CONVERT OPUS → AU (OPUS to AU)
Fast, secure OPUS to AU conversion. No registration required.
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HOW TO CONVERT OPUS → AU
Upload the OPUS
Drop or select your OPUS file. The upload is encrypted and the file is queued for conversion.
Transcode via FFmpeg
FFmpeg decodes the OPUS stream to PCM internally, then re-encodes as AU at the bitrate you select.
Download the AU
The AU is delivered as a direct download; metadata and cover art transfer automatically where possible.
Here is the short version — Opus is the modern low-latency royalty-free codec used in VoIP, streaming, and WebRTC. Hence the need for AU. Need a AU version of a OPUS recording for a podcast host, audio book platform or DAW that refuses the original container? Drop the file above and our encoder produces a clean AU you can drag straight into the destination tool. Metadata such as title, artist and cover art travels with the audio. One more beat. Opus is the modern low-latency royalty-free codec used in VoIP, streaming, and WebRTC. Receiving format: AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines.
Why convert OPUS to AU
Opus Audio is great in its own niche, but Sun AU Audio is either more universally playable or better suited to the device you are targeting. Converting lets you ship the audio without asking listeners to install a codec. The loss in quality between the two is negligible at sensible bitrates.
OPUS vs AU — Technical specifications
Side-by-side comparison of the technical details.
OPUS
- MIME type
- audio/opus
- Extensions
- .opus, .ogg (container)
- Standard
- RFC 6716 (2012)
- Sample rates
- 8, 12, 16, 24, 48 kHz
- Latency
- 5-60 ms (configurable)
- MIME Type
- audio/opus
- Extension
- .opus
- Category
- Audio Files
- Full Name
- Opus Audio
- Conversion Engine
- FFMPEG
AU
- Extensions
- .au, .snd
- MIME Type
- audio/basic
- Extension
- .au
- Category
- Audio Files
- Full Name
- Sun AU Audio
- Conversion Engine
- FFMPEG
- MIME types
- audio/basic, audio/au, audio/x-au
- 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
| Specification | OPUS | AU |
|---|---|---|
| MIME type | audio/opus | — |
| Extensions | .opus, .ogg (container) | .au, .snd |
| Standard | RFC 6716 (2012) | — |
| Sample rates | 8, 12, 16, 24, 48 kHz | — |
| Latency | 5-60 ms (configurable) | — |
| MIME Type | audio/opus | audio/basic |
| Extension | .opus | .au |
| Category | Audio Files | Audio Files |
| Full Name | Opus Audio | Sun AU Audio |
| Conversion Engine | FFMPEG | FFMPEG |
| MIME types | — | audio/basic, audio/au, audio/x-au |
| 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 |
OPUS vs AU — Strengths and limitations
What each format does best, and where it falls short.
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.
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.
Quality & Compatibility
Lossy-to-lossy transcoding (most cross-format audio jobs) loses a tiny amount of quality on each pass — usually inaudible at our default VBR ~190 kbps for music or 96 kbps for speech. Lossy-to-lossless conversions freeze the existing quality but cannot improve it; lossless-to-lossy is only as good as the target bitrate you choose.
Tips for Best Results
- Pick 128 kbps for podcasts and voice, 192–256 kbps for music, 320 kbps only if the audio will be edited further downstream.
- Keep the OPUS master alongside the AU — re-encoding a lossy format twice accumulates audible artefacts.
- For mono voice content, convert to mono AU explicitly to halve file size without any quality loss.
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 AU 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 OPUS container to the AU container automatically where both formats support them. If a tag field has no AU equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
Opus Audio
Source 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.
Sun AU Audio
Target formatAU is a simple audio format from Sun Microsystems, commonly used on Unix systems.
Going the other way? Convert AU back to OPUS with the same engine.
Common Use Cases
Share across platforms
Send AU files to anyone without worrying about whether they have the right software for OPUS.
Embed in documents
Drop AU output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
AU often produces smaller files than OPUS 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.
OPUS vs AU — Typical file sizes
Approximate file sizes for common scenarios.
- Voice call (24 kbps) 180 KB/min
- Podcast (48 kbps) 21 MB/hour
- Music (128 kbps) ~1 MB/min
- 10-second clip (8-bit µ-law, 8 kHz) 80 KB
- 10-second clip (16-bit PCM, 44.1 kHz stereo) ~1.7 MB
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Related comparisons
See these formats side by side to understand which fits your use case best.
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