AU Format Guide

Available Conversions

Convert to AU

About AU Format

AU (Audio) is an audio file format originally developed by Sun Microsystems for use on their Unix-based workstations and servers. Introduced in the late 1980s alongside the SunOS and Solaris operating systems, AU became the standard audio format for Unix systems and was closely associated with the NeXT computer platform as well (where it used the .snd extension). The format uses a simple header structure followed by audio data, supporting various encoding methods including 8-bit mu-law (the most common), linear PCM at 8, 16, 24, and 32 bits, 32-bit IEEE floating point, and compressed formats like G.711 A-law and ADPCM. AU files typically use the .au extension and are identified by the magic number ".snd" (0x2E736E64) at the beginning of the file. The format's simplicity and universal availability on Unix systems made it a foundational audio format in the history of digital audio computing.

History of AU

The AU format was created by Sun Microsystems in the late 1980s as part of their audio infrastructure for SunOS (later Solaris). Sun's SPARCstation workstations were among the first affordable Unix machines with built-in audio hardware, and the AU format was designed to work natively with Sun's audio device drivers (/dev/audio on Solaris). The format was heavily influenced by the telephone industry's mu-law encoding standard, which Sun adopted as the default encoding at 8 kHz sample rate for voice-quality audio. When NeXT Computer, Steve Jobs' company between Apple stints, developed their NeXTSTEP operating system, they adopted essentially the same format with the .snd extension. The AU format gained broader recognition in the mid-1990s when it became one of the first audio formats supported by web browsers, alongside WAV and MIDI. Java's Sound API, introduced with Java 2 in 1998, included native AU format support, ensuring the format's continued relevance in cross-platform development. Although AU has been largely superseded by WAV, MP3, and more modern formats for general use, it remains significant in Unix/Linux audio development, telephony systems, and Java-based audio applications. Sun Microsystems was acquired by Oracle Corporation in 2010, but the AU format specification remains freely available and implemented across virtually all audio processing libraries.

Key Features and Uses

The AU format supports multiple encoding methods, with mu-law (u-law) being the most historically common. Mu-law encoding compresses 14-bit linear samples into 8 bits using a logarithmic companding algorithm, providing a dynamic range of approximately 72 dB while keeping file sizes small. For higher quality, AU supports linear PCM encoding at 8, 16, 24, and 32 bits per sample, as well as 32-bit and 64-bit IEEE floating point. The format supports arbitrary sample rates, though 8000 Hz (telephone quality), 22050 Hz, and 44100 Hz (CD quality) are most common. AU files can contain mono or multi-channel audio, with the channel count specified in the header. The header is remarkably simple: a 24-byte minimum structure containing the magic number, data offset, data size, encoding type, sample rate, and channel count, optionally followed by an annotation string. This simplicity makes AU files easy to parse and generate programmatically, which is why the format is popular in educational settings and embedded systems. AU also supports G.711 A-law encoding (used in European telephone systems) and various ADPCM compression schemes.

Common Applications

AU remains the native audio format for Solaris and many Unix/Linux audio subsystems, where it is used for system sounds, audio device testing, and command-line audio processing. Java applications frequently use AU format through the javax.sound API, as it is one of the guaranteed supported formats across all Java implementations. The format is used in telephony and VoIP systems due to its native mu-law and A-law encoding support, which map directly to the G.711 standard used in telephone networks. Scientific and research applications on Unix systems often use AU for audio data storage and analysis. Many legacy web pages from the 1990s and early 2000s embedded AU files for audio playback, and the format is still supported by modern web browsers for backward compatibility. Audio programming tutorials and educational materials frequently use AU due to its simple header structure, making it ideal for teaching digital audio concepts. The format is also found in embedded systems, particularly those running Unix-like operating systems, where its minimal overhead and simple parsing requirements are advantageous. Linux command-line tools like sox, aplay, and ffmpeg all support AU format natively.

Advantages and Disadvantages