AU to AAC Converter

Convert AU to AAC: What This Tutorial Covers

Sun/NeXT .au files (often saved as .snd) turn up in Solaris and SPARCstation system sounds, Java 1.0 and 1.1 applet assets, university speech corpora and 1990s web archives. Getting them into AAC makes them playable on phones, browsers and car stereos without hunting for a legacy player. What this page adds is the part that decides your result: an .au file is a thin wrapper around several very different encodings, and whether your conversion is a clean transcode or a pointless inflation depends entirely on which one is inside.

How to Convert AU to AAC

1. Upload Your AU File: Drag and drop onto the page or click "Add Files" to select .au or .snd files. Batch conversion is supported, and every file in the queue uses the same settings.
2. Choose a File Compression Mode: The panel opens on Quality Preset, the hands-off mode — and the least predictable control here, with nothing actually pre-selected in it. Use Constant Bitrate (the dropdown runs from 8 kbps to 384 kbps and opens on 128 kbps), Variable Bitrate (five quality-targeted AAC bands from 20k–32k up to 96k–112k), Custom Bitrate or Specific file size instead.
3. Set Audio Channel, Sample Rate, or Trim (Optional): Leave Audio Channel and Audio Sample Rate on "ORIGINAL" to match the source exactly — for a classic telephony .au that means 8000 Hz mono, and raising either setting adds bytes without adding detail. Trim starts on "Unchanged" and takes a start time plus a duration.
4. Convert and Download: Click Convert and save the .aac, individually or as a ZIP. Files upload over an encrypted connection, are processed on our servers, and are deleted automatically a few hours later. No sign-up, no watermark.

Walk-through: Match the Bitrate to What Your AU Actually Contains

The Sun AU header is only 24 bytes and simply declares which encoding follows, so two files with the same extension can be wildly different. The classic Sun default — and the overwhelming majority of .au files still floating around — is 8-bit µ-law at 8000 Hz mono, the same G.711 companding used by digital telephony. That is 64 kbit/s of already-lossy, telephone-bandwidth audio. A minority of .au files hold linear PCM instead, which is a genuinely clean source.

You can read this straight out of the file. The AU header's fourth 32-bit field, at byte offset 12, is an encoding number, and any tool that reports codecs (ffprobe, VLC's codec panel, or a hex editor) will tell you which of these you are holding:

Once you know which one you have, pick from the table below rather than defaulting to a music bitrate:

• If your goal is a smaller file, a 60-second µ-law .au is about 480 KB (8 KB per second). Encoding it at 64 kbps AAC produces the same 480 KB — no saving at all — while 32 kbps mono halves it with little audible cost on speech.
• If your goal is compatibility, stay at the source's own sample rate and channel count. Upsampling 8 kHz to 44100 Hz inflates the file and changes nothing you can hear.
• If your goal is a taggable music file, target AU to M4A instead. A bare .aac file is an unwrapped AAC stream with essentially nowhere to store titles or artwork.

Common Errors and How to Fix Them

• "The AAC is no smaller than the AU" — Your source was 8-bit µ-law, which is already a 64 kbit/s stream. Encoding at 64 kbps or above matches or exceeds it. Drop to 32 kbps mono for a real saving.
• "It still sounds like a phone call" — An 8 kHz source is band-limited to roughly 4 kHz no matter what you encode it to. AAC cannot invent treble the recording never captured. This is expected behaviour, not a conversion fault.
• "My player won't open the .aac file" — Some players and phones expect AAC inside a container rather than as a bare stream. Convert to AU to M4A, which wraps the same AAC audio in an MP4 container that music apps recognise and can tag.
• "The file isn't recognised at all" — A genuine AU begins with the magic bytes 2E 73 6E 64 (ASCII .snd). Check the first four bytes; anything else is not a standard Sun/NeXT AU and needs identifying before it can be converted.
• "The output is silent" — Usually a truncated or zero-length source. Files pulled from 1990s archives were frequently damaged by FTP transfers made in ASCII mode. Confirm the original plays in VLC before converting.
• "My .snd file behaves oddly" — Sun/NeXT .snd is the same container as .au and converts normally. Classic Mac OS System 7 also used .snd for an unrelated sound resource; those rare pre-OS-X files need a Mac-specific extractor first.

