What Is the Audible Format?

headphones plugged into a book
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The Audible format is a proprietary audio format developed by Audible, the spoken-word company. It is designed for the secure distribution and use of audiobooks on various software and hardware devices. The different Audible formats (.aa, .aax, and .aax+) cover a wide range of encoded bitrates. These sound formats are designed to give you a choice as to the sound quality level you want when you download your purchased audiobooks. This flexibility is useful when you have an older portable device that doesn't support certain Audible bitrates or when you need to limit the size of audiobook files due to storage space constraints. The current Audible formats are:

  • Format 2 (.AA) has a bitrate of 8Kbps, and its sound quality is comparable to AM radio quality.
  • Format 3 (.AA) has a bitrate of16Kbps and produces a sound that is on par with FM radio.
  • Format 4 (.AA) sound is encoded at 32Kbps, and the quality of sound is categorized at standard MP3 level.
  • Enhanced Audio (.AAX / .AAX+) has the highest Audible bit rate of 64Kbps and is deemed as having CD-quality sound.

Audible Files Protection and Restrictions

To prevent the unauthorized copying and playing of downloaded audiobooks, the Audible format uses an encryption algorithm usually referred to as DRM copy protection. Interestingly, the actual sound data inside an Audible file is encoded in an unprotected format — either MP3 or ACELP — but is then wrapped up in the encrypted Audible container.

Several restrictions apply when you use this audio format. They are:

  • Playback Restriction: A username and password mechanism are used to prevent the playback of downloaded files on unauthorized devices.
  • Maximum Number of Computers: You can only play back your purchased audiobooks on a maximum of four computers.
  • Limited Audio CD Burning: Even though you can burn audio CDs (via iTunes, for example), you only have the option to burn an Audible download once. This is limited when compared to other DRM systems like Apple's FairPlay protection that is used for certain digital products on the iTunes Store.

How Audible Content Is Distributed and Played

  • Computer (Mac or PC ): For a long time, the only way of purchasing and downloading content from Audible's library was first to download the AudibleManager software, which then allowed you to transfer your purchases to a compatible portable device. Applications that can play back files in the .aa format are relatively sparse compared to compatibilities for other more mainstream formats. Apart from the AudibleManager software program, the only other applications that can play back this format are iTunes and Windows Media Player.
  • Hardware devices: There is a wide selection of hardware devices on the market that can be used to play back files in the Audible format. They include iPod, iPhone, Android smartphones, iPad, portable media players, MP3 players, and streaming media devices.
  • Audible Apps: Rather than using the original method of downloading Audible content to your computer and then transferring files via cable to your mobile devices, you can download an Audible app. This method of distribution uses the software on compatible hardware devices to download DRM copy-protected audiobooks using a Wi-Fi or cellular connection from Audible's library. There are software apps for most smartphone platforms including for iOS and Android.