Convert AU to M4B — Free Online Tool
Convert Sun AU audio files to M4B audiobook format using AAC encoding — transforming legacy Unix PCM audio into a modern, chapter-capable container compatible with Apple Books, iTunes, and podcast apps. This conversion re-encodes raw PCM audio (typically pcm_s16be) into AAC at 128k bitrate, adding the MPEG-4 container features that make bookmarking and chapter navigation possible.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your AU file here
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Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
AU files store raw PCM audio (most commonly 16-bit big-endian signed PCM, or pcm_s16be) with a minimal header — there is no compression, no metadata richness, and no container features like chapters. During this conversion, FFmpeg decodes the raw PCM samples from the AU file and re-encodes them using the AAC codec at 128k bitrate. The encoded audio is then wrapped in an MPEG-4 container (.m4b), which supports chapter markers, bookmarking, and ID3-style metadata. The -movflags +faststart flag reorganizes the MP4 metadata to the front of the file, enabling progressive playback. Because this goes from uncompressed PCM to lossy AAC, the output file will be significantly smaller than the source, with a minor and generally imperceptible quality tradeoff at 128k.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program. In the browser tool, this runs via FFmpeg.wasm (WebAssembly), executing the same logic as the desktop FFmpeg binary entirely within your browser — no server involved. |
-i input.au
|
Specifies the input Sun AU file. FFmpeg reads the AU header to determine the PCM codec (most likely pcm_s16be), sample rate, and channel count, then decodes the raw audio samples for re-encoding. |
-c:a aac
|
Sets the audio codec for the output to AAC (Advanced Audio Coding), the native and most compatible codec for the M4B container. AAC provides efficient lossy compression of the uncompressed PCM audio from the AU source, making it suitable for audiobook distribution and Apple device playback. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second. For speech-heavy content like audiobooks, this is a solid balance between file size and quality. The original AU PCM audio has no bitrate ceiling, so this determines how aggressively the audio is compressed into AAC. |
-movflags +faststart
|
Moves the MPEG-4 container's metadata (moov atom) to the beginning of the output M4B file. This is required for progressive streaming playback and is expected by many podcast platforms and audiobook apps that begin playback before the full file is downloaded. |
output.m4b
|
Specifies the output filename with the .m4b extension. FFmpeg uses this extension to select the MPEG-4 container format, and the .m4b convention signals to Apple Books, iTunes, and compatible audiobook players that this file supports bookmarking and chapter navigation. |
Common Use Cases
- Digitized spoken-word recordings originally captured or archived in AU format on Unix/Linux systems that need to be loaded into Apple Books or iTunes as audiobooks with bookmarking support
- Converting legacy Sun Microsystems workstation audio lectures or educational recordings into a format that supports chapter navigation for structured listening
- Repurposing AU-format audio from early internet archives or Unix-era software distributions into podcast-compatible M4B episodes
- Archivists and librarians converting collections of .au recordings into a more modern, metadata-rich audiobook format for long-term distribution
- Developers working with Unix audio pipelines who need to produce M4B output compatible with iOS audiobook apps from PCM source files
- Converting synthesized speech or text-to-speech output saved as AU files into distributable M4B audiobooks for accessibility tools
Frequently Asked Questions
Yes, this conversion is lossy. AU files using pcm_s16be store audio as uncompressed PCM, which is lossless by nature. Encoding to AAC at 128k introduces compression artifacts, though at 128k AAC the quality loss is minimal and generally inaudible for speech-focused content like audiobooks. If you need to preserve the exact audio fidelity, you can increase the bitrate to 192k or 256k in the FFmpeg command by changing -b:a 128k to -b:a 192k.
The AU format has no concept of chapters — it is a flat, headerless audio stream. The M4B container supports chapters natively, but this conversion tool does not automatically generate chapter markers since there is no chapter metadata in the source AU file. After converting, you can add chapter markers using tools like mp4chaps or Audiobook Builder if you want to define navigation points in the resulting M4B.
AU files with pcm_s16be encoding store every audio sample uncompressed, meaning a one-hour recording can easily exceed 600MB. AAC at 128k is a highly efficient lossy codec and typically achieves compression ratios of 10:1 or better compared to uncompressed PCM. A 600MB AU file could compress to around 55-60MB as an M4B at 128k AAC, making it far more practical for distribution and device storage.
M4B with AAC audio is natively supported by Apple Books, the Podcasts app, and iTunes on macOS and Windows. On iOS and iPadOS, M4B files opened via Apple Books will retain your playback position through bookmarking. Third-party audiobook apps like Bound, Bookmobile, and Prologue also support M4B. Android support varies — apps like Smart AudioBook Player and Voice Audiobook Player handle M4B well, but the native Android player typically does not.
The audio bitrate is controlled by the -b:a flag in the command. To increase quality, replace 128k with a higher value such as -b:a 192k or -b:a 256k. For speech-only content like audiobooks, 64k or 96k AAC is often sufficient and produces very small files, while 192k or above is recommended if the AU source contains music. The command would look like: ffmpeg -i input.au -c:a aac -b:a 192k -movflags +faststart output.m4b
Yes. On Linux or macOS, you can use a shell loop: for f in *.au; do ffmpeg -i "$f" -c:a aac -b:a 128k -movflags +faststart "${f%.au}.m4b"; done. On Windows Command Prompt, use: for %f in (*.au) do ffmpeg -i "%f" -c:a aac -b:a 128k -movflags +faststart "%~nf.m4b". The browser-based tool processes one file at a time, but the FFmpeg command shown is ideal for batch workflows on the desktop — especially useful for large archival AU collections.
Technical Notes
AU (Sun Audio) files use a minimal 24-byte header followed by raw audio data. The most common codec is pcm_s16be (16-bit big-endian signed PCM), though the format also supports pcm_s8, pcm_u8, pcm_alaw, and pcm_mulaw. None of these codec variants carry metadata like title, artist, or album — the AU format was never designed for rich tagging. When wrapping the re-encoded AAC audio in the M4B (MPEG-4) container, FFmpeg can embed iTunes-compatible metadata tags using -metadata flags if desired. The M4B container is structurally identical to M4A; the .m4b extension is a convention that signals audiobook intent to compatible players, enabling bookmarking features. The -movflags +faststart flag moves the moov atom to the beginning of the file, which is important for streaming playback and is required by some podcast platforms. Note that AU files sourced from telephony systems may use pcm_alaw or pcm_mulaw at 8kHz sample rates — in those cases, the output quality will be limited by the source regardless of the AAC bitrate chosen.