Add your AU file
Drop one or more .au files onto the converter above, or browse for them. They load into browser memory only — nothing is uploaded, so there is no size-based pricing and no server queue.
AU to AAC
Re-encoding .au audio as .aac changes how the sound is compressed and which players will accept it. Free, private, and validated — the file never leaves your browser.
100% private for supported formats
Runs on your deviceConvert supported files
Drop AU files here
Batch files can each use a different output. Nothing uploads for local conversions.
Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.About converting Sun audio to AAC audio
Re-encoding .au audio as .aac changes how the sound is compressed and which players will accept it. An .au file is the Sun audio format, one of the earliest structured audio files, created for Unix workstations. A minimal header followed by sample data, classically 8 kHz mu-law telephony audio, makes .au about as simple as an audio format can get.
An .aac file holds raw Advanced Audio Coding data, usually as an ADTS stream. AAC was designed as MP3's successor and delivers audibly better quality at the same bitrate, though the bare .aac extension is rarer than AAC audio traveling inside .m4a or .mp4 files. For this route the practical draw is noticeably more efficient than MP3 at equal bitrates and hardware decoding in essentially every phone and TV — balanced against raw ADTS files handle tags and cover art poorly, which is worth knowing before you commit a large batch.
Why convert AU to AAC?
The practical trigger for this conversion is usually a mismatch: with .au, essentially obsolete outside legacy systems. Switching to .aac buys you noticeably more efficient than MP3 at equal bitrates, which is why it is the better fit for receiving the audio track copied out of MP4 video without re-encoding. Because the conversion runs locally, trying it costs nothing but a few seconds of compute on your own machine.
AU vs AAC at a glance
| Aspect | Sun audio (.au) | AAC audio (.aac) |
|---|---|---|
| Format type | Lossless — every pixel or sample is preserved exactly | Lossy — discards detail it judges invisible to shrink files |
| How it stores data | a 24-byte header carries a '.snd' magic number, data offset, encoding type, sample rate, and channel count | a pure MDCT transform codec, free of the hybrid filterbank compromise that limited MP3 |
| Strongest at | maintaining sound assets in vintage Unix and Java software | receiving the audio track copied out of MP4 video without re-encoding |
| Weak spot | essentially obsolete outside legacy systems | raw ADTS files handle tags and cover art poorly |
| Metadata | an optional annotation field in the header can hold a short text string; no formal tag structure exists | no standard tag container exists for raw ADTS streams; ID3 tags are sometimes prepended but poorly supported, so wrapping in .m4a is the reliable way to tag AAC |
Pick AAC as the output
Select .aac in the output menu next to each file. The menu only offers targets this engine can genuinely produce, so if AAC is selectable, the route is real and validated.
Convert on your device
Press Convert. The transcode runs on an FFmpeg engine compiled to WebAssembly, so the audio never leaves your device.
Verify and download
Each result is signature-checked before the download unlocks, so a failed encode can never masquerade as a valid AAC file. Outputs keep the original filename with the .aac extension.
AU to AAC questions, answered
Will converting AU to AAC lose quality?
Some. Since au is lossless-oriented and aac is lossy-oriented, the encoder will discard detail it judges least visible; the quality slider controls how aggressive that is. Keep the original .au if you may need a pristine master later.
Is it private to convert AU files here?
Yes — the .au file is processed inside your browser tab and never uploaded. The transcode runs on an FFmpeg engine compiled to WebAssembly, so the audio never leaves your device. Close the tab and the file is gone from memory.
Where will the AAC file work?
AAC decoding is built into every browser, phone, and operating system, whether the audio arrives as a raw .aac stream or inside an .m4a wrapper. The last holdouts are elderly MP3-only car stereos and bargain portable players.
Will the AAC file be bigger or smaller than my AU?
Smaller, usually dramatically so — that is the main reason this route exists. The quality slider trades size against fidelity; the default sits at the point where most people cannot spot the difference.
What happens to the metadata in my AU file?
This converter strips container metadata deliberately (the FFmpeg command runs with metadata mapping disabled) so exported files do not leak recording details. In the source, an optional annotation field in the header can hold a short text string; no formal tag structure exists; none of it is transmitted anywhere because the file never leaves your browser.