Audio file introduced by Sun Microsystems; used primarily on Sun or other Unix-based machines and has been adopted by several audio programs, including Adobe Audition and QuickTime; stores the data in three parts: a header (24 bytes), a variable length annotation block, and the actual audio data.
The AU file extension is associated with Audacity audio editing application. Behind the scenes, large audio files are broken up into chunks of just over a megabyte in size, which Audacity calls block files with .au file extension. The Audacity .au files are not compatible with .au Unix audio files.