Convert AAC to MP3

Going from the better codec to the more compatible one

AAC beats MP3 at every bitrate — it was designed a decade later specifically to do so, and the margin is widest at the low end. So this conversion is a downgrade, taken deliberately for compatibility, and the honest way to judge it is not "did it sound as good" but "did the thing that refused the AAC now play it". Car stereos, older DAP players, gym equipment, voice recorders and a surprising amount of broadcast software still want MP3 and nothing else.

Both are lossy, so this is a transcode: the AAC is decoded back to sound and encoded again by a codec that disagrees with it about what is inaudible. The MP3 encoder cannot tell which parts of what it receives were the original recording and which were the AAC encoder's approximations, so it faithfully spends bits on both. Encoding at a little above the source bitrate — 192 kbps for a 128 kbps AAC — is what keeps the second generation from adding anything you can hear; encoding below it compounds two lots of loss and is where transcoded audio gets its bad reputation.

One case converts noticeably worse than the rest, and it is worth recognising. AAC has a low-bitrate mode — used for streaming and internet radio, typically at 32 to 64 kbps — that reconstructs the high frequencies from a compact description rather than storing them. MP3 has no equivalent mechanism, so those frequencies must be encoded literally, and the result is either a much larger file or an audibly duller one. If a converted stream recording sounds flat, this is why, and no setting here fixes it.

Two hard limits come from MP3 itself rather than from anything here: at most two channels, so a surround track is folded down to stereo, and a maximum sample rate of 48 kHz. Tags and cover art are not carried across yet, so an album's worth of files will arrive unlabelled — worth knowing before converting a library rather than after.

When you want something else

Check first whether anything actually needs the MP3. Every phone, computer, browser and current car system made in the last fifteen years plays AAC, and keeping the original is smaller and better. The conversion is worth doing for a specific device that has refused the file — not as housekeeping.

For a whole library, ffmpeg preserves what this does not: ffmpeg -i in.m4a -c:a libmp3lame -q:a 2 -map_metadata 0 out.mp3 carries the tags and artwork across, and -q:a 2 is variable-bitrate encoding that spends bits where the music needs them rather than spreading them evenly. fre:ac and foobar2000 do the same with a front end, and both handle a few thousand files unattended.