AU to M4A Converter

AU vs M4A — Which Should You Convert To?

.au is the Sun Microsystems / NeXT audio format from the Unix-workstation and early-Java era; M4A is AAC audio in an MPEG-4 container, the file type an iPhone, an Android phone and Apple Music all treat as native. If the recording needs to play on anything made this century, M4A wins that argument outright. The honest caveat is what the conversion can carry over: an AU holding telephone-companded µ-law audio will play everywhere afterwards but still sound like a phone call, while an AU holding linear PCM turns into a clean, compact AAC copy of genuinely full-quality audio.

AU vs M4A — Side-by-side

When to Pick M4A

• The file has to play on an iPhone, iPad, or in Apple Music / iTunes without anyone installing anything — AAC in an MPEG-4 container is Apple's native audio format.
• You want small files with wide device support: AAC plays on essentially every modern phone, and in every current mainstream browser (Firefox leans on the operating system's decoder for it).
• Your AU holds linear PCM and you want a compact, taggable copy for listening rather than a bulky lossless one for archiving.
• You want proper metadata. AU has no tag fields at all; M4A gives these recordings a title and artist for the first time.

When Another Target Fits Better

• You need a bit-exact archive of a linear-PCM AU — use AU to FLAC, which preserves every sample and still supports tagging.
• You want the single most universally playable lossy format, including on decades-old software — AU to MP3 remains the safest common denominator.
• You are loading the audio into an editor that prefers uncompressed input — AU to WAV gives you a plain PCM working copy.
• The destination is the modern web or a voice pipeline where efficiency matters most — AU to Opus beats AAC at low bitrates.

How to Convert AU to M4A

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, Java 1.0/1.1 applet audio and old Unix speech clips all work, and several files can be queued and converted with the same settings.
2. Choose How the AAC Is Sized: File Compression opens on Quality Preset, the hands-off mode — and the least predictable control here, with nothing actually pre-selected in it. Use Variable Bitrate (five AAC bands from 20k-32k up to 96k-112k), Constant Bitrate (a dropdown running from 8 kbps to 384 kbps that opens on 128 kbps), Custom Bitrate (starts at 128 Kbps) or Specific file size (starts at 8 MB) when you have an exact target to hit.
3. Adjust Audio Sample Rate, Audio Channel or Trim (Optional): Leave Audio Sample Rate and Audio Channel on "ORIGINAL" to keep the source rate and layout, or force mono and a lower rate to shrink a speech recording. Trim takes a Start time and Duration in seconds or HH:MM:SS.sss.
4. Convert and Download: Click Convert and save the .m4a individually or 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.

Choosing a Bitrate for Your AU Source

The right setting depends entirely on which AU flavour you have — spending bits on telephone-grade audio buys nothing.

Frequently Asked Questions

Will converting my AU to M4A improve the sound quality?

No. AAC is a lossy codec — it can modernise and shrink your audio but cannot add detail that was never captured. If your AU holds 8-bit µ-law or A-law (Sun's telephone-companded default), the M4A sounds like the source: the same narrow bandwidth, just in a container phones actually open. If it holds linear PCM, you get a clean single-generation AAC encode of full-quality audio at a fraction of the size. What changes is compatibility and file size, not fidelity.

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 read Tools → Codec Information. A codec line of pcm_mulaw or pcm_alaw means 8-bit companded audio; pcm_s16be or deeper means linear PCM, where the AAC encode preserves genuinely full-quality sound. An 8000 Hz, single-channel signature is the fingerprint of a classic Sun recording — fine to convert, but do not expect hi-fi out of it.

Why won't my .au file play on my phone or in modern music apps?

AU predates almost every consumer device on the market. It was a Unix-workstation and early-Java standard, and phone and desktop music apps were never built to parse its .snd header — some also misinterpret the bare .au extension as something else entirely. M4A solves both problems at once, because AAC in an MPEG-4 container is exactly what iOS, Android and every music library expect to find.

What bitrate should I choose for the M4A?

Match it to the source rather than to a habit. For a µ-law speech clip, 32–64 kbps mono captures everything that is actually there — the original is only a 64 kbps stream, so anything higher inflates the file without adding information. For linear-PCM music at 44.1 kHz, 160–256 kbps is where AAC becomes hard to distinguish from the source for most listeners. If you would rather not think about it, leave Constant Bitrate on its preselected 128 kbps — that names the rate explicitly, whereas the Quality Preset dropdown is the hands-off route and the least predictable control here, showing "Highest" on load with nothing genuinely selected behind it.

Is M4A the same as MP4, and will it play on Android?

They share a container. .m4a is the audio-only profile of the same MPEG-4 file format .mp4 uses — Apple adopted the distinct extension so music files would open in a music player instead of a video player. Android has decoded AAC in MP4 containers since its earliest releases, so .m4a files play natively there too. If a stubborn app refuses the extension, renaming the file to .mp4 usually convinces it, since the bytes are the same.

Does the M4A keep any metadata the AU had?

There is essentially none to lose — the AU header offers only an optional annotation string, with no artist, title or album fields at all. What you gain is the point: the M4A supports full MP4 tags, so afterwards you can add title, artist, album, year and embedded cover art in Mp3tag, Kid3 or MusicBrainz Picard and have the recording appear correctly in a music library.

Should I pick M4A or MP3 for an old .au archive?

M4A sounds better at the same bitrate — AAC was designed as MP3's successor and is noticeably more efficient below roughly 128 kbps, which is exactly the range legacy speech recordings live in. MP3 wins only on reach into old software: turn-of-the-century hardware players, ancient car stereos and some embedded systems know MP3 and nothing newer. Converting for a phone, a modern car or a music library? Use M4A. Handing the file to something old or unknown? AU to MP3 is the safer bet.

Can I convert an M4A back to AU afterwards?

Yes, M4A to AU goes the other way, but treat it as a one-way trip for quality. AAC is lossy, so rebuilding an .au around it wraps already-compressed audio in an older container, and our AU output defaults to 8-bit G.711 µ-law unless you choose a PCM encoding — which would compand it a second time. Keep the original .au (or a FLAC copy) if you may need the source fidelity again.

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. Our AAC encodes are produced with the Fraunhofer FDK AAC encoder, which is why a modest bitrate holds up well on speech. The practical limit on very long recordings is upload size and time rather than the encode: a 60-second 8 kHz 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 AAC compresses it.