Convert AAC to MP3

Where bare AAC files come from

AAC (Advanced Audio Coding) took over from MP3 in phones, streaming and video. Most of the time it sits inside an .m4a or .mp4 file, but some programs save it bare, as an .aac file:

• some voice and call recorder apps on Android phones,
• radio and stream recorders,
• tools that copy the sound track of a video into its own file without converting it.

A bare .aac file has no index, so some players can't show its length or jump around in it properly, and many upload forms and car stereos won't take it at all. An MP3 has none of those problems, which is why it is the usual way out. The conversion takes the AAC sound as it plays and writes it again as MP3, so whatever played before plays the same afterward, only in more places.

Keeping the quality

AAC is more efficient than MP3: at 128 kbps, AAC usually sounds better than MP3 does. So when you convert, don't go below the AAC's own rate. As it is takes care of that by copying the rate across: a 256 kbps AAC stays at 256, and anything under 128 kbps is lifted to 128, so a 96 kbps file doesn't lose more on the way. HE-AAC, the low-bit-rate flavor some radio streams and voice apps use, converts like any other AAC and lands on that floor: 128 kbps, or 64 for a mono file.

For music worth keeping, High (320 kbps) leaves the most margin for the move from one lossy format to another. For recorded calls and memos, Small is clear and compact: 128 kbps for stereo, 64 for a one-channel recording. Sample rate and channels stay as they were unless you change them under More settings, where you can also set an exact bit rate between 320 and 64 kbps for the whole batch. Unsure? Convert one file at two settings and compare them in your usual player before doing the whole folder.

Reading the file list

Each row in the list tells you what a file really holds, for example “12:04 · AAC · 96 kbps · 44.1 kHz · 8.6 MB”. That is the bit rate As it is starts from, and it is a quick way to spot a file that isn't what its name says: an .aac that turns out to hold MP3 sound, say, or a recording that is lower quality than you thought.

If a file can't be read at all, its row says so before you convert, and the rest of the list is unaffected. You can take it out with Remove and carry on. Above the list, Octoolo also adds up how many files you've added and their total size, so you know how big the job is before you start.

You may not need MP3 at all

If a device or app plays .m4a but refuses your .aac file, there is a lossless fix: convert to M4A with As it is. Octoolo moves the same AAC sound into an .m4a file without re-encoding it, and the row says Copied as it is, nothing re-encoded. It is instant and loses nothing, and the new file gets a proper index, so players show its length and can skip around in it. It works the other way too: an .m4a can become a bare .aac the same way, without re-encoding, if some device insists on .aac.

The difference between the two kinds of AAC file is explained on MP3 to AAC. For the sound of a video, use MP4 to MP3 instead.