What an AAC file is
AAC (Advanced Audio Coding) is the lossy codec that replaced MP3 in many places. Apple Music, YouTube, most streaming services and almost every phone use it, because it sounds better than MP3 at the same bitrate. Usually AAC sits inside an MP4 container with the extension .m4a. A file ending in .aac is different: it is a raw AAC stream, often called ADTS. You get such files from screen recorders, some voice recorder apps, radio streams, Bluetooth tools and audio extracted from videos.
Raw AAC has no container, so it carries no tags, no cover art and no index for seeking. Some players show a wrong duration or cannot jump to the middle of the file, and many programs, including older versions of Windows Media Player and some video editors, refuse .aac files completely. Converting fixes these problems.
Which format to convert AAC to
- Choose AAC to MP3 when the file must play on old players, car radios or any software at all.
- Choose AAC to M4A to keep the AAC codec but gain a proper container with tags and reliable seeking.
- Choose AAC to WAV or AAC to AIFF for editing software that only takes uncompressed audio.
- Choose AAC to FLAC if your library or player is set up for FLAC.
- Choose AAC to OGG or AAC to OPUS for games, web projects and chat apps.
- Choose AAC to WMA only for old Windows devices that need it.
- Choose AAC to M4R for an iPhone ringtone, or AAC to MMF for a classic feature phone.
- Choose AAC to MIDI to get the notes of a melody, or AAC to MP4 to make a video for upload.
Quality: what changes and what does not
AAC is already lossy. Every conversion first decodes it to plain audio, then encodes again. Two cases follow from that:
To a lossless format (WAV, AIFF, FLAC) nothing more is lost, but nothing is gained either. A 128 kbps AAC saved as WAV still sounds like a 128 kbps AAC, it is just about ten times larger.
To another lossy format (MP3, OGG, WMA, or AAC again inside M4A) there is a second round of compression. MP3 is less efficient than AAC, so give it a higher bitrate: a 128 kbps AAC needs about 192 kbps as MP3 to sound the same. For Opus the same bitrate as the source is enough.
| Source AAC | Suggested MP3 | Suggested Opus |
|---|---|---|
| 96 kbps | 160 kbps | 96 kbps |
| 128 kbps | 192 kbps | 128 kbps |
| 256 kbps | 320 kbps | 192 kbps |
Batch conversion
You can add dozens of AAC files at once, for example all the clips from a recorder. They are converted one after another with the same settings, and each finished file can be saved right away or together with the others in one ZIP file. The engine loads once and then works offline. Sample rate and channels are kept unless you change them; switching speech recordings to mono lets you lower the bitrate without a loss you can hear.
Common problems
If an AAC file shows the wrong length in your player, converting it to M4A or MP3 usually fixes that, because the new file gets a proper header. HE-AAC files (a low-bitrate variant used by internet radio) decode fine, but they often sound thin; do not expect the converted file to sound fuller. Files protected by DRM, such as downloads from streaming apps, cannot be opened by any converter. For simple trimming after conversion, use Cut audio.