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AU → FLAC
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Fast, secure AU to FLAC conversion. No registration required.
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 FLAC. A AU to FLAC transcode is mostly about compatibility, not fidelity. At sensible default bitrates you cannot tell the two apart by ear; what you get is a file that actually opens on the hardware or website you were aiming at. FFmpeg handles the heavy lifting and we stream the result straight back as a download. Worth knowing: AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines. Meanwhile FLAC is the Free Lossless Audio Codec, offering 40–60% compression with zero quality loss.
Sun AU Audio
Source formatAU is a simple audio format from Sun Microsystems, commonly used on Unix systems.
FLAC Audio
Target formatFLAC is an open-source lossless audio codec that compresses audio to roughly 50-60% of its original size without any quality loss. It is the preferred format for audiophiles and music archival.
Why convert AU to FLAC
Moving from AU to FLAC 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 FLAC codec supports to avoid compounding compression.
HOW TO CONVERT
AU → FLAC
Provide the audio file
Drag the AU onto the uploader. Files up to 100 MB run on the free tier without registration.
ffmpeg handles the conversion
Our ffmpeg-based pipeline reads sample rate and channel layout, then writes a matching FLAC with ID3 tags intact.
Save the output
Click to download the FLAC. Batch uploads are bundled into a ZIP for single-click retrieval.
Common Use Cases
Transcription pipelines
ASR services like Whisper and AssemblyAI prefer FLAC for deterministic decoding before feature extraction.
Video-editor soundtracks
Premiere, Final Cut and DaVinci Resolve ingest FLAC as a clean track on the timeline — AU sometimes drops frames on long files.
DJ software libraries
FLAC parses quickly in Rekordbox, Serato and Traktor so BPM detection and waveform analysis finish in seconds.
Audio book delivery
ACX, Findaway and Audible spec FLAC with specific bitrate, sample rate and channel-count requirements.
AU vs FLAC — 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.
FLAC Strengths
- Lossless — decoded audio is bit-exact identical to the source.
- 40-60% smaller than uncompressed WAV/AIFF.
- Free, patent-free, open-source reference implementation.
- Built-in error detection via MD5 checksums.
- Streaming-friendly — seek tables let you jump to any timestamp instantly.
Limitations
- File sizes still large compared to lossy codecs (5-10× bigger than AAC for same audio).
- Not suitable for low-bandwidth scenarios like streaming on mobile data.
- Older MP3 players and car stereos may not decode FLAC.
AU vs FLAC — 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
FLAC
- MIME type
- audio/flac
- Extension
- .flac
- Standard
- Open-source reference implementation (Xiph.Org)
- Max bit depth
- 32 bits per sample
- Max sample rate
- 655 350 Hz
- Max channels
- 8
| Specification | AU | FLAC |
|---|---|---|
| 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/flac |
| Extension | — | .flac |
| Standard | — | Open-source reference implementation (Xiph.Org) |
| Max bit depth | — | 32 bits per sample |
| Max sample rate | — | 655 350 Hz |
| Max channels | — | 8 |
AU vs FLAC — Typical file sizes
Approximate file sizes for common scenarios.
AU
- 10-second clip (8-bit µ-law, 8 kHz) 80 KB
- 10-second clip (16-bit PCM, 44.1 kHz stereo) ~1.7 MB
FLAC
- 3-min song (CD quality) 20-30 MB
- Full album (10 tracks, CD) 250-400 MB
- 3-min song (hi-res 24-bit/96 kHz) 80-120 MB
- Live concert recording (24-bit) 2-10 GB
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 FLAC. 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 FLAC 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 FLAC 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 FLAC container automatically where both formats support them. If a tag field has no FLAC equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
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Your files are encrypted during transfer, processed in isolated containers, and automatically deleted within 60 minutes. We never read, share, or store your data.