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 defaults to 8-bit µ-law companding, which is telephony-grade — and even the optional 16-bit PCM output only preserves what the AAC already had, at 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 — the lossy companded format SunOS exposed through /dev/audio, and the exact profile early Java's java.applet.AudioClip required. That is what this converter writes by default: 8-bit µ-law (encoding code 1), at whatever sample rate and channel layout you choose. If you need full-fidelity output instead, select 16-bit big-endian linear PCM (PCM_S16BE, encoding code 3) in the advanced options; 8-bit A-law is also available.

Because your source is AAC — a lossy codec — converting to AU does not recover any detail AAC already discarded, and the default µ-law pass packs each sample into 8 logarithmic bits (~14-bit dynamic range), a further telephony-grade fidelity cut. Even so the file usually grows: µ-law stores one byte per sample per channel, about 5.3 MB per minute at 44.1 kHz stereo regardless of how small the AAC was, and selecting 16-bit PCM doubles that to roughly 10 MB per minute. You are paying bytes for a 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. Converting to AU cannot bring that detail back — and the default 8-bit µ-law output is telephony-grade companding, so it actually shaves a little more off. Selecting 16-bit PCM (PCM_S16BE) in the advanced options gives a faithful, sample-for-sample copy of exactly what the AAC contained — no better, just much larger. 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 stores a full byte for every sample. AAC packs stereo music into roughly 128–256 kbps; the default µ-law AU spends one byte per sample per channel, which works out to about 5.3 MB per minute at 44.1 kHz stereo — and 16-bit PCM, if selected, doubles that. A 3–4 minute AAC song can therefore land around 16–21 MB as µ-law AU, twice that as PCM. The extra bytes are raw sample 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?

Yes — that is the default. 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 8-bit µ-law by default at whatever sample rate you choose, so if a legacy program requires the exact classic profile, also set Audio Sample Rate to 8000 Hz and Audio Channel to Mono. A-law and 16-bit big-endian linear PCM (PCM_S16BE) are the selectable alternatives in the advanced options.

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 optional 16-bit PCM output (PCM_S16BE) follows that; the default µ-law samples are single bytes, so byte order doesn't arise for them. 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, 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 5.3 MB, since the default 8-bit µ-law output runs about 5.3 MB per minute at 44.1 kHz stereo no matter how compact the source AAC was — selecting 16-bit PCM doubles that.