CONVERT AU → OPUS (AU to OPUS)
Fast, secure AU to OPUS conversion. No registration required.
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HOW TO CONVERT AU → OPUS
Provide the audio file
Drag the AU onto the uploader. Files up to 25 MB run on the free tier without registration; paid plans go up to 2 GB.
ffmpeg handles the conversion
Our ffmpeg-based pipeline reads sample rate and channel layout, then writes a matching OPUS with ID3 tags intact.
Save the output
Click to download the OPUS. Batch uploads are bundled into a ZIP for single-click retrieval.
Here is the short version — AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines. Hence the need for OPUS. A AU to OPUS conversion is typically about compatibility: some players refuse AU, many accept OPUS. The audio payload makes the round trip with minimal artefacts when bitrate is left at sensible defaults. Drop a AU file into the uploader and the OPUS comes back in seconds. Keep in mind AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines. And remember that Opus is the modern low-latency royalty-free codec used in VoIP, streaming, and WebRTC.
Why convert AU to OPUS
Moving from AU to OPUS usually buys compatibility or a friendlier file size. For spoken-word content the difference is inaudible; for high-resolution music pick the highest bitrate the OPUS codec supports to avoid compounding compression.
AU vs OPUS — Technical specifications
Side-by-side comparison of the technical details.
AU
- MIME types
- audio/basic, audio/au, audio/x-au
- Extensions
- .au, .snd
- Header
- 24 bytes (magic, offset, size, encoding, rate, channels, info)
- Codecs
- PCM 8/16/24/32-bit, µ-law, A-law, IEEE float
- Byte order
- Big-endian
- MIME Type
- audio/basic
- Extension
- .au
- Category
- Audio Files
- Full Name
- Sun AU Audio
- Conversion Engine
- FFMPEG
OPUS
- Extensions
- .opus, .ogg (container)
- MIME Type
- audio/opus
- Extension
- .opus
- Category
- Audio Files
- Full Name
- Opus Audio
- Conversion Engine
- FFMPEG
- MIME type
- audio/opus
- Standard
- RFC 6716 (2012)
- Sample rates
- 8, 12, 16, 24, 48 kHz
- Latency
- 5-60 ms (configurable)
| Specification | AU | OPUS |
|---|---|---|
| MIME types | audio/basic, audio/au, audio/x-au | — |
| Extensions | .au, .snd | .opus, .ogg (container) |
| Header | 24 bytes (magic, offset, size, encoding, rate, channels, info) | — |
| Codecs | PCM 8/16/24/32-bit, µ-law, A-law, IEEE float | — |
| Byte order | Big-endian | — |
| MIME Type | audio/basic | audio/opus |
| Extension | .au | .opus |
| Category | Audio Files | Audio Files |
| Full Name | Sun AU Audio | Opus Audio |
| Conversion Engine | FFMPEG | FFMPEG |
| MIME type | — | audio/opus |
| Standard | — | RFC 6716 (2012) |
| Sample rates | — | 8, 12, 16, 24, 48 kHz |
| Latency | — | 5-60 ms (configurable) |
AU vs OPUS — Strengths and limitations
What each format does best, and where it falls short.
AU Strengths
- Trivially simple format — 24-byte header, then samples.
- µ-law 8-bit variant fits hours of speech in kilobytes.
- Stable since 1988; every major audio library reads it.
- Streaming-friendly: size field is optional.
Limitations
- Aging — obsolete outside legacy and compatibility scenarios.
- No metadata beyond a single annotation string.
- No native multi-channel surround support.
OPUS Strengths
- Best-in-class quality across the entire bitrate range.
- Royalty-free and patent-free.
- Ultra-low latency — suitable for live voice and music.
- Handles speech and music equally well — no need to switch codecs.
- Mandatory codec in WebRTC, so supported in every browser by design.
Limitations
- Very low hardware decoder adoption — software-only on most phones.
- Older platforms (legacy Windows apps, old cars) may not play .opus files.
- Container semantics confusing — Opus lives inside Ogg, WebM, or MP4.
Quality & Compatibility
Sample rate, channel layout and bit depth are preserved by default: a 44.1 kHz stereo AU becomes a 44.1 kHz stereo OPUS. Metadata — title, artist, album, cover art — travels where both formats support it. Protected DRM content cannot be converted legally and is rejected.
Tips for Best Results
- Check the podcast host specification before choosing bitrate — some mandate CBR 64 kbps, others accept VBR up to 192 kbps.
- Preserve ID3 tags by editing them before conversion; Mp3tag and MusicBrainz Picard handle round-tripping cleanly.
- If the AU is 24-bit studio audio, the OPUS at 16-bit is sufficient for listening; higher is wasted on consumer playback gear.
Frequently Asked Questions
Lossy-to-lossy conversions (most combinations) re-compress the audio, which technically introduces some loss. At a 192 kbps or higher target it is inaudible on normal equipment. Lossy-to-lossless conversions freeze the existing quality but cannot improve it; lossless-to-lossy transcodes are only as good as the target bitrate you choose.
For voice content (podcasts, audiobooks, lectures) 128 kbps is indistinguishable from higher bitrates. For music, 192-256 kbps covers most listening; 320 kbps is the ceiling for OPUS and the right choice for audio you plan to edit further. Above that, prefer a lossless target instead.
Yes. Title, artist, album, year and cover art travel from the AU container to the OPUS container automatically where both formats support them. If a tag field has no OPUS equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
Sun AU Audio
Source formatAU is a simple audio format from Sun Microsystems, commonly used on Unix systems.
Opus Audio
Target formatOpus is a versatile, open-source audio codec optimized for both speech and music at very low bitrates. It is the standard for WebRTC voice calls and excels at real-time communication.
Going the other way? Convert OPUS back to AU with the same engine.
Common Use Cases
Share across platforms
Send OPUS files to anyone without worrying about whether they have the right software for AU.
Embed in documents
Drop OPUS output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
OPUS often produces smaller files than AU for web, email and storage.
Archive & future-proof
Store in a widely-supported format that will still open on future operating systems without legacy plugins.
AU vs OPUS — Typical file sizes
Approximate file sizes for common scenarios.
- 10-second clip (8-bit µ-law, 8 kHz) 80 KB
- 10-second clip (16-bit PCM, 44.1 kHz stereo) ~1.7 MB
- Voice call (24 kbps) 180 KB/min
- Podcast (48 kbps) 21 MB/hour
- Music (128 kbps) ~1 MB/min
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Related comparisons
See these formats side by side to understand which fits your use case best.
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