Everything you need to know about AAC
AAC (Advanced Audio Coding, .aac) is the successor to MP3, standardized as part of MPEG-2 in 1997 and refined in MPEG-4 (2003). It's the default audio codec in iTunes, Apple Music, YouTube, and most modern streaming. AAC produces noticeably better quality than MP3 at the same bitrate, especially below 128 kbps.
How it works under the hood
- Subbands and MDCT. AAC uses a more sophisticated frequency analysis than MP3 - more subbands, better psychoacoustic models, longer transforms.
- Profiles. AAC-LC (Low Complexity) is the standard. HE-AAC adds Spectral Band Replication for low bitrates. xHE-AAC (Extended) is the 2014 version optimized for variable bandwidth streaming.
- Container variants. AAC streams come in two wrappers: ADTS (raw audio with transport headers) and MP4 (.m4a). M4A is what iTunes uses; ADTS is what radio streams use.
- Patent-free since 2017. Most AAC patents have expired. Some HE-AAC patents are still active, but base AAC-LC is now royalty-free for most use cases.
Where you'll actually use it
- Apple Music, iTunes purchases, Apple Podcasts (M4A wrapper)
- YouTube's preferred audio (AAC-LC at 128 kbps)
- DAB+ radio broadcasting (HE-AAC v2)
- Modern streaming services that need quality over compatibility
How it compares to alternatives
AAC vs MP3: AAC sounds better at every bitrate, especially below 128 kbps. AAC vs Opus: Opus wins for voice and low bitrates; AAC is better supported on Apple devices. AAC vs FLAC: Use AAC for distribution, FLAC for archival.
Things that will trip you up
- Raw .aac (ADTS) won't play in some media libraries - wrap it in M4A (.m4a) for broader compatibility
- iTunes has historically used 256 kbps AAC for purchases - that's where the 'iTunes Plus' label came from
- HE-AAC and HE-AAC v2 add complexity at the encoder side - older devices may decode them as plain AAC-LC
Test it yourself: iTunes/Music app on Apple devices, foobar2000 on Windows, FFmpeg's `aac_at` (Apple AudioToolbox) encoder for best quality on macOS.