What is a .AAC file?
The typical usage of the file is to serve an elementary stream of Advanced Audio Coding. Usually, the codec's file extension does not represent an actual file container, being encapsulated in ADTS instead of MP4/M4A files that hold both audio and video streams along with a rich metadata. These files store the compressed sound track and nothing more; chapters, artwork and container metadata are not found inside an AAC file.
One can get such a file by capturing a broadcasted stream in the raw AAC format, or extracting it from an MP4/TS stream, mobile recordings, web downloads. It can come from the result of audio transcoding software or some media players with the ability to save its output stream in AAC files. Typically, it should be presented in ADTS format, which is defined with the presence of a brief frame header. There is one less frequently met variant called ADIF format, which starts with an ASCII sequence ADIF and goes on with the file header followed by the audio payload. LATM LOAS can also appear with .AAC extension, mostly used in streaming scenarios.
The formats difference could be seen in files with the help of MediaInfo/FFprobe and some hex editor tools. ADTS files start with FF F1 or FF F9 headers; ADIF - with 41 44 49 46 and LOAS with 0x2B7 pattern sequence.
How to open a .AAC file
Open a .AAC file on Windows
- 1 Go to the folder with your file in File Explorer. Usually, downloaded files end up in Downloads.
- 2 Double-click your .AAC file if you have an app for .aac files on your computer. In case it does not open, move on to step three.
- 3 Right-click your .AAC file -> select .
- 4 Choose VLC media player in the list (Windows 11: choose Always). For Windows 10 -> put a check on "Always use this app to open .aac files" and click OK.
Still not opening? See common problems and fixes, or view it in your browser.
Programs for Windows
- VLC media player open-source
- foobar2000 free
- Audacity open-source
- Windows Media Player free
- AIMP free
- MediaInfo open-source
- Adobe Audition paid
- EZ CD Audio Converter paid
Open a .AAC file on macOS
- 1 In the Finder app, locate your .AAC file.
- 2 Just double-click your .AAC file in order to automatically launch the program which is connected with it.
- 3 Need another app on your Mac? Control-click the file (or right-click with a mouse). In the pop-up menu select .
- 4 If it isn't in the list, choose and find the application manually.
Still not opening? See common problems and fixes, or view it in your browser.
Programs for macOS
- VLC media player open-source
- foobar2000 free
- Audacity open-source
- Apple Music freemium
- QuickTime Player free
- MediaInfo open-source
- Adobe Audition paid
- SoX open-source
Open a .AAC file on Linux
- 1 Launch your file manager. Depending on the desktop it may be named Files (GNOME), Dolphin (KDE Plasma), Nemo (Cinnamon) or Thunar (Xfce).
- 2 Double-click the file.
- 3 Or choose another program by right-clicking the file -> .
- 4 Install VLC media player (if it is not installed on your computer) from the software centre, repository or Flathub.
Still not opening? See common problems and fixes, or view it in your browser.
Programs for Linux
- VLC media player open-source
- Audacity open-source
- MediaInfo open-source
- SoX open-source
- FFmpeg open-source
Open a .AAC file on Android
- 1 Launch Files or another file management app.
- 2 In Downloads, find the .AAC file and open it by tapping.
- 3 A list of apps capable of opening the file will be shown. Tap VLC media player and press Always to permanently connect the file type with this app.
- 4 No apps able to open this file? Download VLC media player from the Google Play Store.
Still not opening? See common problems and fixes, or view it in your browser.
Programs for Android
- VLC media player open-source
- foobar2000 free
- Apple Music freemium
- AIMP free
- MediaInfo open-source
Open a .AAC file on iPhone / iPad
- 1 Use the Files app and go to On My iPhone, iCloud Drive or Downloads.
- 2 Tap the .AAC file you want to open.
- 3 If the file can be opened by a built-in iOS app (supported format), it opens automatically.
- 4 Touch and hold the file to get the context menu -> choose -> select VLC media player from the list of apps.
- 5 If there is no such option, download the app first from the App Store and then tap the document again.
Still not opening? See common problems and fixes, or view it in your browser.
Programs for iPhone / iPad
- VLC media player open-source
- foobar2000 free
- Apple Music freemium
- MediaInfo open-source
Open a .AAC file online
- 1 Move to the online viewer at the top of this page.
- 2 Drag the file there or select it, and its contents will appear in your browser tab. The file is not uploaded.
Still not opening? See common problems and fixes, or view it in your browser.
Programs for Online
- MediaInfo open-source
More details about opening .AAC files
For windows, the most suitable apps that allow playing the ADTS .aac files with the beginning code “FF F1” include VLC media player, foobar2000, and AIMP for free, Windows Media Player, Adobe Audition, and EZ CD Audio Converter that payed software. However, it is still possible that double click will not do its magic, in case like this one open the VLC media player (or Audacity) manually and choose main menu.
