CONVERT
AU → M4A
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Fast, secure AU to M4A 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 M4A. Moving audio from AU into M4A is a routine job for podcasters, musicians, transcribers and anyone who needs a file to play somewhere the original would not. KaijuConverter reads the AU once, re-encodes through FFmpeg at the bitrate you choose, and returns a polished M4A in seconds. In practice AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines. On the other end, M4A wraps AAC audio in an MP4 container, Apple's default for iTunes and voice memos.
Sun AU Audio
Source formatAU is a simple audio format from Sun Microsystems, commonly used on Unix systems.
M4A Audio
Target formatM4A is an MPEG-4 audio container typically containing AAC or ALAC encoded audio. It is the standard format for iTunes purchases and Apple Music downloads.
Why convert AU to M4A
The motivation for a AU → M4A conversion is almost always practical: a playback device, hosting platform or editing suite that insists on M4A. The audio quality trade-off is controllable via bitrate; the compatibility win is immediate and unambiguous.
HOW TO CONVERT
AU → M4A
Give us the AU
Select a AU (or several for batch). We read the header to pick decoder settings automatically.
Re-encode to M4A
The audio is decoded, optionally resampled, and re-encoded as M4A at transparent default bitrate.
Retrieve your M4A
Grab the download as soon as it is ready. Typical jobs finish in seconds for short clips.
Common Use Cases
Share across platforms
Send M4A files to anyone without worrying about whether they have the right software for AU.
Embed in documents
Drop M4A output into Word, Google Docs, PowerPoint, Notion or a website without conversion warnings.
Optimize size
M4A 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 M4A — 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.
M4A Strengths
- Superior audio quality to MP3 at the same bitrate (AAC codec).
- Native support across Apple, iOS, Android, and Windows.
- Carries rich metadata: album art, chapters, lyrics, podcast bookmarks.
- Same container as MP4 — tooling overlaps with video workflows.
- Lossless variant (ALAC inside M4A) for audiophile archiving.
Limitations
- AAC patents still active in some jurisdictions — licensing fees apply for encoders.
- Seeking in variable-bitrate M4As can drift without an index atom.
- Less universal than MP3 on older hardware (pre-2010 car stereos, cheap MP3 players).
AU vs M4A — 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
M4A
- Codecs
- AAC-LC, HE-AAC, ALAC
- MIME type
- audio/mp4
- Extension
- .m4a (and .m4b for audiobooks, .m4p for legacy DRM)
- Container
- ISO Base Media File Format (ISOBMFF)
- Max sample rate
- 96 kHz
| Specification | AU | M4A |
|---|---|---|
| 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 | AAC-LC, HE-AAC, ALAC |
| Byte order | Big-endian | — |
| MIME type | — | audio/mp4 |
| Extension | — | .m4a (and .m4b for audiobooks, .m4p for legacy DRM) |
| Container | — | ISO Base Media File Format (ISOBMFF) |
| Max sample rate | — | 96 kHz |
AU vs M4A — 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
M4A
- 4-minute song (AAC 128 kbps) 4-5 MB
- 4-minute song (AAC 256 kbps) 8-10 MB
- 1-hour podcast (64 kbps) 28 MB
- 4-minute song (Apple Lossless) 25-35 MB
Quality & Compatibility
The M4A output is as good as the AU source allows. If the AU was encoded at 96 kbps, the M4A cannot reconstruct detail the encoder already dropped; picking a very high M4A bitrate just produces a larger file. Match M4A bitrate to the AU quality for the best balance.
Tips for Best Results
- Sample-rate mismatches between AU and target device (48 kHz phone output from a 44.1 kHz track) are handled automatically; no manual resampling needed.
- For audiobook delivery, match the platform spec exactly — ACX requires 192 kbps CBR 44.1 kHz stereo, for example.
- Batch-convert an album in one job so every track shares identical encoder settings and loudness normalisation.
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 M4A 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 M4A container automatically where both formats support them. If a tag field has no M4A equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
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