Short answer
Use MP3 when the file has to play on anything, including old hardware. Use AAC for general use — better quality than MP3 at the same bitrate and supported almost as widely. Use Opus when you want the smallest file, especially for speech, where it stays clear at bitrates that make MP3 sound broken.
MP3 vs AAC / Opus, side by side
| Aspect | MP3 | AAC / Opus |
|---|---|---|
| Compatibility | Universal, including old devices | AAC broad; Opus modern only |
| Quality at 128 kbps | Acceptable | Noticeably better |
| Quality at 64 kbps | Poor | Opus still clear for speech |
| Speech at low bitrate | Weak | Opus is exceptional |
| Licensing | Patents expired — free | AAC licensed; Opus royalty-free |
| Editing software support | Everywhere | AAC good; Opus patchy |
| Best for podcasts | Still the norm | Better, if your platform accepts it |
Where the differences actually show
At 256 kbps and above, all three are transparent for most listeners on most equipment — the differences are academic. The gap opens as the bitrate falls. At 128 kbps AAC is clearly ahead of MP3; at 64 kbps Opus is in a different class entirely, staying intelligible and natural where MP3 becomes watery and metallic.
That is why Opus is what voice and video calling applications use. It was designed for exactly the low-bitrate speech case, and it is unmatched there.
Bitrate matters more than codec
For spoken word, 64–96 kbps is effectively transparent in any modern codec, and mono halves that again with no perceptible loss. Music generally wants 128 kbps or more in stereo.
Going above the source bitrate never recovers quality. Re-encoding a 128 kbps MP3 at 320 kbps produces a larger file containing exactly the same damaged audio.
Compatibility, honestly
MP3's patents have expired, it plays on everything, and for a file you are handing to someone else it remains the safe answer. AAC is the sensible default inside video files and for Apple platforms. Opus is superb but still not accepted by every podcast host, car stereo or editing suite — check before committing a library to it.
Frequently Asked Questions
Is AAC really better than MP3?
Yes, at the same bitrate — the encoder is substantially more modern. The practical difference is small at high bitrates and significant at low ones. If your files are already 256 or 320 kbps MP3, converting them to AAC will not improve anything and will lose a little through re-encoding.
What bitrate should I use for a podcast?
64–96 kbps mono is transparent for speech and keeps episodes small. Stereo is only worth it if you have genuine stereo content such as music beds — a single voice carries no stereo information worth spending bitrate on.
Can I convert MP3 to a lossless format to improve it?
No. Converting MP3 to WAV or FLAC produces a much larger file containing exactly the same audio, damage included. Lossless formats preserve what they are given — they cannot restore what an earlier encoder discarded.