CONVERT
M4A → AU
Tap to choose your fileDRAG. DROP. DONE.
Upload any file and our engines will handle format detection automatically.
Max 100 MB · Free plan · No signup required
Convert to:
Detecting available formats...
Optimize for
Leave empty to use original name. Extension added automatically.
Uploading...
Processing your file...
Fast, secure M4A to AU conversion. No registration required.
Why this pair exists — M4A wraps AAC audio in an MP4 container, Apple's default for iTunes and voice memos. Ergo, the AU route. Turn your M4A audio into a widely-supported AU file. The conversion happens server-side through FFmpeg — the same engine behind every major audio editor — so the output plays cleanly on phones, car stereos, DJ software and streaming tools. Worth knowing: M4A wraps AAC audio in an MP4 container, Apple's default for iTunes and voice memos. Meanwhile AU is the Sun/NeXT audio format, a historical PCM container still found in older pipelines.
M4A Audio
Source 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.
Sun AU Audio
Target formatAU is a simple audio format from Sun Microsystems, commonly used on Unix systems.
Why convert M4A to AU
The motivation for a M4A → AU conversion is almost always practical: a playback device, hosting platform or editing suite that insists on AU. The audio quality trade-off is controllable via bitrate; the compatibility win is immediate and unambiguous.
HOW TO CONVERT
M4A → AU
Give us the M4A
Select a M4A (or several for batch). We read the header to pick decoder settings automatically.
Re-encode to AU
The audio is decoded, optionally resampled, and re-encoded as AU at transparent default bitrate.
Retrieve your AU
Grab the download as soon as it is ready. Typical jobs finish in seconds for short clips.
Common Use Cases
Cross-platform music libraries
Moving libraries between iTunes, foobar2000 and Plex is smoother when tracks are standardised on AU.
Streaming uploads
SoundCloud, Bandcamp and YouTube Music accept AU directly; M4A triggers a transcoding step and a delay.
Legacy hardware playback
Older car head units, portable players and boomboxes often decode AU exclusively — a lasting compatibility guarantee.
Ringtones and notifications
iOS, Android and Windows all accept AU as a system sound or custom ringtone with no further conversion.
M4A vs AU — Strengths and limitations
What each format does best, and where it falls short.
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 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 vs AU — Technical specifications
Side-by-side comparison of the technical details.
M4A
- MIME type
- audio/mp4
- Extension
- .m4a (and .m4b for audiobooks, .m4p for legacy DRM)
- Container
- ISO Base Media File Format (ISOBMFF)
- Codecs
- AAC-LC, HE-AAC, ALAC
- Max sample rate
- 96 kHz
AU
- Codecs
- PCM 8/16/24/32-bit, µ-law, A-law, IEEE float
- MIME types
- audio/basic, audio/au, audio/x-au
- Extensions
- .au, .snd
- Header
- 24 bytes (magic, offset, size, encoding, rate, channels, info)
- Byte order
- Big-endian
| Specification | M4A | AU |
|---|---|---|
| MIME type | audio/mp4 | — |
| Extension | .m4a (and .m4b for audiobooks, .m4p for legacy DRM) | — |
| Container | ISO Base Media File Format (ISOBMFF) | — |
| Codecs | AAC-LC, HE-AAC, ALAC | PCM 8/16/24/32-bit, µ-law, A-law, IEEE float |
| Max sample rate | 96 kHz | — |
| MIME types | — | audio/basic, audio/au, audio/x-au |
| Extensions | — | .au, .snd |
| Header | — | 24 bytes (magic, offset, size, encoding, rate, channels, info) |
| Byte order | — | Big-endian |
M4A vs AU — Typical file sizes
Approximate file sizes for common scenarios.
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
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
Quality & Compatibility
The AU output is as good as the M4A source allows. If the M4A was encoded at 96 kbps, the AU cannot reconstruct detail the encoder already dropped; picking a very high AU bitrate just produces a larger file. Match AU bitrate to the M4A quality for the best balance.
Tips for Best Results
- Sample-rate mismatches between M4A 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 AU 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 M4A container to the AU container automatically where both formats support them. If a tag field has no AU equivalent, it is dropped silently. Use any tag editor (Mp3tag, MusicBrainz Picard) to fine-tune afterwards.
RELATED CONVERSIONS
Other popular pairs involving M4A or AU
More from M4A
More ways to reach AU
Related comparisons
See these formats side by side to understand which fits your use case best.
Related Guides
Opus Audio Format: The Complete Technical Guide
Complete technical guide to Opus audio: SILK and CELT engines, hybrid mode, bandwidth modes, bitrate reference from 6 to 510 kbps, Ogg Opus container, latency, browser support, and FFmpeg opusenc encoding commands.
Read guideAAC Audio Format: The Complete Technical Guide
Complete technical guide to AAC: AAC-LC, HE-AAC v1/v2, AAC-ELD profiles, MDCT filter bank, TNS, PNS, joint stereo, bitrate reference, M4A vs ADTS containers, and FFmpeg libfdk_aac encoding commands.
Read guideFLAC: The Complete Guide to Free Lossless Audio Codec
Complete technical guide to FLAC: file structure, STREAMINFO MD5, linear predictive coding, Rice entropy coding, compression levels 0–8, ReplayGain, FLAC vs ALAC, and high-resolution audio encoding commands.
Read guideSecure & Private Conversion
Your files are encrypted during transfer, processed in isolated containers, and automatically deleted within 60 minutes. We never read, share, or store your data.