Convert AIFF to M4B — Free Online Tool
Convert AIFF audio files to M4B format, transcoding the uncompressed PCM audio (pcm_s16be) into AAC and packaging it into an MPEG-4 container with full support for chapters and bookmarking — ideal for turning high-quality audio recordings into audiobook or podcast files.
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FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
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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
AIFF stores audio as raw, uncompressed PCM data (typically 16-bit big-endian), which means files are large but perfectly lossless. During this conversion, FFmpeg decodes that PCM audio and re-encodes it using the AAC codec at 128kbps, then wraps the result in an MPEG-4 container with the .m4b extension. This is a full transcode — not a remux — because AAC and PCM are fundamentally different codecs. The M4B container is specifically designed for audiobooks and podcasts: it supports chapter markers and playback bookmarking (remembering where you left off), neither of which AIFF supports. The -movflags +faststart flag reorganizes the MP4 metadata to the front of the file, enabling playback to begin before the full file has loaded — useful for streaming scenarios.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool. In the browser version of this tool, FFmpeg.wasm runs this same command engine compiled to WebAssembly, so no files leave your device. |
-i input.aiff
|
Specifies the input file — your source AIFF containing uncompressed PCM audio. FFmpeg reads the raw PCM stream (commonly pcm_s16be) and prepares it for re-encoding. |
-c:a aac
|
Sets the audio codec to AAC (Advanced Audio Coding), which is the standard codec for M4B audiobook files. This triggers a full transcode from lossless PCM into AAC's perceptual compression scheme, significantly reducing file size while maintaining speech intelligibility. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second. For spoken-word audiobook content converted from an AIFF master, this bitrate provides a good balance between file size and clarity; increase to 192k or 256k if the source contains music or requires higher fidelity. |
-movflags +faststart
|
Moves the MP4 MOOV metadata atom to the beginning of the output file. This is important for M4B audiobooks that may be streamed or progressively downloaded, as it allows compatible players to begin playback before the entire file has been received. |
output.m4b
|
Defines the output filename with the .m4b extension. The M4B extension signals to Apple devices and audiobook-aware players that this MPEG-4 file contains audiobook content with support for chapters and playback bookmarking — features that the source AIFF format cannot provide. |
Common Use Cases
- Recording a spoken-word audiobook session in a DAW that exports AIFF, then converting it to M4B for distribution on Apple Books or compatible podcast apps
- Archiving a high-fidelity AIFF master of a multi-chapter audio course and converting it to a bookmarkable M4B so listeners can resume exactly where they stopped
- Converting AIFF recordings from a professional microphone setup into M4B files to import into Podcast chapters apps for chapter-marker editing
- Reducing the file size of a large AIFF audio drama production for mobile distribution, where the uncompressed source is impractical for end users to download
- Preparing an AIFF voiceover recording for submission to an audiobook aggregator that requires AAC audio inside an M4B container
- Converting AIFF lecture recordings into M4B format so students can use the bookmarking feature in Apple Podcasts to track their progress through long sessions
Frequently Asked Questions
Yes, some quality loss is unavoidable because AIFF is lossless PCM and M4B uses the lossy AAC codec. However, AAC at 128kbps — the default used here — is widely considered transparent for speech and voice content, which is the primary use case for M4B audiobooks and podcasts. For music or critical listening, bumping the bitrate to 192k or 256k in the FFmpeg command will produce a noticeably higher-quality result.
The M4B container fully supports chapters, but this conversion tool processes a single AIFF file without embedded chapter data. The output M4B will not automatically contain chapter markers unless you add them afterward using a tool like Chapter and Verse, Audiobook Builder, or by editing the FFmpeg command to include a chapter metadata file via the -map_metadata flag. The container is chapter-ready — you just need to supply the chapter information.
AIFF stores audio as uncompressed PCM, meaning a one-hour stereo recording at 16-bit/44.1kHz occupies roughly 600MB. AAC at 128kbps compresses audio using perceptual coding — discarding frequency information the human ear is unlikely to detect — reducing that same hour of audio to around 55–60MB. For speech-focused audiobook content, this roughly 10:1 size reduction comes with minimal perceptible quality difference.
Yes. Replace '128k' in the -b:a flag with a higher value such as '192k', '256k', or '320k'. For audiobook-quality speech, 128k is typically sufficient, but if your AIFF source contains music or sound design, 192k or 256k will preserve more of the original detail. The command would look like: ffmpeg -i input.aiff -c:a aac -b:a 256k -movflags +faststart output.m4b
FFmpeg will attempt to carry over any ID3-style metadata embedded in the AIFF file to the M4B container's MP4 atom tags, but AIFF metadata support is limited and inconsistently written by different applications. You may find that some tags transfer correctly while others are missing or malformed. For clean audiobook metadata, it is best to tag the M4B output explicitly afterward using a tool like MP3Tag or Kid3.
The displayed command processes one file at a time, but you can adapt it for batch processing in a shell script. On macOS or Linux, use: for f in *.aiff; do ffmpeg -i "$f" -c:a aac -b:a 128k -movflags +faststart "${f%.aiff}.m4b"; done — this loops through every AIFF in a folder and produces a matching M4B for each. On Windows, the equivalent can be achieved with a simple for loop in Command Prompt or PowerShell.
Technical Notes
AIFF's default codec in this context is pcm_s16be — 16-bit signed big-endian PCM — though AIFF files can also carry 24-bit or 32-bit PCM variants. FFmpeg will decode any of these PCM depths correctly before encoding to AAC, so high-resolution AIFF sources are handled without manual intervention. The AAC encoder used here is FFmpeg's native aac encoder, which is functional and widely compatible; if you have access to the libfdk_aac encoder in your local FFmpeg build, substituting -c:a libfdk_aac will generally yield higher quality at the same bitrate. M4B is structurally an MP4 file with an .m4b extension — Apple devices and apps use the extension to recognize it as an audiobook container and enable bookmark/chapter features. The -movflags +faststart flag moves the MOOV atom to the beginning of the file, which is best practice for any MP4-derived format intended for streaming or progressive download. Note that M4B does not support multiple audio tracks, so if your AIFF source somehow has multichannel content beyond stereo, only the default audio stream will be encoded.