What is AA, AAX, AAX+ and M4B?


There are several different audiobook formats that are in use which cover a wide range of encoded bitrates. The Audible format (represented by the file extension .aa, .aax, and .aax+) is a proprietary audio scheme developed by Audible, Inc. while the M4B file is developed by Apple, Inc. In this post, we will show you what different audio book formats such as AA, AAX, AAX+ and M4B are so that you can choose the right one for your needs.

What Is AA File?

The AA file extension is a data file format associated to Audible Audio Book, a spoken audio recording of Audible. The AA file extension and Audible Audio Book were developed by Audible. These files contain the digital audio clips of the audiobooks with Digital Rights Management encryption. The Audible Download Manager is in charge of downloading the AA files to the computer. AA file format are supported by several devices like Apple’s iPods, Microsoft Zune, Creative Zen, Amazon Kindle and other. These files can be converted to MP3, WMA or MPEG format to ensure compatibility and portability with other devices. AA files can also be stored in a Compact Disc in case there is a need to remove it from the hard drive. Files in AA format can be opened with Apple iTunes in Mac Operating systems and with Audible AudibleManager, Nero 11, RapidSolution Tunebite 9, SoundTaxi Media Suite and TuneCab in Microsoft Windows-based systems.

Also Read: Audiobooks: How to Convert Audible AA file to MP3

What Is AAX File?

The AAX file extension is a data file format associated to the Audible Enhanced Audiobook. AAX file extension and Audible Enhanced Audiobook were developed by Audible. These files are multimedia digital audiobook which include images, links, videos and timeline. They are considered as enhanced format of AA files. Listening to an enhanced audible audiobook would be like listening to the narrator from a recording booth. Aside from being a reading material, it is also used as a supplemental material for interactive learning and widely used in children’s books. Children’s books are also known as Audible Kids which released two enhanced audiobooks namely, Picture Books and Graphic Novels. AAX files can be played by iTunes, Apple iPod, Microsoft Zune, Creative Zen, and the Amazon Kindle 2. To be able to play enhanced audiobook from supported devices, it requires having an Audible account and a minimal subscription fee. Files in AAX format can be opened with Apple iTunes in Microsoft Windows-based and Mac operating systems.

Also Read: How to Convert AAX to MP3 on Windows & Mac


  • Format 2 (.AA): this has a bitrate of 8 Kbps and its sound quality is classed as AM radio.
  • Format 3 (.AA): the bitrate for this format is 16 kbps and produces a sound that is on a par with FM radio.
  • Format 4 (.AA): Sound at this level is encoded at 32 kbps and the quality of sound is categorized at standard MP3 level.
  • Enhanced Audio (.AAX / .AAX+): this format has the highest Audible bit rate of 64 kbps and is deemed as having CD quality sound.

What Is M4B File?

A file with the M4B file extension is an MPEG-4 Audio Book file. They’re often seen used by iTunes to store audio books. Some media players use the M4B format to store digital bookmarks along with the audio, letting you pause playback and then resume later. This is one reason they’re preferred over MP3s, which can’t save your place in the file.

The M4A audio format is basically identical to M4B except that those types of files are used for music instead of audio books. Apple’s iPhone uses the MPEG-4 Audio format for ringtones too, but those files are instead saved with the M4R extension.

Also Read: How to Convert M4B to MP3 in Bulk

Zoey Wesson

Zoey Wesson is passionate about providing digital media solutions to people who want to create innovative content via Apple music conversion, online audio recording, etc.