When This Doesn't Work

A few files in old archives are not really standard Sun/NeXT audio. Headerless raw PCM dumps with an .au extension, Mac System 7 snd resources, and files whose bytes were mangled by a legacy ASCII-mode transfer all fail the magic-number check and cannot be decoded reliably by any converter. If a file will not go through, identify what it truly is first — inspect those leading four bytes, try opening it in VLC or a hex editor — and repair or re-source it before trying again. For genuinely headerless PCM you need a tool that lets you declare the sample rate, bit depth and byte order by hand, because the file itself no longer says.

Frequently Asked Questions

Will converting an old µ-law AU to AAC improve its sound quality?

No, and no format can. The classic Sun .au default is 8-bit µ-law at 8000 Hz mono — telephone fidelity, with usable bandwidth capped near 4 kHz and dynamic range already compressed away by the µ-law companding at the moment of recording. AAC will reproduce that intelligibility faithfully and compactly, but it has nothing to restore. The reason to convert is that the result plays on modern devices, which .au largely does not.

My AU holds 16-bit PCM, not µ-law — does that change things?

Substantially, yes, and this is the good case. A .au containing linear PCM is an intact, uncompressed source, so encoding to AAC is a clean first-generation lossy compression — the same situation as ripping a CD. At 128 kbps and up, stereo AAC is widely judged transparent for music, so you keep essentially all the perceived quality at a fraction of the PCM size. If you would rather stay uncompressed for editing, use AU to WAV; for compressed lossless with tagging, AU to FLAC.

Is AAC actually better than MP3 for these files?

At the same bitrate, generally yes, and the margin is widest at low bitrates — which is exactly where 8 kHz speech archives live. AAC was standardised in 1997 as MPEG-2 Part 7 and later extended within MPEG-4, explicitly as a more efficient successor to MP3, with coding tools that hold together better when bits are scarce. Above roughly 256 kbps the audible difference between the two narrows to almost nothing. If your priority is playback on genuinely ancient hardware rather than efficiency, AU to MP3 remains the lowest common denominator.

Should I output .aac or .m4a?

Pick .m4a unless something specifically demands a raw stream. Both carry identical AAC audio; the difference is packaging. A .aac file is an unwrapped AAC bitstream, which some players and most music libraries handle poorly and which has essentially nowhere to keep titles, artists or artwork. AU to M4A puts the same audio in an MP4 container with proper metadata support, and it is what phones, iTunes-style libraries and car head units expect.

Where does AAC actually play?

Very widely. caniuse records AAC playback at about 96.7% of global browser usage, and it is native on every iPhone, iPad and Mac, on Android, and on essentially all car stereos and Bluetooth speakers from the last decade. The one recorded caveat is Firefox, listed as partial support because it defers to the operating system's codecs — which can matter on minimal Linux installs. That is a far better position than .au, which no mainstream browser plays at all.

Should I keep the file mono?

Yes, if the source is mono — and nearly every legacy .au is. Leaving Audio Channel on "ORIGINAL" preserves that. Forcing a mono recording to stereo simply duplicates the same signal into two channels and doubles the bitrate needed for identical audio. The reverse is also worth knowing: for a genuinely stereo PCM .au holding speech rather than music, folding down to mono halves your bitrate requirement with no meaningful loss.

Does the .snd extension also convert?

Yes, for Sun and NeXT .snd files — they use the identical container and header as .au and are handled the same way. The trap is that classic Mac OS System 7 also used .snd, for a completely unrelated resource-fork sound format. Those pre-OS-X Mac files are not Sun audio, will not decode here, and need a Mac-specific extraction step before any converter can read them.

Why does my AAC file sound worse than the AU at a low bitrate?

Because you have stacked a second lossy encode on top of an already-lossy one. µ-law is itself a lossy companding scheme, so an aggressive AAC setting compresses audio that has already lost information, and the artifacts of the two stages compound. Keep the bitrate at or above roughly half the source's effective rate — 32 kbps mono is a sensible floor for 64 kbit/s µ-law speech — and avoid resampling at the same time, since that adds another processing stage to the chain.

How are my files handled, and how long do you keep them?

Your file travels over an encrypted connection, is converted on our servers, and is deleted automatically a few hours afterwards. Nothing is shared or made public, there is no account to create, and no watermark is applied. Batches of small legacy audio files are the easy case for the pipeline — the practical constraint is transfer time rather than any per-file limit.