CONVERT AU → AAC (AU to AAC)
Fast, secure AU to AAC conversion. No registration required.
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HOW TO CONVERT AU → AAC
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 AAC with ID3 tags intact.
Save the output
Click to download the AAC. 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 AAC. A AU to AAC 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. Background. AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines. Destination side, AAC is the Advanced Audio Codec, more efficient than MP3 and ubiquitous in modern streaming.
Why convert AU to AAC
Moving from AU to AAC 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 AAC codec supports to avoid compounding compression.
AU vs AAC — 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
AAC
- Extensions
- .aac, .m4a, .mp4 (container-dependent)
- MIME Type
- audio/aac
- Extension
- .aac
- Category
- Audio Files
- Full Name
- AAC Audio
- Conversion Engine
- FFMPEG
- MIME type
- audio/aac
- Standard
- ISO/IEC 14496-3
- Variants
- AAC-LC, HE-AAC, HE-AACv2, AAC-LD, xHE-AAC
- Sample rates
- 8-96 kHz
| Specification | AU | AAC |
|---|---|---|
| MIME types | audio/basic, audio/au, audio/x-au | — |
| Extensions | .au, .snd | .aac, .m4a, .mp4 (container-dependent) |
| 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/aac |
| Extension | .au | .aac |
| Category | Audio Files | Audio Files |
| Full Name | Sun AU Audio | AAC Audio |
| Conversion Engine | FFMPEG | FFMPEG |
| MIME type | — | audio/aac |
| Standard | — | ISO/IEC 14496-3 |
| Variants | — | AAC-LC, HE-AAC, HE-AACv2, AAC-LD, xHE-AAC |
| Sample rates | — | 8-96 kHz |
AU vs AAC — 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.
AAC Strengths
- Better quality than MP3 at equal bitrate — the industry standard since 2000s.
- Universally supported on every smartphone, OS, and browser.
- Efficient on battery thanks to widespread hardware decoding.
- Scales from 8 kbps speech (HE-AACv2) to lossy-transparent 320 kbps.
- Five-channel + LFE surround support out of the box.
Limitations
- Patent-encumbered — encoders have licensing fees, which is why open alternatives (Opus, Vorbis) exist.
- Slightly more complex to encode than MP3.
- Raw .aac streams carry no seek index — tooling often prefers M4A/MP4 containers.
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 AAC. 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 AAC 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 AAC 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 AAC container automatically where both formats support them. If a tag field has no AAC 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.
AAC Audio
Target formatAAC is a lossy audio codec that delivers better sound quality than MP3 at similar bitrates. It is the default audio format for Apple Music, YouTube, and most streaming services.
Going the other way? Convert AAC back to AU with the same engine.
Common Use Cases
Share across platforms
Send AAC files to anyone without worrying about whether they have the right software for AU.
Embed in documents
Drop AAC output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
AAC 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 AAC — 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
- Speech podcast (64 kbps) 1 MB/min
- 3-min music track (128 kbps) 3 MB
- 3-min music track (256 kbps) 6 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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