What is AU? Sun Audio Format Explained

How AU Works

The AU format, also seen with the .au and .snd extensions, is one of the oldest and simplest audio file formats in common computing history. Its structure is deliberately minimal: a short header at the start of the file describes the audio, followed immediately by the audio data itself. This bare arrangement made it easy to read and write on early systems with limited resources, which is a large part of why it endured for so long in the Unix world.

The header carries just a few essential fields, including a magic number that identifies the file as AU, the offset at which the audio data begins, the encoding used, the sample rate, and the number of channels. Because the data offset is stored explicitly, the format allows a variable-length information region between the header and the audio, but in practice the header remains compact. There is no elaborate chunked container here, simply enough metadata to interpret the samples that follow.

AU's most historically characteristic encoding is 8-bit mu-law, the companding scheme associated with G.711 telephony. Mu-law is a nonlinear encoding that allocates more resolution to quiet sounds and less to loud ones, matching how human hearing perceives level, so it captures usable, telephone-quality voice in only eight bits per sample. This is why so many classic AU files carry that distinctive narrowband, telephony-grade sound.

The format is not limited to mu-law, however. AU also supports linear PCM at various bit depths and several other encodings, so it can hold higher quality uncompressed audio as well as the compact telephony format. In all cases the underlying philosophy is the same: a tiny descriptive header followed by a straightforward stream of samples, with little of the container complexity found in later formats.

History and Standardization

AU originated at Sun Microsystems and was closely associated with Sun's Unix workstations and with the NeXT platform, which is why it is sometimes referred to as the Sun or NeXT audio format. It dates from an early period of networked Unix computing, when a simple, self-describing sound file that any workstation could handle was genuinely useful, and it became a de facto default for audio on those systems.

Its reach extended beyond Unix workstations through its adoption in early Java, where AU was among the sound formats the platform could play out of the box. For a time this gave the format meaningful presence on the web and in cross-platform applications, since Java's write-once portability carried AU support along with it wherever the runtime went.

AU was never governed by a heavyweight multimedia standards process; it grew instead from the conventions of the systems that used it. As richer and more capable formats took over on consumer platforms, AU gradually became a legacy format. It is still recognized by many audio tools and remains historically important, but it is rarely chosen for new work today.

AU Compared to WAV and AIFF

The closest modern analogues to AU are WAV and AIFF, the two dominant uncompressed audio containers. WAV, developed in the Microsoft and IBM world, and AIFF, associated with Apple, are both built on the more elaborate chunked container style, which stores audio alongside a flexible set of tagged metadata blocks. AU, by contrast, is far simpler, with essentially just a header and a data stream, which makes it lighter but also less extensible.

That simplicity is both AU's charm and its limitation. Its minimal header is trivial to parse and was ideal for constrained early systems, but it lacks the richer metadata and broad tooling ecosystem that grew up around WAV in particular. Over time WAV became the overwhelmingly dominant uncompressed format on personal computers, while AIFF held a strong position in the Apple audio world, leaving AU as the older Unix-heritage option.

The honest tradeoff today is that AU is a historically significant but largely superseded format. Its ties to early Unix, NeXT, and Java give it real heritage value, and its mu-law telephony encoding remains a compact way to store voice, but for contemporary work the wider compatibility and richer metadata of WAV and AIFF make them the practical defaults. AU is best understood as a clean, elegant artifact of an earlier computing era rather than a format to reach for in new projects.