How to use the ACX Check plugin in Audacity to pass ACX

The ACX Check plugin for Audacity is a free analysis tool that measures a chapter's peak level, RMS loudness and noise floor and shows whether each meets ACX's limits. You download it from Audacity's plugin site, install it with the Nyquist Plugin Installer, then select a whole chapter and run Analyze > ACX Check. If a reading fails, you fix it with loudness normalization, a limiter or a quieter recording.

ACX is Audible's self-publishing service, and its quality team rejects files that miss its audio standard. This guide is for authors narrating their own book in Audacity. The figures come from ACX's audio submission requirements, dated April 15, 2026, and Audacity's own pages, read in November 2026.

Below: what ACX requires, how to install ACX Check, how to fix each failing reading, and what the plugin does not check.

What audio levels does ACX require?

ACX requires each chapter file to measure between -23 dB and -18 dB RMS, with peaks below -3 dB and a noise floor below -60 dB RMS.

RMS is a measure of average loudness across the whole file. Peak is the single loudest moment. Noise floor is the level of the background hiss and room sound when nobody is speaking.

ACX Check compares RMS, peak and noise floor against ACX's limits, and the noise floor is the reading to aim well past the limit:

ACX Check also warns when a selection is not at 44.1 kHz or runs over ACX's 120-minute limit. The other rules on the same ACX page are yours to check: 192 kbps or higher constant bit rate MP3, all files mono or all stereo, one chapter per file, 1 to 5 seconds of room tone at the start and end, opening and closing credits, and a retail sample of 5 minutes or less.

How do you install ACX Check in Audacity?

ACX Check is installed as a Nyquist plugin: download the file, add it with the installer, restart Audacity, enable it, then find it under Analyze. A Nyquist plugin is a small script file that adds a tool to Audacity's menus.

1. Download the ACX Check file (it ends in .ny) from Audacity's analysis plugins page.
2. In Audacity, open Tools > Nyquist Plugin Installer and select the file. The installer's manual page explains each option.
3. Restart Audacity, then enable ACX Check in the Plugin Manager.
4. Select the whole chapter (Ctrl+A or Cmd+A) and choose Analyze > ACX Check.

If ACX Check still does not appear, restarting Audacity before looking in the Plugin Manager is the fix given in an Audacity forum thread. On a Mac, also check that the full file name ends in .ny.

These steps are for Audacity 3.x. Audacity 4.0 came out in September 2026, and its release notes say Nyquist plugins are supported but some analyzers and effects are still missing and the Macro Manager has not arrived yet. Audacity's pages do not yet confirm that ACX Check runs in version 4, and the mastering macro below needs the Macro Manager, so Audacity 3.x is the safer choice for mastering for now.

How do you fix a reading that fails ACX Check?

A failing ACX Check reading is fixed by processing the chapter in a set order and then checking again.

Audacity's own audiobook mastering guide gives this chain. An equalizer (EQ) is an effect that turns frequency ranges up or down, loudness normalization sets the average level of a file, and a limiter is an effect that stops any peak from going past a set level:

1. Filter Curve EQ with the "Low roll-off for speech" preset, to remove low rumble.
2. Loudness Normalization set to RMS -20 dB, which puts RMS in the middle of ACX's range.
3. Limiter set to Soft Limit with a limit of -3.5 dB, which keeps peaks below -3 dB.
4. Analyze > ACX Check to confirm all three readings pass.

If the noise floor still fails, the same guide suggests Noise Reduction at gentle settings, starting at 6, 6, 6 (noise reduction in dB, sensitivity and frequency smoothing) and trying 9, 6, 6. It warns that 12, 6, 6 can make a voice sound hollow and may itself fail ACX's listening check. In Audacity 3.x the whole chain is available as a downloadable macro.

Two habits save rework. Keep and process your WAV masters, and export MP3 only at the end, because each MP3 export compresses the audio again and loses detail. Also aim for a noise floor of about -65 dB rather than just under -60. On the Audacity forum, a narrator whose 26 files passed both ACX Check and Audio Lab was still rejected for noise, and the book was approved after the chapters that measured between -60 and -65 dB were remastered to -65 dB.

What does ACX Check not catch?

ACX Check does not catch anything you can only hear, and it does not check bit rate, mono or stereo, credits or the retail sample.

• Noise you can hear. Mouth clicks, breaths, a dog barking or a fridge hum can pass the numbers and still fail ACX's human review.
• Digital silence. A noise gate is an effect that mutes audio below a set level. Muting pauses to total silence sounds unnatural to listeners, and ACX asks for room tone, not silence, at the start and end of each file.
• File settings. Bit rate, bit-rate method, sample rate and mixed channels are checked by ACX Audio Lab on upload, not by the plugin. ACX's FAQ for Audio Lab, dated October 2026, says it does not detect noise floor or editing errors, so ACX Check is the noise measurement to take before the human check.
• Credits and the retail sample. Listen for them yourself.

Mastering takes time on top of recording. Our guide on how long it takes to record an audiobook puts numbers on the whole job, and what ACX is and how it works covers its deals and royalties.

The bottom line

ACX Check is a free Audacity plugin that tells you whether each chapter meets ACX's three level limits: RMS between -23 and -18 dB, peaks below -3 dB and a noise floor below -60 dB. Install it with the Nyquist Plugin Installer, run Audacity's mastering chain of EQ, RMS normalization to -20 dB and a -3.5 dB limiter, and aim for a noise floor a few decibels below the limit. Your next step: master one finished chapter, run ACX Check and upload it to ACX Audio Lab before you record the rest.