
| Channel support | Header channel-count field supports any value; mono (1) and stereo (2) are the predominant real-world uses |
| Released | Late 1980s, on NeXT and Sun (SunOS/Solaris) systems |
| Latest version | Stable, minimal header format; effectively unchanged for decades |
| Open standard | Yes · royalty-free |
| Specification | en.wikipedia.org |
AU conversions
Community Q&A
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