AU Compressor

How to Compress AU Audio Online

1. Upload Your AU Files: Drag and drop, or click "+ Add Files" to select one or more .au files from your device. Batch is supported — every file in the queue inherits the same settings.
2. Pick Audio Codec: The default keeps the AU container but lets you swap the encoding inside. Choose pcm_mulaw (μ-law, 8-bit ~half the size of 16-bit PCM at 8 kHz, ITU-T G.711 — the historical AU default), pcm_alaw (A-law, the European G.711 variant), or stay on pcm_s16be / pcm_s16le (16-bit linear PCM, lossless but ~4x larger than μ-law).
3. Set Quality, Bitrate, or Target Size (Optional): Use the Audio Quality Preset (Lowest, Low, Medium, High, Highest), set a Constant Bitrate (8 to 320 kbps) or Variable Bitrate, target an exact file size in KB/MB, or shrink by percentage. Adjust Audio Sample Rate (8000, 16000, 22050, 24000, 44100, 48000 Hz) and Audio Channel (Original, Mono, Stereo). Trim with start time and duration if you only need a segment.
4. Compress and Download: Click Compress. Files process in your browser session — no sign-up, no watermark, no upload to a third party.

Why Compress AU?

AU (also written .snd) is a Sun Microsystems / NeXT audio container introduced in 1992 with a 24-byte big-endian header followed by raw sample data. The same container can hold tiny 8-bit μ-law voicemail or 32-bit floating-point studio masters, so two AU files with identical playtime can differ in size by 8x or more. Compressing AU usually means picking a denser encoding inside the same container, not changing the file extension.

• Java Sound and legacy IVR systems — javax.sound.sampled reads AU natively, and many older interactive-voice-response platforms still expect 8 kHz mono μ-law AU. Compressing a 44.1 kHz stereo PCM AU down to 8 kHz mono μ-law cuts file size by roughly 22x and matches what the system actually plays.
• Email and chat attachments — Gmail's 25 MB per-message attachment cap and Discord's 10 MB free-tier upload limit (raised at Nitro tiers) will reject a 60-second uncompressed 16-bit stereo AU at 44.1 kHz (10 MB) much sooner than a μ-law version of the same clip (480 KB).
• Telephony and voice notes — μ-law at 8 kHz is the same encoding used in North American digital phone networks under G.711; A-law is its European counterpart. Voice quality stays intelligible at roughly 64 kbps.
• Web archives — early 1990s web pages embedded AU because browsers shipped with built-in playback. Compressing those archive AU files keeps the original format for authenticity but trims hosting cost.
• Scientific and academic datasets — Praat, SoX, and many speech-corpus tools accept AU. Smaller files mean faster batch processing on lab clusters without re-encoding to MP3 (which would alter spectral content).
• Solaris/SPARC and embedded Unix — devices that ship with /dev/audio and expect AU benefit from staying in-format rather than introducing a new decoder dependency.

AU vs WAV vs MP3 — Format Comparison

Encoding and Bitrate Guide for AU

For music, stay at 16-bit PCM and 44.1 kHz. For speech, μ-law or A-law at 8 kHz mono gives the smallest file with intelligible voice. For an exact target size, pick the percentage or target-size option instead of guessing a bitrate.

Frequently Asked Questions

Why is my AU file so much larger than a comparable MP3?

Most AU files are uncompressed PCM, which stores every sample at full bit depth. A 16-bit stereo PCM AU at 44.1 kHz runs ~1.4 Mbps — roughly 10 MB per minute — while a 128 kbps MP3 of the same audio is about 940 KB per minute. Switching the AU's internal encoding to μ-law cuts size by ~4x; converting to AU to MP3 cuts it further but leaves the AU world.

Should I pick μ-law or A-law?

Use μ-law if the destination system is in North America or Japan (G.711 μ-law is the digital-telephony standard there). Use A-law for European telephony and most ITU international links. Both pack 16-bit dynamic range into 8 bits per sample at 8 kHz; μ-law has slightly wider dynamic range, A-law has less proportional distortion for quiet signals.

Will compressed AU still play in Java's javax.sound.sampled?

Yes for PCM (8/16-bit signed) and μ-law and A-law variants — those are the encodings the Java Sound SPI handles natively. If you encode the AU as Vorbis, FLAC, or some other non-PCM codec the JRE's default mixer won't play it without an additional SPI plugin.

Can I shrink an AU without changing the encoding?

Partially — you can downsample (e.g., 44.1 kHz to 22.05 kHz halves size) or downmix stereo to mono (halves size again). Both are lossy in the sense that they discard frequency or channel information, but the encoding itself stays PCM. Combining both on a voice file roughly quarters the size with little perceptible loss.

Does compressing AU damage audio quality?

Lossless options (lowering bit depth from 24 to 16, or staying at PCM with a smaller sample rate that still covers your audio's frequency range) lose nothing audible for most material. μ-law and A-law are technically lossy — they keep about 13.5 effective bits in the encoded 8-bit byte — but voice and most non-musical content sound unchanged to the ear.

What is the maximum file size XConvert accepts?

Free-tier users can compress AU files up to the size cap shown on the upload widget; signed-in tiers raise the cap. Processing is browser-session based, so very large files are limited mostly by your browser's available memory rather than a hard server quota.

Why does my AU file get bigger after I open it in some editors?

Some editors decode μ-law or A-law to 16-bit PCM on import and re-save the file as PCM AU. The container looks identical but the encoding inside has been promoted to lossless PCM. Re-running it through this compressor with pcm_mulaw selected restores the smaller size.

Should I just convert to WAV or MP3 instead of compressing AU?

If the consumer is a modern app or device, yes — AU to WAV keeps it lossless, AU to FLAC gives lossless plus ~50% size reduction, and AU to MP3 gives the smallest size for sharing. Compress within AU only when the destination explicitly requires the .au format (legacy Java apps, Solaris tooling, IVR systems).

Can I batch-compress many AU files at once?

Yes. Add multiple files to the queue and they all share the codec, bitrate, sample rate, and channel settings you pick. Each file is processed and downloaded independently, so a partial failure on one file does not block the rest. For the reverse direction see WAV to AU or the general Audio Compressor.