The file extension AC3 identifies audio files in the so-called Adaptive Transform Coder 3 format. It is a lossy coding process from Dolby Laboratories for digital multichannel audio streams.
The AC3 sound format is mainly used for digital sound transmission in the TV area as well as DVDs and BluRays. It supports up to 6 channels (Dolby Digital EX up to 8), with 3 front, 2 rear channels and one for the subwoofer. Therefore, the usual 5.1 is often indicated with 3 / 2.1.
On DVDs and BluRays AC3 files are hard to find, as the audio streams are contained in the respective container (vob or M2ts). Ac3 files are mainly used in the production of video files, DVDs and BluRays for integration into other containers.
Only for music files AC3 is often used directly, as many media boxes as well as DVD and BluRay players can play AC3 files directly.
It should be noted that some programs can not load AC3 files directly, but need a plugin, such as Quicktime or Windows Media Player (ac3filter).