AAC (Advanced Audio Coding)
The audio format that made streaming music sound better than MP3 at half the file size.
| Full name | Advanced Audio Coding |
| Extension | .aac |
| MIME type | audio/aac |
| Developer | Fraunhofer IIS, AT&T Bell Laboratories, Dolby Laboratories, Sony, Nokia |
| Released | 1997 (ISO/IEC 13818-7, MPEG-2 Part 7) |
| Type | Lossy audio compression |
| Max channels | 48 audio channels |
| Common bitrates | 96, 128, 192, 256 kbps |
What is a AAC file?
AAC is a lossy audio compression format standardized in 1997 as part of the MPEG-2 specification. It was designed as the successor to MP3, offering better sound quality at the same or lower bitrates. Apple adopted it as the default format for iTunes and the iPod, which made it one of the most widely used audio formats in the world.
AAC stores audio by discarding sounds the human ear is least likely to notice, then encoding the rest with high efficiency. It supports up to 48 audio channels, compared to MP3's two-channel stereo limit. The format comes in several profiles, with AAC-LC (Low Complexity) being the most common for music and podcasts. HE-AAC (High Efficiency AAC) extends the base format with Spectral Band Replication, making it suited for low-bitrate streaming.
History
AAC was developed through a joint effort by Fraunhofer IIS, AT&T Bell Laboratories, Dolby Laboratories, Sony, and Nokia, and was declared an international standard by the Moving Picture Experts Group in April 1997. It was first published as MPEG-2 Part 7 (ISO/IEC 13818-7), then incorporated into the broader MPEG-4 standard in 1999 as ISO/IEC 14496-3. Apple's adoption of AAC for the iTunes Music Store in 2003 drove its mass-market spread, and it remains the default audio codec for Apple devices, YouTube, and many streaming platforms today.
How it works
AAC files are typically wrapped in one of two transport containers: ADTS (Audio Data Transport Stream) or ADIF (Audio Data Interchange Format). ADTS places a short sync header before each audio frame, which allows players to seek to any point in the file and recover from corruption mid-stream. ADIF puts all decoder configuration into a single header at the start, making it efficient for file exchange but not for random seeking. In practice, AAC audio is also commonly stored inside MP4 or M4A containers, which add chapter markers, cover art, and full metadata support.
What it is used for
- Streaming music on Apple Music, YouTube, and similar platforms
- Audio tracks inside MP4 video files for web and mobile playback
- Podcasts and audiobooks distributed to Apple Podcasts listeners
- Low-bitrate voice transmission in digital broadcasting (DAB+, DVB)
How to open it
Most media players open AAC files directly, including Apple Music, VLC, Windows Media Player (with codec), and Android's built-in player. If you need to edit or convert an AAC file, tools like Audacity, FFmpeg, or the converter on this site handle it without needing extra software.
Pros and cons
Strengths
- Better audio quality than MP3 at the same bitrate, especially below 192 kbps
- Native support on Apple, Android, and most modern browsers
- Supports up to 48 channels, covering stereo, surround, and multichannel audio
- HE-AAC variant keeps audio intelligible at very low bitrates (32–64 kbps)
Trade-offs
- Lossy compression means some audio data is permanently removed
- Not as universally supported as MP3 on older or embedded hardware
- Bare .aac files lack metadata support; cover art requires an MP4/M4A wrapper
- Patents historically restricted open-source implementations, though most have expired
Convert AAC files
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From AAC
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AAC FAQ
Is AAC better than MP3?
At the same bitrate, AAC generally produces better sound quality than MP3, particularly at 128 kbps and below. Most listeners cannot tell the difference at 256 kbps in either format, but AAC has a clear technical edge at lower bitrates.
Can I play AAC files on Windows?
Yes. VLC plays AAC files on any platform without extra setup. Windows Media Player can play them with the right codec installed. Modern versions of Windows 10 and 11 also include native AAC support through the Films and TV app.
What is the difference between AAC and M4A?
M4A is a file container (an MP4 file holding only audio) that usually contains AAC-encoded audio inside. The .aac extension refers to a raw AAC bitstream, often wrapped in ADTS. Both play the same audio; M4A adds metadata like track title, artist, and album art.
Does converting MP3 to AAC improve quality?
No. Converting between lossy formats never recovers lost audio data. The result is a file that has been compressed twice, which can introduce extra artifacts. If you need AAC, encode from the original uncompressed source whenever possible.