Convert WebM to AU — Free Online Tool
Convert WebM video files to AU audio format, extracting the Opus or Vorbis audio track and re-encoding it as 16-bit big-endian PCM — the native codec of the Sun AU container. This tool is ideal for Unix-oriented workflows or legacy systems that require raw PCM audio in the classic .au format.
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FFmpeg Command
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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
WebM files typically carry audio encoded with Opus or Vorbis — both compressed, lossy codecs optimized for web streaming. The AU format, developed by Sun Microsystems, has no support for these codecs and stores audio as raw PCM data. During this conversion, FFmpeg discards the WebM video stream entirely (since AU is audio-only) and fully decodes the compressed Opus or Vorbis audio track, then re-encodes it as 16-bit signed big-endian PCM (pcm_s16be) inside the AU container. This is a lossy-to-lossless-PCM pipeline: the original compression artifacts from Opus/Vorbis encoding are preserved in the decoded waveform, but no additional lossy compression is applied. The resulting .au file contains uncompressed audio with a simple fixed-length header, making it trivially readable by Unix audio tools and legacy software.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary, the open-source multimedia processing engine that runs under the hood in this browser tool via WebAssembly (FFmpeg.wasm) and can also be run directly on your desktop command line. |
-i input.webm
|
Specifies the input file — a WebM container that may contain a VP9 video stream and an Opus or Vorbis audio stream. FFmpeg reads the container, demuxes the streams, and makes them available for output mapping. |
-c:a pcm_s16be
|
Tells FFmpeg to encode the audio stream as signed 16-bit big-endian PCM — the standard and most compatible codec for the Sun AU container format, matching the big-endian byte order of the SPARC Unix systems AU was originally designed for. The Opus or Vorbis audio from the WebM is fully decoded and then written as raw uncompressed PCM samples. |
output.au
|
Specifies the output file with the .au extension, which signals FFmpeg to use the Sun AU container format muxer. Since AU is audio-only, FFmpeg automatically drops the WebM video stream and writes only the re-encoded PCM audio track into the simple AU header-plus-raw-data structure. |
Common Use Cases
- Feeding extracted audio from a WebM screen recording into a legacy Unix audio processing pipeline that only accepts .au files
- Preparing audio for Sun Solaris or other Unix workstation applications that were built around the .au format as their native input
- Converting WebM-captured voice memos or interviews into uncompressed PCM AU files for archival in environments that require non-proprietary, minimally-structured audio containers
- Supplying .au audio assets to older Java applets or early-web applications that used the Sun AU format as their default supported audio type
- Extracting the audio layer from a WebM video and producing a flat PCM waveform for analysis or editing in scientific audio tools that parse raw AU headers
- Stripping WebM video to produce an uncompressed AU audio file for import into digital audio workstations or tools that accept .au but not Opus-encoded streams
Frequently Asked Questions
The conversion involves a decode-then-PCM-encode step, not a second round of lossy compression. The Opus or Vorbis audio in the WebM is fully decoded, and the resulting waveform is written as uncompressed 16-bit PCM in the AU file. Any quality degradation that occurred during the original Opus or Vorbis encoding is already baked into the audio signal — this conversion does not add further lossy compression, but it also cannot recover what was lost when the WebM was originally encoded.
Yes, the AU format supports multi-channel PCM audio including stereo, so a stereo Opus or Vorbis track from the WebM will be preserved as stereo pcm_s16be in the output. However, AU has no standardized support for surround or multi-channel configurations beyond what individual applications choose to implement, so if your WebM contains a multi-channel audio track beyond stereo, compatibility with the target AU playback tool is not guaranteed.
All of them are dropped. The AU format is a bare-bones audio-only container with a fixed header and no support for video streams, subtitle tracks, chapter markers, or metadata beyond basic encoding parameters. FFmpeg automatically ignores the video and any other non-audio streams when writing to an AU output. If your WebM contains multiple audio tracks, only the first (default) audio track will be included in the output.
The Sun AU format was developed on SPARC-based Unix systems, which use big-endian byte ordering, and pcm_s16be (signed 16-bit big-endian PCM) is the AU container's most widely compatible and historically standard codec. While the AU spec also supports 8-bit and G.711 compressed variants like pcm_alaw and pcm_mulaw, pcm_s16be offers the best balance of fidelity and compatibility across modern and legacy Unix audio tools. FFmpeg selects it as the default for AU output for this reason.
To change the sample rate, add '-ar 44100' (or your target rate) before the output filename — for example: ffmpeg -i input.webm -c:a pcm_s16be -ar 44100 output.au. To switch to 8-bit PCM, replace pcm_s16be with pcm_s8 or pcm_u8 in the command. Note that pcm_mulaw and pcm_alaw are also valid AU codecs if you need G.711-compatible output for telephony applications, using '-c:a pcm_mulaw' or '-c:a pcm_alaw'.
Yes. On Linux or macOS, you can loop over files in a directory with a shell one-liner: for f in *.webm; do ffmpeg -i "$f" -c:a pcm_s16be "${f%.webm}.au"; done. On Windows Command Prompt, use: for %f in (*.webm) do ffmpeg -i "%f" -c:a pcm_s16be "%~nf.au". This is especially useful for batch-processing large files over 1GB that exceed the browser tool's limit, running the same command locally on your desktop.
Technical Notes
The Sun AU format uses a straightforward fixed or variable-length header (magic number 0x2e736e64, i.e., '.snd') followed by raw PCM sample data, making it one of the simplest audio container formats in existence. Because of this simplicity, AU files carry virtually no metadata — there is no equivalent of ID3 tags, no title, artist, or album fields, and no chapter or subtitle support. Any metadata embedded in the WebM file (such as title, encoder info, or language tags on the audio track) will be silently discarded during conversion. The output file size is entirely predictable: a 16-bit stereo AU file at 48kHz will consume exactly 192,000 bytes per second of audio, making AU files significantly larger than their WebM Opus counterparts. WebM with Opus typically achieves near-transparent quality at 96–128kbps, so a 5-minute stereo Opus stream at 128kbps (~4.8MB) becomes roughly 55MB as uncompressed pcm_s16be AU. If the source WebM was encoded at a sample rate other than what your target application expects (commonly 44100Hz or 8000Hz for legacy tools), you may need to add explicit resampling with '-ar' to avoid playback issues on the receiving system.