Use this article to ensure the audio files you produced comply with the ACX audio requirements.

Note: If you are looking for audiobook cover specs, visit ACX Cover Art Requirements.

Before submitting the entire audiobook, you can use our audio analysis tool to get feedback on a finished audio file and confirm if you have followed the ACX standards.

In this article:

• What are the content requirements for audio files on ACX?
• What are the requirements for file names?
• What are the submission requirements for audio quality?
• What are the submission requirements for bit rate?

What are the content requirements for audio files on ACX?

Include opening and closing credits: Opening Credit announcements must state the audiobook's title, author(s), and narrator(s). These credits announcements must be consistent with the text displayed within the title's metadata and cover art. If you are a producer working with a rights holder, be sure to consult with them to confirm completeness of the stated title credits or align on any changes.

Closing Credits must indicate finality. A best practice to inform the listener that they have reached the end of the book is to state, "You have been listening to...", then restate the audiobook's title, author(s), and narrator(s), and conclude by stating, "The End".

You can add music to the credits, but make sure it doesn't obscure the narration.

Include a retail sample of 5 minutes or less: Consider the following when selecting an audiobook sample:

• Please choose the first five minutes of your audiobook as the sample, which allows for a seamless listening experience into the rest of the title once purchased. You may work with your rights holder to select a different portion of the audiobook as the sample, however, most listeners will hear a sample from the audiobook’s beginning regardless of the selected excerpt.
• Samples must not contain any explicit (e.g., violent or pornographic) material. If the beginning of your audiobook contains this, please select a different sample. For example, if the book is erotic, please do your best to choose a clean but romantic excerpt.
• The sample must come from the audiobook.
• Apple Books automatically uses the first five minutes of the book as their sample.

Each file should be no longer than 120 minutes: Each file must be no longer than 120 minutes. Split longer sections into separate files and include a secondary header for continuity.

Your submitted audiobook must be narrated by a human unless otherwise authorized:  Unauthorized use of text-to-speech, AI, or automated recordings in ACX titles is prohibited. Do not remit payment to a producer if you suspect unauthorized use of text-to-speech. If you need additional support, please contact us here. Audible is working to accept third-party TTS content from publishers and creators who are interested and will share more updates on timing and availability as our offering evolves. 

What are the requirements for file names?

The following are requirements for the file name and formatting of your audiobook.

Include one section or chapter per file: Each file must contain only one chapter or section. Upload each file individually. Opening and closing credits should be separate files.

Label files with chapter or section title: Each file must include the section header (e.g., "Prologue", "Chapter 1"). Missing headers will require revisions and could delay release.

Use only standard US alphabetical/numeric characters in file names: Avoid special characters.

Note: To learn about text requirements for chapter names, visit Edit chapter titles.

What are the submission requirements for audio quality?

When submitting audiobooks to ACX, it's crucial to ensure your audio files meet their technical requirements for encoding, peak levels, and RMS levels. Proper mastering is key to delivering a high-quality listening experience for your audience.

Learn more about these topics in the ACX blog and ACX University videos.

Audio sound must be consistent: Your audiobook should be consistent in sound and formatting. This includes audio levels, tone, noise level, spacing, and pronunciation. Consistent audio makes for a better listening experience and can lead to good reviews and better sales.

There are no extra sounds: Make sure each file is free of extra sounds like plosives, microphone pops, mouse clicks, excessive mouth noise, and outtakes. These sounds distract listeners and can lead to bad reviews.

Files are in either mono or stereo: All files must be in the same channel format (all mono or all stereo). Mixed formats will cause errors.

Volume is between -23dB and -18dB RMS: Each file needs to fall between the specific volume range of -23dB and -18dB RMS for consistent volume.

Root Mean Square (RMS) is a way to measure the average value of an audio signal. By keeping all files within this range, listeners won't have to constantly adjust the volume of their playback device. Visit Mastering Audiobooks with Alex the Audio Scientist to learn more.

Peak levels are less than -3dB: Each file must have peak values no higher than -3dB to avoid distortion. By leaving this headroom you'll reduce the possibility of distortion, which can seriously reduce the quality of the listening experience. This headroom is also needed to ensure the files are successfully encoded. 

Click here for an article from the “Alex the Audio Scientist” series on peak level and how it can be controlled during the mastering process.

Noise floor is less than -60dB RMS: Each file must have a noise floor no higher than -60dB RMS to avoid background noise distractions.

Keep room tone less than 5 seconds: We recommend between 1 and 5 seconds  of room tone at the beginning and end of each file for an ideal listening experience. Room tone spacing must not exceed 5 seconds.

What are the submission requirements for bit rate?

Before going on sale in the Audible store, titles are encoded in a variety of formats that customers have the option of downloading. Constant Bit Rate (CBR) MP3 files are required so this encoding process works without error.

File format is 192 kbps or higher CBR: Each file must be a 192 kbps or higher CBR, 44.1kHz MP3. You may upload 256kbps or 320kbps files if you'd like, but the difference in quality heard by listeners will be negligible.

We've created a helpful guide to encoding your files using fre:ac, a free third-party software. You can find the software at http://www.freac.org. We encourage you to install the most current software version available. After installing the software, please consult our file encoding instructions.