.au File — Sun/NeXT Audio

AU is an audio file format originally developed by Sun Microsystems for their Unix workstations and NeXT computers. The format stores audio with a simple header (magic number, offset, data size, encoding, sample rate, channels) followed by audio data in various encodings including mu-law (8-bit logarithmic), linear PCM (8/16/24/32-bit), IEEE floating point, and compressed formats. The .au format was one of the first audio formats supported on Unix systems and was commonly used for system sounds, telephone recordings, and early web audio (it was one of the few formats Java's AudioClip could play). The mu-law encoding variant provides telephone-quality audio at 8 kHz/8-bit, which was standard for voice applications. While the AU format has been largely superseded by WAV, FLAC, and modern codecs for general audio, it remains encountered in telephony systems, legacy Unix applications, and audio programming examples. The format's simplicity makes it one of the easiest audio formats to parse programmatically, often used in educational contexts for teaching digital audio processing concepts.