.aac File Extension

What kind of file is a .aac?

A .aac file is usually a standalone Advanced Audio Coding stream framed as an Audio Data Transport Stream (ADTS). AAC is a lossy perceptual codec that discards audio information its model predicts will be difficult to hear. Standalone files normally use the audio/aac MIME type.¹

The extension does not cover every file that contains AAC audio. An M4A file commonly stores AAC inside an MP4 container, which adds timing tables and better support for metadata and artwork. It normally uses audio/mp4, not audio/aac.

ADTS frames begin with a 12-bit syncword of all ones, so common files start with bytes such as FF F1 or FF F9. IANA does not register one fixed magic number, however. A reliable check parses consecutive frame headers rather than trusting the suffix or only two bytes.¹

How do I open a .aac on my computer?

VLC opens raw AAC on Windows, macOS, and Linux. FFmpeg can inspect, decode, remux, and convert it.²³ Built-in player support depends on the framing and AAC profile.

Browser support for raw .aac streams is less consistent than support for AAC in MP4. MDN documents ADTS playback in Firefox and Safari, with some decoding supplied by the operating system.⁴ For dependable web delivery, wrap compatible AAC-LC audio in M4A or MP4 and serve it as audio/mp4.

Poly support for .aac

Poly recognizes .aac filenames as audio. It builds a waveform, reads available duration and tags, and uses embedded cover art when present. Spoken content is transcribed for search, while audio similarity helps find recordings that sound alike. Matches can jump to the relevant moment.

The Poly agent can use the audio and its generated transcript. Playback still depends on the browser being able to decode the particular AAC stream, and music without intelligible speech may have little useful transcript text.

History of .aac

AAC was first standardized in 1997 as MPEG-2 Part 7, ISO/IEC 13818-7. MPEG-4 Part 3 later incorporated and expanded the family.¹ The design was not constrained by backward compatibility with MPEG-1 audio, which gave its developers more freedom to improve compression. MPEG's technical overview described transparent quality relative to stereo CD source material at 128 kb/s for its evaluated audio, while also stressing multichannel uses.⁵

Profiles include AAC Low Complexity, HE-AAC, and HE-AAC v2. A decoder that accepts common AAC-LC may not accept every profile or framing.

Working with .aac and alternatives

Choose a target based on the job:

Remux when only the wrapper needs to change. Transcode when the destination cannot decode the profile or you need a different codec, sample rate, or channel layout. Keep the best original because repeated lossy conversion can accumulate artifacts.