How to Master ACX Audio Requirements: Uploading Your Audiobook on Audible

To sell your audiobook on Audible through ACX (Audiobook Creation Exchange), every audio file must meet ACX’s technical requirements. The free ACX Check plugin for Audacity tells you in a few seconds whether your recording passes. In this guide, you’ll find the requirements, the plugin download, and the official Audacity steps to fix levels that fail.

In this article

1. Download the ACX Check Plugin
2. ACX Audio Requirements
3. How to Install ACX Check in Audacity
4. How to Check Your Recording
5. How to Fix Levels That Fail
6. Export Your Files for ACX

Download the ACX Check Plugin

ACX Check is listed on the Audacity Plugin Official Website: https://plugins.audacityteam.org/analyzers/analysis-plugins#acx-check

Download ACX-Check Plugin

The download is a small file called ACX-Check.ny (a Nyquist plugin by Steve Daulton). It works on Windows, macOS and Linux.

ACX Audio Requirements

These are the main technical requirements from the ACX help page:

• Loudness (RMS): between -23 dB and -18 dB RMS.
• Peaks: no higher than -3 dB.
• Noise floor: no higher than -60 dB RMS.
• File format: MP3, 192 kbps or higher, constant bit rate (CBR), 44.1 kHz.
• Channels: all files mono or all files stereo, not mixed.
• Room tone: 1 to 5 seconds at the start and end of each file, never more than 5 seconds.
• Files: one chapter or section per file, each with its section header (for example “Chapter 1”), and no file longer than 120 minutes.
• Credits: opening and closing credits in their own files.
• Retail sample: 5 minutes or less.
• Narration: human-narrated, unless ACX has authorized otherwise.

ACX can update these rules, so check the ACX audio submission requirements before you upload.

How to Install ACX Check in Audacity

1. Download ACX-Check.ny with the button above.
2. Open Audacity and go to Tools > Nyquist Plugin Installer.
3. Click Browse and select the downloaded file.
4. Click Open, then Apply and finally OK.
5. Restart Audacity. ACX Check now appears in the Analyze menu.

Note: If ACX Check doesn’t appear, open Tools > Plugin Manager and make sure it is enabled.

How to Check Your Recording

1. Open your chapter in Audacity and select the whole track (Ctrl + A).
2. Go to Analyze > ACX Check.
3. Read the three results: Peak level, RMS level and Noise floor. ACX Check compares them with the ACX specifications and tells you what passes and what fails.

The noise floor is the RMS level of the quietest 500 milliseconds in your selection, so make sure the file includes some room tone. ACX Check also warns you if the sample rate is not 44100 Hz or the file is longer than 120 minutes.

How to Fix Levels That Fail

Audacity’s official Audiobook Mastering guide uses three effects. Start with an unedited recording, and first export a WAV backup of your raw reading so you never have to record the chapter again.

1. Effect > EQ and Filters > Filter Curve EQ: choose Presets & settings > Factory Presets > Low roll-off for speech and click Apply.
2. Effect > Volume and Compression > Loudness Normalization: normalize RMS to -20 dB and click Apply.
3. Effect > Volume and Compression > Limiter: choose Soft Limit with a limit of -3.5 dB and click Apply.
4. Run Analyze > ACX Check again.

After these steps, the peak and RMS readings should pass. If the noise floor still fails (louder than -60 dB), see the noise section of the Audacity Audiobook Mastering guide.

Warning: Don’t over-process your voice. A file can pass ACX Check and still fail ACX’s human quality control if it sounds distorted or over-processed. The quality of the original recording matters most: record at a reasonable volume in a quiet room with no echoes.

Export Your Files for ACX

Export each chapter as a separate MP3 file with a constant bit rate of 192 kbps or higher and a 44.1 kHz sample rate. Keep all files mono, or all stereo, and you’re ready to upload them to ACX.