Convert M4B to AU — Free Online Tool
Convert M4B audiobook files to AU format, decoding AAC audio into raw PCM (16-bit big-endian) for use on Unix systems and legacy audio pipelines. This tool runs entirely in your browser using FFmpeg.wasm — no upload required.
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FFmpeg Command
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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
M4B files store audio as AAC-encoded streams inside an MPEG-4 container, along with chapter markers, bookmarks, and ID3-style metadata. AU is a minimalist format developed by Sun Microsystems that carries raw, uncompressed PCM audio with a simple fixed-length header — it has no concept of chapters, metadata, or bookmarks. During this conversion, FFmpeg decodes the AAC audio stream from the M4B container and re-encodes it as 16-bit big-endian PCM (pcm_s16be), which is the AU format's default and most compatible codec. All M4B-specific features — chapters, bookmarks, podcast metadata — are discarded because AU has no mechanism to store them. The result is a larger but completely uncompressed audio file suitable for Unix audio tools, legacy systems, and raw PCM audio pipelines.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary, the open-source multimedia processing engine that handles decoding the M4B container, decoding the AAC audio stream, and re-encoding it as raw PCM into the AU format. |
-i input.m4b
|
Specifies the input file — an M4B audiobook or podcast file containing an AAC audio stream along with MPEG-4 container metadata such as chapters and bookmarks. |
-c:a pcm_s16be
|
Sets the output audio codec to 16-bit signed big-endian PCM, which is the native and default encoding for the AU format. This decodes the compressed AAC audio from the M4B and writes it as raw, uncompressed samples in the byte order required by the Sun AU specification. |
output.au
|
Specifies the output filename with the .au extension, which tells FFmpeg to write a Sun AU container. The file will contain only the raw PCM audio stream with AU's minimal header — no chapters, metadata, or M4B-specific features are carried over. |
Common Use Cases
- Feeding audiobook audio into Unix command-line audio processing tools (like SoX or aplay) that natively read AU files without additional decoding steps
- Archiving or analyzing the raw PCM audio waveform from an M4B audiobook for audio forensics, quality inspection, or academic research
- Converting M4B podcast audio to AU for playback or processing on legacy Sun Microsystems workstations or older Unix-based systems
- Extracting uncompressed audio from an M4B file as an intermediate step in a professional audio mastering or transcription workflow that requires PCM input
- Providing raw audio data to embedded systems or audio hardware interfaces on Unix platforms that expect big-endian 16-bit PCM in AU container format
Frequently Asked Questions
No. The AU format has no support for chapters, bookmarks, or any extended metadata — it uses a minimal fixed-length header that stores only sample rate, encoding type, and channel count. All M4B-specific features like chapter titles, bookmark positions, and podcast tags will be permanently discarded during this conversion. If preserving chapters is important, consider converting to a format like MP4 or MKV that supports chapter metadata.
M4B stores audio using AAC, a highly efficient lossy codec that typically achieves compression ratios of 10:1 or more compared to raw audio. AU with pcm_s16be stores every audio sample uncompressed, so a one-hour M4B audiobook that might be 50MB could expand to 600MB or more as an AU file. This size increase is expected and unavoidable — it is the nature of converting from compressed AAC to raw 16-bit PCM.
The original AAC encoding in your M4B was a lossy process, so some quality was already lost when the audiobook was first created. The conversion to AU with pcm_s16be is a lossless representation of whatever audio data is decoded from that AAC stream — FFmpeg does not apply any further lossy compression. The resulting AU file will be a faithful uncompressed representation of the AAC-decoded audio, with no additional degradation introduced by this specific conversion.
PCM stands for Pulse-Code Modulation, which is raw uncompressed digital audio. The 's16' part means each audio sample is stored as a signed 16-bit integer, giving a dynamic range of 96dB — equivalent to CD-quality audio. The 'be' suffix means the byte order is big-endian, which is the byte order native to Sun Microsystems hardware and required by the AU format specification. This is the default and most universally compatible audio encoding for AU files.
Yes. The AU format supports several codecs including pcm_mulaw (G.711 mu-law, common in telephony), pcm_alaw (A-law companding), pcm_s8 (8-bit signed PCM), and pcm_u8 (8-bit unsigned PCM). To use mu-law encoding, replace '-c:a pcm_s16be' with '-c:a pcm_mulaw' in the command. Keep in mind that mu-law and A-law are lossy companding codecs designed for voice frequencies, so they are not ideal for music or high-fidelity audiobook audio.
The command as shown processes a single file, but you can wrap it in a shell loop for batch processing. On Linux or macOS, use: 'for f in *.m4b; do ffmpeg -i "$f" -c:a pcm_s16be "${f%.m4b}.au"; done'. On Windows PowerShell, use: 'Get-ChildItem *.m4b | ForEach-Object { ffmpeg -i $_.FullName -c:a pcm_s16be ($_.BaseName + ".au") }'. Batch processing on the desktop is especially useful if you have files larger than 1GB, which exceed the browser tool's processing limit.
Technical Notes
The AU format's header encodes sample rate, channel count, and encoding type but nothing else — there is no room for title, artist, duration estimates, or any of the rich metadata that M4B carries. FFmpeg will write a valid AU header with the correct sample rate (matching your M4B's original sample rate, typically 44100Hz or 22050Hz for audiobooks) and channel count (usually mono or stereo). Because AU uses big-endian byte ordering for pcm_s16be, the resulting files may need byte-swapping if consumed by little-endian systems expecting standard WAV-style PCM — though most modern AU-aware tools handle this transparently. The AU format has no maximum file size limit in its specification, but very long audiobooks converted to uncompressed 16-bit stereo at 44100Hz will produce files of several gigabytes. There is no audio quality parameter to configure for pcm_s16be since it is an uncompressed format — the bit depth (16-bit) is fixed by the codec choice itself. If your M4B source was encoded at a low AAC bitrate (64k or 96k), the AU output will be large but will not recover any audio detail that was lost during the original AAC encoding.