M4B (audiobook)
Also known as: m4b file, audiobook format, m4b vs m4a
M4B is an audio format built for audiobooks. It uses the same AAC audio and container as M4A, but adds chapter markers and bookmarking so a long recording can be navigated by chapter and resume where you stopped.
- AAC audiobook format with chapters and bookmarks
- Same container as M4A, tagged as an audiobook
- Long titles can take significant storage
What makes M4B different
An `.m4b` file holds AAC-encoded audio in the same MPEG-4 container used by M4A. The difference is purpose: M4B carries chapter markers and supports remembering your position, which players treat as the signature of an audiobook rather than a music track.
Because of that extra structure, audiobook apps file M4B titles in a separate library and keep your place across sessions, instead of restarting from the beginning each time.
Size and storage
Audiobooks are long, so an M4B for a full-length title can be a sizable file even at a modest, speech-friendly bitrate. Several books can quietly take up real space on a phone.
The audio is already compressed, so M4B files do not shrink meaningfully if you try to zip them; managing space is about removing finished titles rather than re-compressing.