Convert AU to WEBA — Free Online Tool
Convert Sun AU audio files to WEBA format, re-encoding raw PCM audio (typically 16-bit big-endian) into Opus at 128kbps for efficient, web-ready playback. WEBA's Opus codec delivers significantly smaller file sizes than AU's uncompressed PCM while maintaining excellent perceptual quality.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your AU file here
or click to browse
Free — no uploads, no signups. Your files never leave your browser.
Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
AU files store audio as raw PCM data with a minimal header — no compression, just uncompressed samples in formats like 16-bit big-endian or 8-bit signed. During this conversion, FFmpeg decodes the PCM samples from the AU container and passes them through the libopus encoder, which applies perceptual audio compression to produce an Opus audio stream wrapped in a WebM container (saved with the .weba extension). This is a full re-encode — not a remux — because Opus is a lossy compressed codec while AU's PCM is uncompressed. The result is a file dramatically smaller than the original AU, optimized for streaming and HTML5 audio playback in modern browsers.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg command-line tool. In this browser-based tool, the equivalent FFmpeg.wasm (WebAssembly) binary runs entirely client-side, so no audio data leaves your device. |
-i input.au
|
Specifies the input AU file. FFmpeg reads the AU header to detect the PCM encoding variant (e.g., pcm_s16be, pcm_mulaw) and sample rate, then decodes the raw audio samples accordingly. |
-c:a libopus
|
Selects the libopus encoder to produce Opus-compressed audio in the output WEBA file. Opus replaces AU's uncompressed PCM with efficient perceptual coding, dramatically reducing file size while maintaining high perceived quality. |
-b:a 128k
|
Sets the Opus encoder's target audio bitrate to 128 kilobits per second. This is the default balance point for WEBA output — sufficient for transparent-sounding music and voice from an AU source, and far smaller than AU's uncompressed PCM bitrate (which is typically 705kbps for mono 44.1kHz or 1411kbps for stereo). |
-vn
|
Explicitly disables any video stream in the output. While AU is audio-only, this flag ensures FFmpeg does not attempt to pass through or generate a video track in the WebM container, keeping the output a clean audio-only WEBA file. |
output.weba
|
Defines the output filename with the .weba extension, which signals an audio-only WebM container. FFmpeg infers the WebM container format from this extension and muxes the Opus audio stream into it for browser-compatible playback. |
Common Use Cases
- Publishing legacy Unix system sounds or NeXT audio samples on a modern website where browsers cannot natively play AU files
- Reducing the file size of uncompressed AU recordings for distribution — AU's PCM audio can be 10x larger than the equivalent Opus-encoded WEBA file
- Preparing AU audio assets from old Sun workstation archives or early internet sound clips for use in a web application using the HTML5 audio element
- Converting AU-format voiceovers or narration exported from legacy audio software into a streaming-friendly format for embedding in a web page
- Archiving or republishing early internet-era .au sound files (common on 1990s websites) in a modern codec without needing server-side processing
- Encoding raw PCM AU audio captured from Unix telephony or IVR systems into Opus for low-latency web-based playback or VoIP integration
Frequently Asked Questions
Yes — this conversion introduces lossy compression. AU files store audio as raw, uncompressed PCM (such as 16-bit big-endian), which is a perfect digital representation. Opus encoding at 128kbps applies perceptual compression that discards audio information the human ear is less sensitive to. At 128kbps, Opus is generally considered transparent for most speech and music content, but the conversion is irreversible — you cannot recover the original uncompressed PCM from the WEBA output.
AU files contain raw PCM audio with no compression whatsoever — a one-minute stereo AU file at 16-bit/44.1kHz occupies roughly 10MB. The Opus codec in WEBA applies advanced perceptual compression, achieving high perceived quality at 128kbps, which reduces that same minute of audio to about 1MB. The size difference is entirely due to AU's uncompressed nature versus Opus's efficient lossy encoding.
WEBA files with Opus audio are natively supported in Chrome, Firefox, Edge, and Opera via the HTML5 audio element. Safari added Opus support in version 16 (2022), so modern Safari works too. Desktop media players like VLC and mpv also handle WEBA natively. However, older media players and some Apple applications (like QuickTime) may not support Opus in a WebM container without additional codecs.
AU files support only a minimal text annotation field in their header — they have no structured metadata tags like ID3 or Vorbis comments. FFmpeg will attempt to transfer any annotation string present in the AU header, but because AU's metadata capabilities are so limited, most AU files carry no meaningful tags to preserve. The resulting WEBA file will likely contain no artist, title, or album metadata regardless.
Replace the '128k' value in the '-b:a 128k' flag with your desired bitrate. For example, use '-b:a 64k' for smaller files suitable for speech, '-b:a 192k' for higher-fidelity music, or '-b:a 320k' for near-transparent quality. Opus is very efficient even at low bitrates — 64kbps is typically sufficient for voice content, making it well-suited for telephone-quality AU sources encoded with pcm_mulaw or pcm_alaw.
Yes — Vorbis is the other audio codec supported in the WEBA/WebM container. Replace '-c:a libopus' with '-c:a libvorbis' in the command. However, Opus is generally preferred: it outperforms Vorbis at equivalent bitrates (especially below 128kbps), has lower latency, and has broader modern browser support. Vorbis may be useful if you need compatibility with older WebM-supporting applications that predate Opus.
Yes — on Linux or macOS, you can use a shell loop: 'for f in *.au; do ffmpeg -i "$f" -c:a libopus -b:a 128k -vn "${f%.au}.weba"; done'. On Windows Command Prompt, use: 'for %f in (*.au) do ffmpeg -i "%f" -c:a libopus -b:a 128k -vn "%~nf.weba"'. The browser-based tool on this page processes one file at a time, so the FFmpeg command is the recommended approach for bulk conversions.
Technical Notes
AU (Sun Audio) files use a straightforward 24-byte header followed by raw audio data, supporting a fixed set of PCM encodings including signed 16-bit big-endian (pcm_s16be), 8-bit signed/unsigned, and the telephony codecs G.711 µ-law (pcm_mulaw) and A-law (pcm_alaw). FFmpeg handles all of these AU variants as inputs and decodes them to a common internal PCM representation before passing to the Opus encoder. The Opus encoder (libopus) internally works at 48kHz, so if your AU file was recorded at a non-48kHz sample rate (common AU files use 8kHz, 22.05kHz, or 44.1kHz), FFmpeg will resample automatically. The '-vn' flag is included to explicitly suppress any video stream processing, which is appropriate since AU is audio-only and prevents any unexpected behavior in edge cases. WEBA is technically a WebM container restricted to audio-only content, and Opus within it supports 1–255 channels, though most AU sources are mono or stereo. One notable limitation: if your AU source was recorded with µ-law or A-law compression at 8kHz (common in Unix telephony), the Opus output at 128kbps will be substantially higher quality than the source warrants — consider using '-b:a 32k' or '-b:a 64k' in such cases to right-size the output.