What is a .aac file?
Advanced Audio Coding · Audio · MPEG · 1997
AAC is the lossy audio format that succeeded MP3, offering better sound at the same bitrate. It powers the iTunes Store, YouTube and most streaming, and often arrives inside an .m4a or .mp4 file.
What is a .aac file used for?
AAC compresses more efficiently than MP3, so a 128 kbps AAC generally sounds better than a 128 kbps MP3. That is why Apple, YouTube and many streaming services adopted it.
You often see it wrapped as .m4a for music or inside .mp4 video. The .aac extension itself is less common than the container files that hold AAC audio.
A raw .aac file is an ADTS stream, which starts with the sync bytes FF F1 or FF F9 and carries the audio with almost no metadata. That is why a raw .aac often shows no title or cover art, while the same audio inside an .m4a keeps tags, artwork and gapless-playback data. If you want your music library to look right, .m4a is the better container.
For bitrate, 256 kbps AAC is effectively transparent for most listeners, and even 128 kbps holds up well because of AAC's efficiency. Converting AAC to MP3 is easy but pointless for quality, since you are going from a better codec to an older one; only do it for a device that cannot play AAC. Compatibility is otherwise excellent on modern hardware, though very old players may still prefer MP3.
How to open a .aac file
The .aac format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list. For a step-by-step walkthrough, see how to open a .aac file.
Windows
- Media Player
- VLC
- iTunes
macOS
- Music
- QuickTime
- VLC
Linux
- VLC
- mpv
Web
- Most browsers
How to convert a .aac file
You can convert a .aac file to MP3, WAV, FLAC using a conversion tool or the export menu of an app that opens it.
.aac file signature (magic bytes)
Programs recognise a .aac file by the bytes at the start of the file, not by its name. These are the signatures for AAC. For a full breakdown, see the AAC file signature page.
| Hex signature | Offset | Note |
|---|---|---|
| FF F1 | 0 | ADTS AAC |
| FF F9 | 0 | ADTS AAC |
| 66 74 79 70 4D 34 41 20 | 4 | ftyp M4A (in .m4a) |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .aac file?
AAC is the lossy audio format that succeeded MP3, offering better sound at the same bitrate. It powers the iTunes Store, YouTube and most streaming, and often arrives inside an .m4a or .mp4 file.
Is M4A the same as AAC?
Almost. M4A is a container that usually holds AAC audio (sometimes lossless ALAC). AAC is the codec inside.
How do I open an .aac file?
Play it in VLC, iTunes/Music or Windows Media Player, all of which support AAC. On mobile, the built-in music apps play it directly.
Is AAC better than MP3?
At the same bitrate, generally yes: AAC sounds cleaner because it compresses more efficiently. MP3 only wins on compatibility with very old hardware.
How do I convert AAC to MP3?
Use VLC's convert feature, iTunes/Music, or ffmpeg (ffmpeg -i song.aac song.mp3). Only do it for devices that cannot play AAC, since you gain nothing in quality.
Comparisons
Related formats
Sources
Details on this page were checked against authoritative references: