OPUS to AU Converter

Convert OPUS to AU Online

Opus is a modern, highly compressed audio codec — the format behind most voice notes and web audio. AU (.au / .snd) is Sun Microsystems' minimal Unix-era sound format. Converting one to the other decodes the compressed Opus stream into the simple Sun audio file that legacy Unix tooling, older Java applications, and scientific pipelines expect. The honest trade-off, spelled out below, is that the .au comes out much larger than the Opus without sounding any better — you are unlocking compatibility, not gaining fidelity. If you just want to play or edit the audio, Opus to WAV and Opus to MP3 are the better targets.

OPUS Format at a Glance

AU Format at a Glance

Why the AU Gets Bigger but Not Better

Opus has already thrown away the inaudible parts of the signal during recording — that is why a one-minute voice note can be only a few hundred kilobytes. This converter decodes that stream and writes 8-bit G.711 µ-law into the .au by default — the format's telephony heritage encoding, which packs roughly 14 bits of dynamic range into 8 with a logarithmic curve. That is a small additional quality cut layered on top of the Opus compression, barely audible on speech but noticeable on music. What no encoding can do is rebuild detail the original Opus encode already discarded. The result is still a much larger file (µ-law stores one byte per sample per channel — about 706 kbps at 44.1 kHz stereo, 128 kbps at 16 kHz mono) that sounds no better than the Opus. If you want a faithful, full-fidelity copy of the decoded waveform, select 16-bit big-endian PCM (PCM_S16BE) in the advanced options — at double the size.

How to Convert OPUS to AU

1. Upload Your OPUS File: Drag and drop your .opus file onto the page, or click "+ Add Files" to browse. You can queue several voice notes or clips and convert them all with the same settings.
2. Set Audio Sample Rate (Optional): The Audio Sample Rate dropdown sits on Original, which copies the source rate into the .au. Leave it there unless the consuming tool needs a specific rate (classic AU pipelines often expect 8000 Hz).
3. Set Audio Channel or Trim (Optional): Audio Channel also defaults to Original; set it to mono if a legacy reader expects single-channel audio. Use Trim (start time plus duration) to keep only part of a long recording.
4. Convert and Download: Click Convert and save your .au file individually or as a ZIP. No sign-up, no watermark.

Frequently Asked Questions

What encoding does this put inside the .au file?

8-bit G.711 µ-law by default — AU's historical telephone encoding, written after the fixed 24-byte .snd header that Sun defined. µ-law is lossy companding built for voice, so it does add a slight second quality reduction on top of the Opus compression — fine for voice notes, less so for music. If you need a faithful, uncompressed copy of the decoded audio, select 16-bit big-endian linear PCM (PCM_S16BE) in the advanced options; A-law is also available. Either way the file is clean enough for a Unix tool or Java application to read, just much larger than the original.

Will the AU sound better than the original Opus?

No, and that is an honest limit rather than a tool flaw. Opus is lossy, so it discarded inaudible (and, at low voice-note bitrates, some audible) detail during recording. Re-encoding cannot rebuild what was thrown away — and the default 8-bit µ-law output adds a slight telephony-grade cut of its own. You end up with a file that is much larger yet sounds no better than the Opus. (Selecting 16-bit PCM in the advanced options avoids the µ-law cut but still cannot exceed the Opus quality.) Converting buys compatibility with .au-only software, not fidelity.

Why would anyone convert Opus to AU in 2026?

Because some software only speaks .au. The realistic cases are narrow but real: legacy Unix sound tooling and shell pipelines, older Java applications (the classic AudioClip and early Java Sound era expected audio/basic), and scientific, embedded, or instrumentation systems built around the format's trivially simple 24-byte header. If a pipeline like that needs to ingest a voice note or web-audio clip, decoding it to a plain .au unlocks the workflow. For anything modern or cross-platform, AU is the wrong target.

Why is my AU file so much bigger than the Opus?

Because the .au stores one full byte per sample per channel while the Opus was highly compressed. The default 8-bit µ-law output runs at sample rate × channels bytes per second — about 5.3 MB per minute at 44.1 kHz stereo, or roughly 0.96 MB per minute for a 16 kHz mono voice note — and selecting 16-bit PCM doubles that. A one-minute Opus voice note, by contrast, can be a few hundred kilobytes. The size jump is the cost of a raw-sample, legacy-friendly format. If you would rather keep things small and playable, convert to a lossy format with Opus to MP3 instead.

What plays an AU file once I've converted it?

Unix-native sound tools, classic Java media frameworks, VLC, and ffmpeg all read .au directly. What generally does not play it: smartphones, web browsers, and most consumer media apps — AU never became a mainstream consumer format. So treat .au as a feed for the specific legacy or scientific tool that asks for it, and convert to Opus to WAV (for editing) or Opus to MP3 (for everyday playback) when you need the audio to play broadly.

Should I convert to AU, or to WAV instead?

For most people, WAV. Both AU and WAV can carry uncompressed 16-bit PCM, but WAV is the de-facto interchange format that virtually every audio editor and player on every platform accepts, whereas AU is tied to Unix, Java, and scientific niches (and this AU converter defaults to 8-bit µ-law — you must select PCM_S16BE for PCM). Pick .au only when the downstream tool specifically requires that extension or its minimal audio/basic structure. Otherwise Opus to WAV gives you an uncompressed decode in a far more widely supported file. If you later need to compress back to an efficient format, the reverse tool is AU to Opus.

How are my files handled, and how long are they kept?

Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — no sign-up, no watermark, never shared or made public.

Can I set the AU to mono or a specific sample rate for a legacy tool?

Yes. The Audio Sample Rate and Audio Channel dropdowns both default to "Original," which copies the Opus stream's rate and layout into the .au. If the software consuming the file expects a particular setup — classic Sun pipelines often assume 8000 Hz mono — set the sample rate and channel accordingly before converting. Matching what the reader expects avoids resampling surprises on the receiving end.