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 plain PCM audio 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 16-bit big-endian linear PCM into the .au (the standard uncompressed encoding for the format). Decoding Opus to PCM adds no new generation of loss — the decoder faithfully reconstructs the waveform Opus describes. What it cannot do is rebuild detail the original Opus encode already discarded. The result is a file that is many times larger (uncompressed 16-bit stereo PCM runs around 1.4 Mbps regardless of source) but carries exactly the quality the Opus already had. The default is plain PCM, not the format's 8-bit µ-law heritage, so there is no additional telephone-grade quality cut layered on top — but the conversion still cannot turn a low-bitrate voice note into hi-fi.

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?

16-bit big-endian linear PCM — the standard uncompressed encoding for the AU container, written into the fixed 24-byte .snd header that Sun defined. This converter does not default to AU's historical 8-bit µ-law telephone encoding, so you are not adding a second, telephone-grade quality reduction on top of the Opus compression. You get a faithful, uncompressed PCM copy of whatever the Opus stream contained — 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. Decoding it to uncompressed PCM inside an .au reproduces that audio faithfully but cannot rebuild what was thrown away. You end up with a file that is much larger — PCM is uncompressed — carrying exactly the quality the Opus already had. 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 PCM .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 holds uncompressed PCM while the Opus was highly compressed. A one-minute Opus voice note can be a few hundred kilobytes; the same minute as 16-bit linear PCM runs into the megabytes (about 1.4 Mbps for 16-bit/44.1 kHz stereo, less for mono at a lower rate). The size jump is the cost of an uncompressed, legacy-friendly format. If you would rather keep things small and playable, convert to a lossy format with Opus to MP3 instead. In our testing, a 60-second 16 kHz mono Opus voice note (about 480 kB) decoded to a roughly 1.9 MB 16-bit PCM .au.

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 the same 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. Pick .au only when the downstream tool specifically requires that extension or its minimal audio/basic structure. Otherwise Opus to WAV gives you the same lossless 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.