The most appropriate programs on Mac OS includes VLC, Audacity, FFmpeg and Apple's own applications - QuickTime and Apple Music. Although Quick time music player and Apple Music are great options for usual playback (they have a lot more functions). And only for check purpose of files we should use VLC, Audacity and FFmpeg.
Linux-based machines require us to install certain media codecs before we are able play the AAC with VLC, and some other GStreamer or FFmpeg bases programs. There are some problems with this file type on android, the best choices of software is the following: VLC, foobar2000, AIMP (some build-in media libraries may work as well).
The best ways to preview the file on iOS devices are Apple's own software, VLC media player (on iOS devices VLC works as app), or foobar2000. It should be noted that support of web browser's raw format is inconsistent. HTML5-audio is a good solution for playing AAC inside the container like .m4a and .mp4. However, before any manipulation with file, please, run such program as MediaInfo/ffprobe. Do not assume every .AAC is an Apple Music download. Apple's Music has its format (it is usually AAC in a container) inside the .mp4/.m4a containers (not .aac file).
Technical details
In AAC codec the transform, perceptually motivated lossy encoding makes use of a modified DCT (discrete cosine transform - MDCT), psychoacoustic bit allocation and scales with possible use of various techniques like TNS (temporal noise shaping). The AAC raw audio file on disc contains audio data represented by an element stream not inside an MP4 (M4A) package. The stream could use either ADTS (audio data transport stream), ADIF or LATM and LOAS (layered open stream) format packaging. For ADTS the header begins with a particular 12-bit number, syncword, FF F1 or FF F9 for no-CRC (Cyclical Redundancy Check) MPEG 4-AAC streams. The ADTS frame has a 7-byte header, or a 9-byte header when using CRC, plus an AAC access unit. For ADIF the package consists of "ADIF"-encoded header that places stream information data closer to its beginning without frame headers in frames. The synchronization word of LOAS/LATM is an 11-bit value 0x2B7 = 0b01010110111 or 0x56 E0 through 0x56 FF.
There are popular stream profile types: AAC LC (Low Complexity), HE-AAC v1 with Spectral Band Replication (SBR) (high-efficiency MPEG-4 audio), v2 including Parametric Stereo, AAC-ELD (enhanced low delay) and AAC-LD and other AAC types with a set of object configuration. The sampling frequencies of 44.1kHz and 48 kHz; mono, stereo, multichannel layouts and the possible bitrate varies from 24 to 320 kbps for voice and music respectively. AAC specifications are available on ISO/IEC 14496-3 standard up to 2019 updates.
Metadata amounts in a raw aac audio file are lesser compared with the M4A package - more detailed metadata about the album, picture file, gapless playback information and precise timing data should be added to an ISO BMF (base media file) package. Raw (elementary) audio data file uses the "audio/aac" registered MIME, despite usage of "audio/aacp" for streaming HE-AAC audio.
The ".aac" audio format file (stream data) has no human-readable text elements.
How to convert .AAC files
Changing .aac format to .m4a format keeps unchanged AAC bitstreams enclosed in MPEG-4 format but provides improved support for storing the metadata. The use of stream copy option in FFmpeg command line is constrained by compatibility of the source and target files: ffmpeg -i input.aac -c:a copy output.m4a.
Save file with .mp3 extension in case of legacy players and software or car stereo. In FFmpeg, re-encode using libmp3lame plugin to output mp3 audio stream. For this task, the following command should be used: ffmpeg -i input.aac -codec:a libmp3lame -b:a 192k output.mp3. It represents lossy to lossy compression process leading to loss in quality of the audio.
The .wav or .aiff extensions are preferable for accurate editing of DAW tracks. These two file formats contain PCM audio data that ensures correct placement of the samples during trimming and bouncing. An AAC file could be opened in Audacity and edited and then exported as WAV, MP3, FLAC, OGG, or M4A if the corresponding encoder is installed.
