What Is AAC? The Codec Inside M4A Files Explained

AAC vs MP3: what actually changed

MP3 was standardised in 1993. AAC followed in 1997 as part of the MPEG-2 standard, then improved further under MPEG-4. The goal was to fix the known limitations of MP3: better handling of high frequencies, improved stereo coding, support for up to 48 channels, and a more efficient psychoacoustic model.

The practical result is measurable: AAC typically matches MP3 quality at about 70% of the bitrate. A 128 kbps AAC file generally sounds better than a 128 kbps MP3. A 256 kbps AAC file is considered transparent (indistinguishable from the original) by most listeners — the same threshold for MP3 sits closer to 320 kbps.

The AAC family: LC, HE, HEv2

"AAC" refers to a family of profiles, not a single codec. The three most common are:

AAC-LC (Low Complexity)

The standard profile used for music and general audio. What you get when you encode to AAC in iTunes, Adobe Audition, or most converters. Best quality at mid-to-high bitrates (96–320 kbps).

HE-AAC (High Efficiency, v1)

Adds Spectral Band Replication (SBR) — a technique that encodes the upper frequencies separately at very low cost. Used for streaming and broadcast at bitrates from 32–128 kbps. Sounds dramatically better than AAC-LC at low bitrates.

HE-AAC v2

Adds Parametric Stereo on top of HE-AAC v1 — encodes stereo information as a single mono channel with spatial parameters. Used for very-low-bitrate streaming (16–48 kbps). Common in digital radio and internet radio broadcasting.

AAC and M4A: container vs codec

AAC is the codec — the compression algorithm. M4A is the container — the file wrapper. An M4A file almost always contains AAC audio, but the file extension and the codec are technically separate things. The same AAC audio can also be stored in an MP4 container (common for video with audio) or in an ADTS stream (.aac file extension).

This matters practically: when you convert an M4A file to MP3, you're decoding the AAC codec, discarding the M4A container, re-encoding to MP3, and wrapping it in an MP3 container. You're changing both the codec and the container in one step.

Bitrate guide for AAC

Bitrate	Profile	Quality	Best for
32–64 kbps	HE-AAC v2	Acceptable for voice	Internet radio, voice streaming
64–96 kbps	HE-AAC	Good for music streaming	Low-bandwidth streaming, podcasts
128 kbps	AAC-LC	Transparent for most — better than 128k MP3	General music distribution
192–256 kbps	AAC-LC	Transparent — indistinguishable by most listeners	Streaming platforms, downloads
320 kbps	AAC-LC	Maximum quality	Archiving lossy masters, audiophile use

Where AAC is used

AAC is the dominant codec in the Apple ecosystem — every song on Apple Music, every audio track in a video purchased from iTunes, every voice memo recorded on an iPhone is AAC. iOS and macOS treat AAC as the default lossy codec.

Beyond Apple, AAC is the audio track format for most MP4 video, the codec used by YouTube (typically 128–256 kbps AAC-LC), and the audio format for DAB+ digital radio (HE-AAC). Android has supported AAC natively since version 3.1. Most modern Bluetooth headphones support AAC as a high-quality Bluetooth audio codec alongside SBC.

When to use AAC and when to use MP3

Use AAC when:You're in the Apple ecosystem, uploading to YouTube or a streaming platform, or distributing to devices manufactured after 2010 where AAC support is near-universal.
Use MP3 when:Compatibility is the priority — sharing with unknown recipients, uploading to services that specifically request MP3, or targeting legacy hardware (car stereos, old MP3 players) that may not support AAC.
Use neither when:Archiving or editing. Both AAC and MP3 are lossy — they discard audio data permanently. Use FLAC or WAV for anything you'll need to re-edit or re-export at full quality.

Converting to and from AAC

Converting a lossless source (WAV, FLAC) to AAC produces a clean result at the chosen bitrate — no generation loss, no accumulated artifacts. This is the correct way to produce a distribution copy from an archive master.

Converting AAC to MP3 (or any other lossy format) involves decoding the AAC and re-encoding to MP3. Both codecs introduce their own artifacts, and the re-encoding step compounds them — the output will be lower quality than either a direct MP3 encode or the original AAC. Do this only when a specific format is required and no lossless source is available.

What Is AAC?