About AU Files
Sun AU Audio
AU is a simple audio format from Sun Microsystems, commonly used on Unix systems.
Family
Audio Files
Extension
.au
MIME Type
audio/basic
Can Use As
41 55 46 4D 00 01 00 00 4B 41 49 4A
ASCII: AUFM....KAIJ
📈 Target Minimum File Size (Guaranteed ≥ KB)
Lossless JPEG Q100 + standard COM segment padding for government & exam portals
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HOW AU CAME TO BE.
AU — sometimes called Sun au or NeXT/Sun audio — was the native uncompressed audio format on Sun SPARC workstations and NeXT computers in the late 1980s. The header is 24 bytes, minimalist: magic number, data offset, size, encoding code, sample rate, channels, and an optional text annotation. After that comes raw audio samples, usually 8-bit µ-law or 16-bit PCM.
AU achieved minor immortality in the Java world because the early Applet API used it as the default audio format for java.applet.Applet.play(). Every "Web 1.0" applet that played a sound played a .au file. Outside Java and Sun workstation archives, AU is effectively extinct — supplanted by WAV on Windows, AIFF on Mac, and OGG/MP3 on the web. FFmpeg and sox still read and write it for compatibility.
Technical Specifications
- MIME types
- audio/basic, audio/au, audio/x-au
- Extensions
- .au, .snd
- Header
- 24 bytes (magic, offset, size, encoding, rate, channels, info)
- Codecs
- PCM 8/16/24/32-bit, µ-law, A-law, IEEE float
- Byte order
- Big-endian
Typical Sizes & Weights
10-second clip (8-bit µ-law, 8 kHz)
80 KB
10-second clip (16-bit PCM, 44.1 kHz stereo)
~1.7 MB
STRENGTHS & LIMITATIONS.
Strengths
- Trivially simple format — 24-byte header, then samples.
- µ-law 8-bit variant fits hours of speech in kilobytes.
- Stable since 1988; every major audio library reads it.
- Streaming-friendly: size field is optional.
Limitations
- Aging — obsolete outside legacy and compatibility scenarios.
- No metadata beyond a single annotation string.
- No native multi-channel surround support.
- Limited to 8 codecs, none modern.
CURIOSITIES & TRIVIA.
Java applets used .au as their default audio format via Applet.play() — every Web 1.0 applet chime came from a .au file.
The AU header is just 24 bytes — one of the simplest audio file formats ever designed.
Sun workstations shipped with cowbell.au, crash.au, ding.au, and rooster.au as standard system sounds — now internet nostalgia artifacts.
NeXT computers (including Steve Jobs' post-Apple workstation) used AU as the native audio format.
The format supports µ-law compression — the same logarithmic PCM encoding used in North American telephony.
Specialized Utilities for .AU Files
MP3 Audio Compressor — Reduce MP3 File Size
Compress MP3, WAV, M4A, and FLAC audio files by adjusting bitrate (64k, 96k, 128k, 192k), sample rate, and mono/stereo channels.
Online Audio Cutter & Trimmer
Trim MP3, WAV, FLAC, M4A, and OGG audio tracks to exact start and end timestamps with built-in browser playback and custom bitrate.
Extract Audio from Video
Extract the audio track from any video file and save it as MP3, WAV, or other audio formats. Perfect for creating podcasts, music rips, or voice-only recordings.
Video to MP3 Audio Extractor
Extract high-bitrate MP3 audio tracks from MP4, MOV, MKV, AVI, and WebM video recordings in seconds—ideal for podcasts, lectures, and interviews.
CONVERT FROM AU
Common Use Cases
Unix audio, telephony
Related Formats
Popular comparisons
AU vs FLAC
Differences, file size, and when to choose each format.
AU vs AAC
Differences, file size, and when to choose each format.
AU vs M4A
Differences, file size, and when to choose each format.
AU vs OGG
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AU vs OPUS
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AU vs WMA
Differences, file size, and when to choose each format.
Popular AU conversions
The most-requested destinations when starting from AU.
Frequently Asked Questions
AU (Sun AU Audio) is an audio file format used to store sound recordings — music, voice, podcasts, sound effects. The format defines how the audio samples are compressed (or stored raw), what bitrates are supported, and how metadata such as title, artist, album, and cover art is embedded. It is part of the audio files family.
VLC, foobar2000, and the default media players on Windows and macOS handle AU natively. On mobile, iOS Music and Android media apps vary in their support — popular formats work everywhere; niche ones may need a dedicated app. If playback fails on a device, converting to MP3 or AAC usually solves it.
Upload the AU to KaijuConverter and pick MP3, WAV, FLAC, AAC, OGG, or any other target. Our FFmpeg pipeline decodes the audio and re-encodes to the target format at sensible default bitrates (VBR ~190 kbps for music, 96 kbps for speech). Metadata and cover art travel with the audio where both formats support them.
AU can be lossy or lossless depending on the specific variant. Lossy variants (smaller files) discard some audio detail during compression in ways tuned to be inaudible; lossless variants preserve every sample exactly but produce larger files. For distribution, lossy at high bitrate is standard; for archival, lossless wins.
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