AU to AAC Converter
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Supports: AU
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Turn legacy Sun/NeXT AU recordings (.au / .snd) into AAC — the modern MPEG codec that plays on iPhones, Android, browsers, and car stereos without extra software. AAC is the codec Apple Music, YouTube, and Spotify stream, and it beats MP3's sound quality at the same bitrate, which matters most when you encode old 8 kHz speech archives at low bitrates. Files convert on our servers; no app, no sign-up, no watermark.
How to Convert AU to AAC
- Upload Your AU File: Drag and drop or click "Add Files" to select
.auor.sndfiles — Sun/NeXT workstation sounds, classic Java 1.0/1.1 applet assets, and old speech corpora all work. Batch is supported. - Pick a Quality Preset: The Quality Preset dropdown (Very High is the recommended default) controls AAC quality without you touching numbers. For exact control, switch to Custom Bitrate, Constant Bitrate, or Variable Bitrate instead.
- Set Audio Sample Rate, Audio Channel, or Trim (Optional): Leave these on their original values to match the source, or downmix to mono, resample, and trim a clip with start time plus duration.
- Convert and Download: Click Convert. Output downloads individually or as a ZIP — no sign-up, no watermark.
What AAC Bitrate to Use for Your AU Source
The right bitrate depends on what the AU actually contains — telephone-grade µ-law or clean linear PCM. Encoding above the source's real bandwidth only wastes bytes; it cannot add detail the old recording never captured.
| Source AU type | Suggested AAC setting | Why |
|---|---|---|
| 8 kHz 8-bit µ-law (classic Sun default) | 48–64 kbps mono | Source bandwidth tops out near 4 kHz (Nyquist of 8 kHz). AAC is efficient here — 64 kbps already exceeds what the µ-law speech holds. |
| 11.025 / 22.05 kHz PCM speech | 64–96 kbps mono | Captures the full source band with AAC's low-bitrate advantage over MP3. |
| 44.1 kHz 16-bit PCM music | 128–192 kbps stereo | 128 kbps AAC is widely judged transparent for stereo music; 192 kbps adds headroom. |
| Archival listening copy of clean PCM | 256 kbps stereo | Near-ceiling AAC quality. Use only when the source is genuinely high quality. |
Frequently Asked Questions
Will converting an old µ-law AU to AAC improve its sound quality?
No. Encoding can never add information the source never held. The classic Sun .au default is 8-bit µ-law at 8 kHz mono — telephone fidelity, with usable audio bandwidth capped near 4 kHz. AAC will reproduce that intelligibility faithfully and compactly, but it cannot restore highs or dynamic range that the µ-law compander discarded when the file was recorded. The reason to convert is compatibility, not a quality gain.
My AU file holds 16-bit PCM, not µ-law — does that change things?
Yes, that is the good case. A modern .au containing linear PCM (often 16-bit) is effectively a lossless source, so encoding to AAC is a clean first-generation lossy compression — comparable to ripping a CD track. At 128 kbps and up, AAC stereo is widely judged transparent for music, so you keep nearly all perceived quality at a fraction of the original PCM size. If you would rather stay uncompressed for editing, convert to WAV instead.
Is AAC better than MP3 for these files?
At the same bitrate, AAC generally sounds better than MP3, and the gap is largest at low bitrates — exactly where 8 kHz µ-law speech archives live. AAC (standardized as MPEG-2 Part 7, ISO/IEC 13818-7, in 1997 and extended in MPEG-4 Part 3) was designed as MP3's successor with a more modern encoder. At 256 kbps and above the audible difference between the two narrows to near nothing. If you specifically need maximum hardware compatibility on very old players, MP3 is still the lowest common denominator.
Where does AAC actually play?
Broadly. AAC is natively supported in Chrome, Edge, and Safari, plays on every iPhone, iPad, and Mac, and on Android and modern car stereos. The one caveat: Firefox's AAC playback relies on the operating system's codecs, so it can be partial on some Linux setups. caniuse reports AAC at roughly 96% global browser support. For a self-contained, taggable music file you can also target M4A, which wraps the same AAC audio in an MP4 container.
Does the .snd extension also convert?
Yes for Sun/NeXT .snd files — they use the same container as .au and are handled identically. Note that classic Mac OS System 7 also used .snd for an unrelated sound-resource format; those rare pre-OS-X files need a Mac-specific extractor first and will not decode here.
How are my files handled, and how long do you keep them?
Files are uploaded over an encrypted connection, converted on our servers, and deleted automatically a few hours after conversion — never shared, never made public, with no sign-up and no watermark. In our testing, a 60-second 8 kHz µ-law AU (about 480 KB) converted to a 64 kbps mono AAC of roughly 480 KB at full speech intelligibility, while a 60-second 16-bit/44.1 kHz stereo PCM AU dropped from about 10 MB to a 128 kbps AAC near 1 MB with no audible loss.