What is the M4b Format?

Learn where, when, and why the M4b format is used

Files ending with the M4b extension can be identified as audiobooks. They are normally purchased from the Apple iTunes Store and are similar (but not identical) to files ending in the M4a extension.

M4a also uses the MPEG-4 Part 14 container format, commonly referred to as MP4. The MP4 format is a metafile wrapper that can hold any type of data (both video and audio) and acts as a container for M4b audio streams. Incidentally, the MP4 container format is based on the Apple QuickTime platform, but it differs slightly by having extended MPEG features and Initial Object Descriptor (IOD) support. That means it includes elements to access MPEG-4 content.


The audio in an M4b file is encoded with the AAC compression format and can be protected with the Apple FairPlay DRM copy protection system. This restricts access to only computers and iOS devices that have been authorized via iTunes.

Advantages of the M4b Format for Audiobooks

The main advantage of listening to M4b audiobooks is that, unlike MP3, WMA, and other commonly used audio formats, you can bookmark a recording at any point. If, for example, you are listening to a book on your iPhone that you purchased from the Apple Music store, you can conveniently pause or "bookmark" the location and resume where you left off at another time.

This is more convenient than skipping through the whole book trying to find the exact point you got to. Audiobooks can be several hours long and so the M4b format is the perfect choice due to its bookmarking feature.

Another advantage of the M4b format is that it enables a large audiobook to be split into chapters like a physical book. Using chapter markers, a single M4b file can be segmented into manageable chunks for the listener to use, like the chapters of a book.

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