AU is the Sun audio format, older than almost everything that still opens it. MP3 is MPEG-1 Audio Layer III, lossy, and the one audio format that every device made since 1995 can play. ffmpeg sits between the two, and this page says exactly what happens to an AU file on the way to becoming an MP3 one.
What runs when an AU file becomes MP3
ffmpeg held to a single thread, on a machine we rent and watch. Your AU file is uploaded once, ffmpeg runs once, the MP3 comes back, and neither file is kept. AU to MP3 is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the AU into the MP3
The running time and the channel layout. A stereo AU comes back as a stereo MP3 of the same length: this is a re-encode, not an edit, and nothing here normalises, trims or boosts the AU.
The whole of the AU, start to end. MP3 is a lossy encoding, but it encodes all of the AU rather than a shortened or quieter version of it, and the loudness you hear is the loudness you sent.
What AU to MP3 costs you
Exactness, once and for good. The AU holds every sample; MP3 keeps what its model believes you can hear and discards the rest. Going back from MP3 to AU later rebuilds the file size and none of the sound, which is the single most common misunderstanding about this pair.
Album art, where the AU carried any. ffmpeg passes -vn on every audio target here, which counts an embedded cover as a video stream and drops it along with the film it was never part of. That is deliberate rather than careless: keeping it makes the mux fail outright on several audio containers, and hiding the choice would be lying by omission.
Any hope of MP3 sounding better than the AU. Nothing in this pass adds information, and a bitrate higher than the AU deserved only makes a larger file of the same recording.
Where AU and MP3 files come from
AU. Sun Microsystems' format from the 1980s, and the sound of the early web: every `.au` file on a 1994 home page was this. Simple enough that a program can write one in twenty lines.
If a model is going to read the MP3
An MP3 file is a waveform, and a language model reads text, so handing an assistant this MP3 directly gets you nothing. The MP3 becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. AU to MP3 is the step before it: get the sound out of the AU in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running AU to MP3 against the real engine with a real file, ffmpeg wrote 24,468 bytes of MP3 in 0.1 seconds, machine otherwise idle. That is one AU file on one day and not an average, which is why the number is given together with the file that produced it.
The AU to MP3 verdict was reached by ffprobe, meaning the container was actually demultiplexed, recognised under the right name, and carried at least one stream. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the MP3 file has anything inside it.
The same AU file, sent somewhere else
Other outputs the probe measured out of an AU file, so the cost of choosing MP3 can be read against something. Sizes do not compare across engines, because each family was probed with its own AU witness file.
AU to WAV: 264,678 bytes in 0.1 seconds, verified by ffprobe.
AU to FLAC: 44,337 bytes in 0.1 seconds, verified by ffprobe.
AU to OGG: 8,670 bytes in 0.2 seconds, verified by ffprobe.
AU to M4A: 27,633 bytes in 0.2 seconds, verified by ffprobe.
AU to OPUS: 27,591 bytes in 0.2 seconds, verified by ffprobe.
AU to AAC: 27,219 bytes in 0.2 seconds, verified by ffprobe.
Other ways into MP3, and what they measured
Among the published routes into MP3, AU is the number 16 largest output of the 22 measured. The witness files differ, so this ranks the probe run and not your document.
CAF to MP3: 24,468 bytes in 0.1 seconds.
3GP to MP3: 24,780 bytes in 0.1 seconds.
MPG to MP3: 24,468 bytes in 0.1 seconds.
OPUS to MP3: 24,429 bytes in 0.2 seconds.
WAV to MP3: 24,468 bytes in 0.2 seconds.
What we will not pretend about AU to MP3
An AU file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An AU to MP3 run that passes 240 seconds is killed, and the ffmpeg process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
The AU file behind the measurement on this page was a 3 second sine wave at 440 hertz, built by ffmpeg for the probe itself. That is why AU to MP3 came out at 0.1 seconds, and it is not what your file will do. The measurement proves the pair works and says almost nothing about how long yours will take.
AU to MP3 costs half a credit per conversion, whatever the length of the AU. That is the same as an image or a document here, and it is a measurement rather than a gesture: an audio target on this engine is done in under two seconds, where a video target can take minutes.
An AU longer than 60 minutes is refused before encoding. ffprobe measures the AU first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the MP3.
AU to MP3 waits at the audio door, which is 2 slots wide and gives up after 5 seconds rather than making you wait twenty times the length of the work. Until the 30th there was one door for everything, and a single video transcode starved every audio conversion for its whole duration: an MP3 that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the AU. No transcription, no scene detection, no summary: AU to MP3 is a container and codec operation, and the MP3 says exactly what the AU said, in a different wrapper. The sound is the whole payload.
One AU file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
AU to MP3: what people ask
What actually converts my AU file to MP3?
ffmpeg does it, held to a single thread, on a machine we rent and watch. Not a browser trick and not somebody else service: the AU file is uploaded once, ffmpeg runs once, the MP3 comes back, and neither file is kept afterwards.
How long does AU to MP3 take?
The probe measured 0.1 seconds for 24,468 bytes of MP3, on a 3 second file. An audio target scales far better than a video one: three minutes of source through this engine takes under a second, because only the sound is encoded. Past 240 seconds any run is stopped, and an audio one does not get close.
What do I lose going from AU to MP3?
The one to know about first: Exactness, once and for good. The AU holds every sample; MP3 keeps what its model believes you can hear and discards the rest. Going back from MP3 to AU later rebuilds the file size and none of the sound, which is the single most common misunderstanding about this pair.
Will the MP3 sound better than the AU?
No. Nothing in this pass adds information to the AU, so the MP3 is at best an exact carry of what you sent and at worst one more lossy pass over it. Asking for a higher bitrate than the AU deserved just makes a bigger file of the same recording.
Is AU to MP3 free?
There is a free allowance every month, and one AU to MP3 conversion costs half a credit against it however long the AU runs. An audio target is charged per conversion and not per minute, because the engine encodes three minutes of sound in well under a second.