What is the AAC file format?
An .aac file is an Advanced Audio Coding file that implements advanced encoding and compression for storing digital audio. Developed by MPEG (Fraunhofer IIS, Dolby, Sony, and Nokia), AAC is widely supported by players such as Music (formerly iTunes), VLC, and Windows Media Player. The .aac format is similar to MP3 but delivers better sound quality at the same bitrate and handles audio signals at 16 kHz and higher more effectively.
A bare .aac file holds a raw ADTS bitstream - distinct from the .m4a container, which packages AAC audio inside an MPEG-4 file. Common profiles include AAC-LC, HE-AAC v1/v2, and xHE-AAC (USAC), with sample rate support from 8 kHz to 96 kHz.
Security & safety
RISK: LOWRaw AAC files are passive audio data and do not execute code. The main practical risk is fake "AAC to MP3 converter" download sites bundling adware/PUPs - prefer VLC, Audacity or the FFmpeg-based converters above. Files from unknown sources should still be scanned, but the format itself carries no macro/script payload.
Format details
in a nutshellFULL NAMEAdvanced Audio Codingaka AAC audio, MPEG-4 AAC
DEVELOPERMPEG (Fraunhofer IIS, Dolby, AT&T, Sony, Nokia)since 1997 (MPEG-2 Part 7); MPEG-4 AAC in 1999
CATEGORYAudio Files
MIME TYPEaudio/aac
TYPELossy compressed audio (raw AAC bitstream, usually ADTS-framed)
STANDARDOpen · royalty-free
MAGIC BYTES · FILE SIGNATURE
OFFSET
0001
HEX
FFF1
ASCII
··
A raw .aac file is normally an ADTS stream that starts with the 12-bit sync word 0xFFF; the second byte is commonly F1 (MPEG-4, no CRC) or F9 (MPEG-2). Some .aac files are ADIF streams beginning with the ASCII "ADIF". AAC stored inside an MP4 container is instead a .m4a/.mp4 file ("ftyp" at offset 4), not a raw .aac file.
Programs that open AAC files
Windows4 apps
macOS2 apps
Linux2 apps
Android1 app
iOS1 app
Technical details
deep spec| MIME type | audio/aac; also audio/aacp, audio/x-aac, audio/x-hx-aac-adts |
| Magic bytes | FF F1 (MPEG-4, no CRC) or FF F9 (MPEG-2, no CRC) at offset 0; ADIF streams begin with ASCII "ADIF" |
| File structure | Raw ADTS bitstream: sequential self-syncing frames each with a 7-byte header (9 bytes with optional CRC) followed by one coded audio block |
| Frame duration | 1024 PCM samples per ADTS frame, approximately 21.3 ms at 48 kHz |
| Byte order | Big-endian |
| Compression | Lossy; Modified Discrete Cosine Transform (MDCT) with perceptual noise shaping and temporal noise shaping (TNS) |
| Audio Object Types | AAC-LC (type 2), HE-AAC v1/SBR (type 5), HE-AAC v2/PS (type 29), xHE-AAC/USAC (type 42) |
| Sample rate support | 8 kHz to 96 kHz; most common values are 44.1 kHz and 48 kHz |
| Channel support | Mono, stereo, 5.1 surround, and up to 48 discrete channels |
| Typical bitrate | 96-320 kbit/s for stereo music (AAC-LC); HE-AAC effective at 32-80 kbit/s; xHE-AAC usable from 12 kbit/s |
| Typical file size | Approximately 1 MB per minute at 128 kbit/s stereo |
| CRC integrity | Optional 16-bit CRC per ADTS frame; presence signalled by the protection_absent flag in the frame header |
| DRM support | Absent in raw .aac files; DRM applied only when AAC is wrapped in a protected MPEG-4 container (.m4p) |
| Container distinction | A bare .aac file is a raw ADTS elementary stream; AAC inside MPEG-4 is distributed as .m4a, .mp4, or .m4b |
| Released | 1997 (MPEG-2 Part 7); MPEG-4 AAC in 1999 |
| Latest version | MPEG-4 AAC (ISO/IEC 14496-3); profiles incl. AAC-LC, HE-AAC v1/v2, xHE-AAC (USAC, 2012) |
| Open standard | Yes · royalty-free |
| Specification | www.iso.org |
AAC conversions
Possible
.AAC .WMAOnline converter
How to convert
.AAC .AC3Online converter
How to convert
.AAC .AMROnline converter
How to convert
.AAC .AIFOnline converter
How to convert
.AAC .APEOnline converter
How to convert
.AAC .3GAOnline converter
How to convert
.AAC .AIFFOnline converter
How to convert
.AAC .MP3Online converter
Yes, AAC converts to MP3 without trouble -- the fastest way is one FFmpeg command: ffmpeg -i file.aac…
How to convert
.AAC .WVOnline converter
How to convert
.AAC .OGGOnline converter
Yes, AAC converts to OGG without problems - the fastest way is VLC (Media > Convert/Save).
How to convert
.AAC .WAVOnline converter
Yes - FFmpeg, VLC, or Audacity converts AAC to WAV in seconds; no further quality is lost during the decode.
How to convert
.AAC .M4AOnline converter
Yes, and in most cases it is a container re-wrap with no re-encode -- the fastest method is one FFmpeg…
How to convert
.AAC .M4ROnline converter
How to convert
.AAC .OPUSOnline converter
How to convert
Community Q&A
asked by usersNo questions yet - be the first to ask about AAC files.
Frequently asked questions
What is an AAC file?
It is a lossy compressed audio file using Advanced Audio Coding, the successor to MP3. It sounds better than MP3 at the same bitrate and is Apple/iTunes' and YouTube's default audio codec.
How do I open an AAC file?
Play it with VLC (any OS), Windows Media Player on Windows 10/11, or the Music app on Apple devices. To edit it, open it in Audacity.
What is the difference between AAC and M4A?
Same audio codec, different container. A raw .aac file is the bare AAC bitstream; an .m4a wraps that AAC audio in an MP4 container with metadata. Most 'AAC' music files are actually .m4a.
Is AAC better than MP3?
Generally yes - at the same bitrate AAC preserves more detail, especially at low bitrates and high frequencies. MP3 is more universally compatible with very old devices.
How do I convert AAC to MP3?
Use VLC (Media - Convert/Save), Audacity, or fre:ac. It is a re-encode, so there is a small quality loss; keep the original if you can.
Why won't my AAC file play on an old device?
Some older players only support MP3 or only AAC-in-MP4 (.m4a). Convert the raw .aac to MP3, or re-wrap it as .m4a, to get it playing.
References
1ISO/IEC 14496-3 - MPEG-4 Audio (AAC)www.iso.org
2Fraunhofer IIS - AACwww.iis.fraunhofer.de
