M4A vs MP3
M4A and MP3 are easy to compare as filenames, but they are not exactly the same kind of thing.
The quick difference
| M4A | MP3 | |
|---|---|---|
| What it identifies | Usually an audio-only MP4-family file/container | MP3 audio format/codec |
| Typical codec | AAC or ALAC | MP3 |
| Can be lossless? | Yes, with ALAC | No |
| Legacy compatibility | Good on modern devices | Extremely broad |
M4A does not identify one codec
A .m4a file commonly contains AAC audio, but it can also contain ALAC. That means the extension alone does not tell you whether the audio is lossy or lossless.
An .mp3 file is much less ambiguous: it contains MP3 audio.
Which sounds better?
If the M4A contains AAC, AAC is generally more efficient than MP3 at comparable bitrates, especially at lower bitrates. But source quality, encoder quality and settings still matter. If the M4A contains ALAC, it is lossless and no longer a like-for-like comparison with MP3.
Where MP3 still wins
MP3 remains the safer choice for unknown or older hardware such as legacy car stereos, basic USB players and embedded devices. M4A/AAC is widely supported on current phones, computers and modern playback software.
Common confusion
AAC is a codec commonly stored inside M4A. M4A can also contain ALAC.
If you re-encode MP3 to AAC, you create another lossy generation. If you simply put compatible audio into another container, that is a different operation.