File extension

.aac

Advanced Audio Coding (AAC)

.aac files are Advanced Audio Coding streams: lossy compressed audio, usually stored as raw ADTS frames, defined by the MPEG-2 and MPEG-4 standards as a successor to MP3.

Known meanings

Formats and reference material for .aac

Advanced Audio Coding (AAC)

Lossy perceptual audio codec from MPEG-2 and MPEG-4, stored as raw ADTS or ADIF streams

  • audio
  • ISO/IEC MPEG
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Details

Format notes

AAC

Advanced Audio Coding (AAC) is a lossy audio codec standardized by ISO/IEC MPEG, first in MPEG-2 (ISO/IEC 13818-7) and then extended in MPEG-4 Audio (ISO/IEC 14496-3). It was designed to give better quality than MP3 at similar bit rates. A file with the .aac extension normally holds a bare AAC stream without a richer container.

Stream Structure

Raw AAC files typically use ADTS (Audio Data Transport Stream), where every frame starts with a header whose first 12 bits are all ones (sync word 0xFFF). The header carries the profile, sampling rate, and channel configuration, so each frame can be decoded independently and the stream can be joined or cut at frame boundaries. A less common variant is ADIF, which has a single header at the start of the file and begins with the ASCII bytes ADIF. Metadata such as ID3 tags is sometimes prepended to ADTS files, but there is no standard tag format for the raw stream.

Adoption

AAC is widely supported by browsers, phones, streaming services, and broadcast systems. Most AAC audio is not distributed as bare .aac files but inside MP4 containers (.m4a, .mp4), in MPEG transport streams, or in HLS segments. The common media type for ADTS is audio/aac.

Preservation And Security Notes

AAC is lossy, so keep an uncompressed or lossless master where one exists and treat AAC as a delivery format. Because the raw stream has no index or standard metadata, wrapping it in MP4 is generally preferable for libraries. Decoders should validate frame lengths and header fields in untrusted input.

Further Reading

  • Library of Congress format description: https://www.loc.gov/preservation/digital/formats/fdd/fdd000114.shtml