AAC to AU Converter

AAC to AU Converter

This tool decodes an AAC audio file and rewrites it as an .au file — the Sun Microsystems audio format (also seen as .snd) from the Unix-workstation and NeXT era. AAC is a modern, lossy codec; AU is a simple, decades-old container that some legacy Unix programs, old Java audio code, and academic sound libraries still expect by name. This conversion exists for that compatibility, not for quality: the AU here holds uncompressed PCM, so it sounds the same as the AAC source while taking up far more space. If you just want an uncompressed copy that plays and edits everywhere, AAC to WAV is the mainstream pick, and AAC to AIFF is the Mac-side equivalent — both are easier to open than AU.

AAC Format at a Glance

AU Format at a Glance

What This Converter Writes Into the AU

The .au most people picture is 8-bit µ-law at 8 kHz — the lossy companded format SunOS exposed through /dev/audio, and the exact profile early Java's java.applet.AudioClip required. This converter does not down-convert to that, and it does not give you a codec dropdown for AU output. It writes 16-bit big-endian linear PCM (encoding code 3) into the AU container, which is the muxer's default and a lossless encoding.

Because your source is AAC — a lossy codec — wrapping it in PCM does not recover any detail AAC already discarded. It is a bit-exact decode of the compressed signal: the .au carries the same audio the AAC did, just stored sample-for-sample instead of compressed. So you get a much larger file that sounds identical to the input. Concretely, CD-quality stereo PCM runs about 10 MB per minute regardless of how small the AAC was, so a few-megabyte song commonly becomes tens of megabytes as AU. You are paying bytes for an uncompressed legacy container, not gaining fidelity.

How to Convert AAC to AU

1. Upload Your AAC File: Drag and drop your .aac onto the page or click "+ Add Files". Several files can be queued and converted with the same settings.
2. Set Audio Sample Rate and Audio Channel: Leave Audio Sample Rate and Audio Channel on "Original" to copy the source track's native rate and layout untouched, or resample (8000–48000 Hz) or downmix to Mono if a legacy target needs a specific profile.
3. Trim (Optional): Use the Trim control to export just a section by setting a start offset and duration, or leave it "Unchanged" to convert the whole clip.
4. Convert and Download: Click "Convert" and download your AU file. No sign-up, no watermark.

Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — never shared or made public.

Frequently Asked Questions

Does converting AAC to AU improve the audio quality?

No. AAC is a lossy format, so detail was already thrown away when the file was encoded. Decoding it and storing the result as AU's linear PCM cannot bring that detail back — the AU is a faithful, sample-for-sample copy of exactly what the AAC contained, no better and no worse. The only thing that changes is the file gets much larger because PCM is uncompressed. Converting is about reaching a tool that needs .au, never about upgrading fidelity.

Why is my .au file so much bigger than the AAC?

Because AAC is compressed and the AU output is not. AAC packs stereo music into roughly 128–256 kbps; the 16-bit PCM written into the AU stores every sample in full, which works out to about 10 MB per minute for CD-quality stereo. A 4 MB AAC song can therefore land around 30–40 MB as AU. The extra bytes are uncompressed data, not added quality. If size matters, keep the AAC or convert to a compressed format instead of AU.

Can I make the AU output 8-bit µ-law like classic Sun .au files?

Not with this converter. The historic .au from Sun workstations was 8-bit µ-law at 8000 Hz — lossy, telephone-grade, and the only format early Java's AudioClip would play. This tool writes 16-bit big-endian linear PCM by default and does not expose a codec selector for AU, so the output is full-bandwidth PCM rather than companded µ-law. If a specific legacy program requires exactly 8-bit µ-law .au, this output will not match that encoding and you would need a tool that lets you force the µ-law codec.

Is the AU output big-endian, and does that matter?

Yes. The AU format stores its header and sample data in big-endian byte order regardless of the host machine, and the 16-bit PCM this converter writes follows that. It matters only if a downstream tool reads the raw bytes assuming little-endian — well-behaved players honor the 24-byte header and handle it correctly. WAV, by contrast, is little-endian, which is one of the real structural differences between the two containers even though both can carry the same PCM samples.

Why convert to AU instead of WAV or AIFF?

Only for compatibility with something that specifically expects .au. Realistic cases are a legacy Unix or NeXT-lineage program that reads the Sun format natively, an old Java application built around the original AU-only sound API, or an academic or retro-computing pipeline that documents .au as its interchange format. For listening, sharing, or editing, uncompressed AU offers no advantage over AAC to WAV (the universal uncompressed choice) or AAC to AIFF on macOS — both are far more widely supported.

What programs can open an .au file in 2026?

VLC, Audacity, SoX, FFmpeg, and most native audio tools on Linux and other Unix systems handle .au directly, and Java's javax.sound.sampled reads it natively. On the desktop, QuickTime and many media players open it as well. Support is broad among media-savvy tools but uneven in everyday consumer apps, which is the main reason WAV or AIFF is the safer choice when you don't specifically need the Sun format.

Is .au the same as .snd?

Practically yes. On NeXT the format was distributed as .snd; on Sun it became .au. The file magic is the same four bytes (.snd, 0x2e736e64), and the two extensions are structurally identical and interchangeable in modern players. If a tool hands you a .snd and another expects .au, renaming is usually enough.

How are my files handled, and how long do you keep them?

Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — never shared, never made public, with no sign-up and no watermark. In our testing, a one-minute stereo AAC track at 256 kbps (about 1.9 MB) produced an .au of roughly 10 MB, since the 16-bit PCM written into AU runs about 10 MB per minute for CD-quality stereo no matter how compact the source AAC was.