AAC and the .aac file
AAC (Advanced Audio Coding) is the successor of MP3. At the same bitrate it usually sounds cleaner, especially below 160 kbps. It is the standard sound format for MP4 videos, YouTube, iPhone and most streaming services. This page saves AAC as a raw stream in a file with the .aac extension.
A raw .aac file has no container around it. That keeps it simple and small, and it works well with tools that expect a plain AAC stream, many car stereos, some Android players and audio processing software. The downside is weak support for tags and cover images, and some music libraries do not import it. For iTunes, Apple Music and phone libraries, the same AAC sound in an M4A container is usually better: see AVI to M4A.
What happens to the AVI sound
AVI files rarely contain AAC. The usual audio codecs in AVI are MP3, AC-3 and uncompressed PCM. So when you convert AVI to AAC, the sound is almost always re-encoded. The tool tells you in advance whether the track can be copied as it is or needs conversion.
Some practical notes:
- Converting MP3 sound to AAC is a second lossy step. Use a bitrate equal to or higher than the original, or the quality will drop.
- From PCM camera sound, AAC at 128 to 192 kbps sounds very close to the original and is about ten times smaller.
- Dolby Digital (AC-3) sound from a DVD rip cannot be stored in a .aac file, so it is always converted.
Choosing between AAC and MP3
Both play on nearly every phone and computer today. AAC gives a little better quality at the same size, while MP3 is supported by even the oldest devices, such as early MP3 players and car radios. If you are not sure the device supports AAC, choose AVI to MP3.
Private and offline
The AVI is read and the sound is encoded on your own computer. Nothing is uploaded, which matters for interviews, lessons and personal recordings. After the page has loaded, the tool also works without an internet connection. You can extract audio from a whole batch of AVI files and download them as one ZIP. Other targets for AVI are on the AVI converter page.