If a lossless copy of the decoded sound wave is desirable, select .flac as an output format; PCM audio stream will be compressed there without losses but it won’t store the original high-quality waveform obtained in the studio. Modern low-bit rate distribution needs converting to OGG Opus with the help of libopus; example command: ffmpeg -i input.aac -codec:a libopus -b:a 128k output.ogg.
| Convert to | Tool | Command / note |
|---|---|---|
| MP3 | FFmpeg | ffmpeg -i input.aac -codec:a libmp3lame -b:a 192k output.mp3 Useful for older players; lossy-to-lossy transcoding reduces quality. |
| WAV | FFmpeg | ffmpeg -i input.aac -codec:a pcm_s16le output.wav Decodes to uncompressed PCM for editing; file size becomes much larger. |
| M4A | FFmpeg | ffmpeg -i input.aac -c:a copy output.m4a Wraps the AAC stream in an MPEG-4 container without re-encoding when compatible. |
| FLAC | FFmpeg | ffmpeg -i input.aac -codec:a flac output.flac Lossless after decoding, but it cannot restore information already lost in AAC compression. |
| OGG Opus | FFmpeg | ffmpeg -i input.aac -codec:a libopus -b:a 128k output.ogg Good for modern streaming and speech; transcoding is lossy. |
| AIFF | Audacity | Open the .AAC file, optionally edit it, then use File > Export Audio > AIFF. |
Common problems and fixes
The file should be either genuinely raw AAC, complete in stream, and not contain encrypted information. The file with .aac ending can simply have another container format, mistakenly called AAC and, accordingly, will be played by some media players as noise. “Invalid data found when processing input” is the typical message in case there are no sync bytes of ADTS, ADIF and LATM/LOAS stream at the start of file, that happens after the stream was partially downloaded or partially captured. “Header missing” or synchronization problems occur, if the recording starts inside an ADTS packet without the original header being registered (7 or 9 bytes); in these cases try recovery from long source files or wrapping the stream using FFmpeg.
Some audio players cannot recognize HE-AAC or LATM/LOAS audio streams in addition to supporting the most basic AAC-LC and, as the result, a file which won't work in one program might just play in VLC. Conversion into the format encapsulated in MP4 container usually solves playback duration and seeks problems for these files: ffmpeg -i input.aac -c:a copy output.m4a. Raw audio streams usually lack duration metadata; consequently, audio players show time of 00:00 or scrub incorrectly, so conversion of audio file into container is required. Renamed .m4a files may cause problems when converting in FFmpeg as they start from MP4 ftyp box and not audio sync; change ending into m4a again, in this situation. If the .AAC files open not in the designated player, then change the program association: VLC media player, Apple Music, WM player. DRM encoded content from iTunes music store is in .m4p; as the result, codec repairers won't help in authorizing.
Security considerations
The AAC files stream doesn't consist of macros, documents scripts, author-run commands, or executable material as well. The greatest potential vulnerability is caused by vulnerabilities of the media software parser and decoder: ADTS headers can be corrupted, wrong in their frame size or simply fuzzed and thus impact FFmpeg, browser media engine, as well as precompiled codecs packages. Keep up to date your VLC program, FFmpeg program, browser media engine, and codec pack which is included into an operating system; refrain from opening unknown AAC files with older applications or outdated codec packages inside the application you use. Raw .aac files contain practically no user-related data although the data can also come from the file's route, or name, and in case it is converted to an M4A file there would be information such as title, performer's name and name of the account or device. Use MediaInfo or ffprobe in the first place before playing the stream.
History and versions
AAC was conceived out of MPEG audio technology in the early '90s as the successor to MP3, with the goals of having improved quality at roughly equal bitrates. Its first incarnation (known officially as 'AAC') appeared standardized in 1997 in form of MPEG-2 AAC as ISO/IEC 13818-7, covering three profile formats (Main Profile, Low Complexity, and Scalable Sample Rate profiles). In 1999, MPEG-4 Audio was defined with MPEG-4 Audio/AAC (ISO/IEC 14496-3), adding a range of object types to this extended AAC codec family. Further profile developments introduced the High Efficiency (aka Enhanced Enhanced AAC, aka HE-AAC and then HE-AAC v1) with Spectral Band Replication (used for improving very low bitrate encodings), followed by HE-AAC v2 which featured the Parametric Stereo functionality (used to encode extremely low-bitrate stereo data streams). It became well-known to consumers via the iTunes/iPod systems developed and released by Apple, however, instead of using the actual .aac files, it preferred packaging its encoded audio in the .m4a format.
Related formats
This combination enables tagging, cover-art inclusion, gapless playback data, and improved seeking features unavailable on a mere elementary .aac stream. The MP3 format is older and the safest choice for older car radios and devices as compared to the AAC which usually gives equally perceived audio qualities despite lower bit rates compared to MP3. The OPUS provides an upper hand in audio quality in cases of speech calls and online streaming despite lower bitrates. In fact AAC is decoded better by many older handheld device decoders than is the case with the Opus. As an open format, the OGG format supports Opus, as well as, can carry Vorbis audio and is good for use on Web applications. However, AAC formats remain more common in Apple software as well as broadcast gear. The WAV is useful in storing audio files meant for editing but generates large audio files as against AAC audio.
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