AU to FLAC Converter

Convert AU to FLAC Online

.au (also seen as .snd) is the Sun Microsystems audio format from the Unix-workstation era — the thing /dev/audio wrote on a SPARCstation and what Java 1.0 applets played through sun.audio.AudioPlayer. FLAC is the modern free lossless codec, standardised by the IETF as RFC 9639 in December 2024. This converter decodes the AU's payload and re-encodes it as FLAC without discarding a single sample, so whatever fidelity the original capture had survives intact — inside a container that current players, phones and browsers actually recognise, and that can finally hold tags.

How to Convert AU to FLAC

1. Upload Your AU File: Drag and drop your .au or .snd file onto the page, or click "+ Add Files" to browse. Sun/SPARCstation system sounds, early Java applet audio and old Unix speech archives all work, and you can queue several files to migrate a whole folder in one pass.
2. Set the Compression Level: Open the options panel and move the Compression level slider (1–12, starting at 12). As the on-screen note says, a lower number encodes faster and leaves a larger file while a higher number squeezes harder — the audio is bit-identical at every setting, so this trades encode time for size only, never quality.
3. Adjust Audio Sample Rate, Audio Channel or Trim (Optional): Leave Audio Sample Rate and Audio Channel on "ORIGINAL" to keep the AU's native rate and layout untouched — that is the genuinely lossless path. Changing either one resamples or remixes the audio. Trim (Start time plus Duration, in seconds or HH:MM:SS.sss) exports only part of a longer recording.
4. Convert and Download: Click Convert and save the .flac file individually or grab everything as a ZIP. Files upload over an encrypted connection, are converted on our servers and are deleted automatically a few hours later. No sign-up, no watermark.

AU Format at a Glance

FLAC Format at a Glance

What the Conversion Can and Cannot Do

FLAC is lossless, which means it preserves exactly what the AU contained — no more. The result therefore depends entirely on which AU flavour you have, and the two are wildly different.

Frequently Asked Questions

Will converting my AU file to FLAC improve the sound quality?

No, and nothing else will either. FLAC is lossless, so it reproduces exactly what the source holds but cannot invent detail that was never recorded. If your AU carries 8-bit µ-law or A-law — Sun's telephone-companded default — the FLAC sounds identical to the AU, with the same narrow ~4 kHz usable bandwidth, just in a container modern software understands. If it carries linear PCM, the FLAC is a genuine bit-exact copy of that full-quality audio at a smaller size. Either way you keep what you had.

How can I tell whether my AU is µ-law or linear PCM before converting?

Run ffprobe file.au, or open the file in VLC and look at Tools → Codec Information, or use the free MediaInfo utility. A codec line reading pcm_mulaw or pcm_alaw means 8-bit companded audio; pcm_s16be or deeper means linear PCM. A 8000 Hz, single-channel signature is the tell-tale fingerprint of a classic Sun recording — perfectly fine to convert, just do not expect hi-fi from it.

Why does the FLAC report 16-bit when my AU was 8-bit µ-law?

Because µ-law is not really 8-bit linear audio — it is a logarithmically companded code word that expands to a 14-bit-equivalent linear value, and every decoder produces 16-bit linear PCM from it. FLAC then stores those 16-bit samples losslessly, so the file header honestly says 16-bit even though only the original companded resolution is in there. We confirmed this on a 60-second 8 kHz mono µ-law sample run through the same FFmpeg-based encode our servers use: the resulting FLAC reported a 16-bit sample depth at 8 kHz. It is not padding or upsampling — it is just how µ-law decodes.

Will the FLAC actually be smaller than my AU file?

For linear-PCM sources, almost always: lossless coding typically removes 30–50% of a CD-quality PCM stream, more on sparse or quiet material. For 8-bit µ-law sources, do not count on it. µ-law is already a compact one-byte-per-sample representation, so FLAC has to losslessly code a 16-bit decode of it and the result can land close to — or occasionally above — the original .au size. If your goal for a µ-law file is a small file rather than a lossless one, AU to MP3 or AU to Opus is the better target.

What FLAC compression level should I pick?

Any of them, as far as sound is concerned — the decoded audio is identical at 1 and at 12. The slider only decides how hard the encoder works: higher values search more prediction orders and produce a slightly smaller file at the cost of encode time, and decode speed and player compatibility are unaffected either way. The slider starts at 12, the smallest-file end, which is the sensible default for archiving. Drop it only if you are converting a very large batch and want it to finish sooner.

Does FLAC keep more metadata than the AU file had?

There is essentially nothing to lose. The AU header carries only an optional annotation string that most files leave blank and most players ignore — there is no artist, title or album field in the format at all. FLAC supports Vorbis comments, so once converted you can add title, artist, album, date and embedded cover art in any tag editor (Mp3tag, Kid3, MusicBrainz Picard) and have these legacy recordings show up properly in a music library for the first time.

What plays a FLAC file once I have converted it?

Far more than plays .au. VLC, foobar2000, Audacity, Winamp, Windows Media Player, the Files and Music apps on Android, and every current desktop browser handle FLAC — Chrome from 56, Firefox from 51, Edge from 16, Safari from 13 on macOS and iOS 11 upward. Most modern car head units and network audio players read it too, which is the practical reason to move a .au archive into FLAC rather than leaving it where only VLC and FFmpeg will touch it.

Can I convert the FLAC back to AU later?

Yes — FLAC to AU reverses the direction, and if the FLAC came from a linear-PCM AU the audio itself survives the round trip intact. Be aware of one asymmetry: our AU output defaults to 8-bit G.711 µ-law, so unless you pick a PCM encoding the returning .au will be companded to telephone grade. Keep the FLAC as your master. If you want a plain uncompressed working copy instead, AU to WAV gives you PCM your DAW will open without a second thought.

How are my files handled, and how long do you keep them?

Your file is uploaded over an encrypted connection, converted on our servers, and deleted automatically a few hours after the job finishes — never shared, never made public, with no sign-up and no watermark. The practical limit on very long recordings is upload size and time rather than the encode itself; a 60-second 8 kHz mono µ-law AU is only about 480 KB, while a 60-second 16-bit/44.1 kHz stereo PCM AU is about 10.6 MB before FLAC gets to it.