What is AU? Sun Audio Format Explained
A simple big-endian audio format from Sun Microsystems, long a de facto Unix sound standard.
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What is AU?
AU is a simple audio file format introduced by Sun Microsystems and historically associated with Unix and NeXT systems, where files typically carry the .au extension (Sun) or .snd extension (NeXT). It became a de facto standard for system sounds on Unix-like platforms and was widely used for short audio clips on early web pages.
An AU file begins with the four-byte magic number 0x2E736E64 (the ASCII string '.snd'), followed by a header of six big-endian 32-bit fields specifying the data offset, data size, encoding, sample rate, and channel count, then optional annotation text and the audio data itself. The original default encoding was 8-bit G.711 mu-law sampled at 8000 Hz, but the encoding field also allows linear PCM, A-law, ADPCM, and floating-point data.
How AU Works
An AU file begins with the four-byte magic number 0x2E736E64, the ASCII string '.snd', followed by six big-endian 32-bit header fields that give the data offset, data size, encoding, sample rate, and channel count, with optional annotation text before the samples.[1] Placing the data offset in the header lets the audio start at a flexible position after the metadata.[2]
History and Standardization
The format originated with Sun Microsystems and NeXT, using the .au extension on Sun systems and .snd on NeXT, and became a de facto standard for system sounds on Unix-like platforms.[1] Its layout is documented in references such as The Open Group's audio file specification.[3]
Encodings and Limitations
The original default was 8-bit G.711 mu-law at 8000 Hz, but the encoding field also permits linear PCM, A-law, ADPCM, and floating-point data.[1] A minimal header with no rich metadata and a missing data-size value (sometimes left unspecified for streams) limit its use compared with modern containers.[2]
MKV Technical Specifications
AU vs Other Audio Formats
| Feature | AU | WAV | AIFF |
|---|---|---|---|
| Compression | Usually uncompressed[1] | Usually uncompressed | Uncompressed |
| Developer/origin | Sun / NeXT[2] | Microsoft / IBM | Apple |
| Byte order | Big-endian[3] | Little-endian | Big-endian |
| Header | Minimal[1] | RIFF chunks | IFF chunks |
| Device support | Limited (Unix) | Wide | Wide |
| Best for | Legacy Unix audio | Editing/archiving | Mac archiving |
AU is a simple legacy Unix sound format; WAV and AIFF offer the same uncompressed quality with far broader modern support.
Advantages & Disadvantages
Advantages
A short, fixed header followed by raw audio makes AU files easy to parse, generate, and stream without complex containers.
As a Unix standard, AU is readable by many cross-platform tools and was natively supported by the Java Sound API and early browsers.
The data size can be left unspecified, allowing audio to be written or transmitted as a continuous stream of unknown length.
The encoding field supports many sample formats, from 8-bit mu-law telephony audio to 16/24/32-bit linear PCM and floating point.
Disadvantages
Beyond an optional free-text annotation field, AU has no structured support for tags, cover art, or rich metadata.
AU has been superseded by WAV, AIFF, and modern codecs, and is rarely used in new audio workflows today.
Its classic 8-bit mu-law, 8000 Hz default offers only telephone-grade quality unless higher-resolution PCM encodings are used.
Common Use Cases
AU appears mainly in legacy Unix audio, programming environments, and historical web content.
Unix system sounds | FileFormer
AU served as the native sound format on Sun, NeXT, and other Unix workstations for alerts and recorded clips.
Java audio | FileFormer
Early Java applets and the Java Sound API used AU as a built-in format for short sound effects and clips.
Early web audio | FileFormer
Small AU files were common for embedded sounds on 1990s web pages before MP3 and streaming formats took over.
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Try Audio Converter FreeFrequently Asked Questions
What is the difference between .au and .snd files?
They are the same format under different conventions: Sun systems typically used the .au extension while NeXT systems used .snd. Both begin with the '.snd' magic number.
Is AU compressed?
Not in the modern sense. AU data is usually uncompressed linear PCM or companded with G.711 mu-law/A-law, which reduces 16-bit samples to 8 bits but is not a true compression codec.
What sample rate do classic AU files use?
The original de facto standard was 8-bit mu-law at 8000 Hz, matching telephone audio, though the header can specify any sample rate such as 44100 or 48000 Hz.
Can modern players open AU files?
Yes. Many cross-platform tools and libraries such as VLC, Audacity, FFmpeg, and SoX can read and convert AU files, even though the format is no longer in common use.
Is AU big-endian or little-endian?
All multi-byte header fields and PCM samples in AU are stored in big-endian byte order, reflecting its origins on Sun's SPARC hardware.