AAC file format
AAC is the codec that replaced MP3 as the default almost everywhere: YouTube, Apple Music, digital radio and every modern phone encode audio with it.
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What is AAC?
AAC (Advanced Audio Coding) was standardised in 1997 by MPEG, with Fraunhofer, Dolby, Sony and AT&T among its developers. It was designed as the direct successor to MP3 and improves on it in the ways that matter: better handling of high frequencies, more efficient coding at low bitrates, and support for up to 48 channels rather than MP3's stereo-plus-workarounds.
In practice AAC at 128 kbps sounds about as good as MP3 at 192 kbps, which is why streaming services chose it when bandwidth was expensive. Apple built the iTunes Store on it, YouTube uses it for most audio tracks, and digital broadcasting standards adopted it. It is also the default audio codec inside MP4 video files.
The confusing part is file extensions. Raw AAC streams get .aac, but the overwhelming majority of AAC audio is wrapped in an MP4 container and ends up as .m4a. Both hold the same codec; the container differs, which matters because some players accept one and reject the other.
| Extension | .aac |
|---|---|
| Category | Audio |
| MIME type | audio/aac |
| Released | 1997 |
| Developed by | ISO/IEC MPEG |
| Compression | Lossy |
AAC: frequently asked questions
- Is AAC better than MP3?
- At the same bitrate, yes. The gain is largest at low bitrates — AAC at 96-128 kbps stays clean where MP3 audibly struggles. At 256 kbps and above both are transparent to most listeners, and MP3 wins on universal compatibility.
- What is the difference between .aac and .m4a?
- The codec is identical; only the packaging differs. A .aac file is a raw stream, while .m4a wraps the same audio in an MP4 container that also carries tags and cover art. Most software prefers .m4a, which is why it is far more common.
- Is AAC lossless?
- No. AAC is a lossy codec — it discards audio information the ear is unlikely to notice, and that data cannot be recovered. Apple's lossless format is ALAC, which also uses the .m4a extension, so check file size if you need to tell them apart.
- Why does my AAC file have an .m4a extension?
- Because it is AAC audio inside an MP4 container, which is how virtually every encoder ships it. Nothing is wrong with the file. Renaming the extension does not convert anything and often breaks playback — convert properly if a player rejects it.
- What bitrate should I use for AAC?
- 256 kbps for music you care about, 128-192 kbps for general listening, and 64-96 kbps for speech, where AAC still sounds decent. Because AAC is more efficient than MP3, you can drop roughly a third off your usual MP3 bitrate for equivalent quality.
- Will AAC play everywhere?
- On anything made in the last fifteen years, effectively yes: phones, browsers, cars, TVs and consoles all decode it. The gaps are old hardware players and some embedded or industrial devices, where MP3 remains the only safe choice.