Add your AAC file
Drop one or more .aac 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.
AAC to AU
Re-encoding .aac audio as .au changes how the sound is compressed and which players will accept it. Free, private, and validated — the file never leaves your browser.
Private for supported formats — processed in your browser
Runs on your deviceConvert supported files
Drop AAC 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 AAC audio to Sun audio
Re-encoding .aac audio as .au changes how the sound is compressed and which players will accept it. 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.
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. For this route the practical draw is so simple that nearly any audio tool can parse it and pCM variants preserve the recording exactly — balanced against essentially obsolete outside legacy systems, which is worth knowing before you commit a large batch.
Why convert AAC to AU?
The practical trigger for this conversion is usually a mismatch: with .aac, raw ADTS files handle tags and cover art poorly. Switching to .au buys you so simple that nearly any audio tool can parse it, which is why it is the better fit for maintaining sound assets in vintage Unix and Java software. Because the conversion runs locally, trying it costs nothing but a few seconds of compute on your own machine.
AAC vs AU at a glance
| Aspect | AAC audio (.aac) | Sun audio (.au) |
|---|---|---|
| Format type | Lossy — discards detail it judges invisible to shrink files | Lossless — every pixel or sample is preserved exactly |
| How it stores data | a pure MDCT transform codec, free of the hybrid filterbank compromise that limited MP3 | a 24-byte header carries a '.snd' magic number, data offset, encoding type, sample rate, and channel count |
| Strongest at | receiving the audio track copied out of MP4 video without re-encoding | maintaining sound assets in vintage Unix and Java software |
| Weak spot | raw ADTS files handle tags and cover art poorly | essentially obsolete outside legacy systems |
| Metadata | 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 | an optional annotation field in the header can hold a short text string; no formal tag structure exists |
Pick AU as the output
Select .au in the output menu next to each file. The menu only offers targets this engine can genuinely produce, so if AU 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 AU file. Outputs keep the original filename with the .au extension.
AAC to AU questions, answered
Will converting AAC to AU lose quality?
No new loss is introduced — since aac is lossy-oriented and au is lossless-oriented, everything the .aac file still contains is carried over exactly. What was already discarded by the original lossy encode cannot be recovered, though; converting to .au does not "restore" quality.
Is it private to convert AAC files here?
Yes — the .aac 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 AU file work?
Audio tools such as Audacity, sox, and ffmpeg open .au readily, and Java's sound APIs still accept it. Consumer players, phones, and browsers dropped meaningful support long ago, so conversion to WAV or MP3 is the practical route.
Will the AU file be bigger or smaller than my AAC?
Larger, sometimes much larger. A lossless .au must store every pixel or sample the decoder produced, without the perceptual shortcuts .aac used. Use this route for editing headroom, not for saving space.
What happens to the metadata in my AAC 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, 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; none of it is transmitted anywhere because the file never leaves your browser.
Can I convert AU back to AAC?
Yes — the reverse route exists as a separate tool. Bear in mind that round-tripping .aac → .au → .aac is not a perfect undo when any lossy step is involved; keep your original if fidelity matters.