The ACX Check plugin is a free analysis plugin for Audacity that measures the three values ACX rejects files for: peak level, no louder than -3 dB; RMS loudness, between -23 and -18 dB; and noise floor, no louder than -60 dB.[1] It does not ship with Audacity. You download it, enable it once, and it appears in the Analyze menu ready to run on any selection.[3]
This guide covers two things the plugin leaves to you: getting it installed, and knowing which Audacity setting moves each number into range. For the full effect chain and the order to apply it in, see the Audacity audiobook mastering workflow.
Where to download the ACX Check plugin for Audacity (Windows and Mac)
Download it from Audacity's own plugins site, on the analysis plugins page, where it is listed under Individual Analyzers as ACX Check.[3] The download is a single 4 KB Nyquist file called ACX-Check.ny, written by Steve Daulton and based on an earlier plugin by Will McCown. That page is the canonical source: Audacity's own audiobook mastering guidance links straight to it.[2]
Two things to be clear about, because both come up constantly in the Audacity forum. First, ACX Check does not ship with Audacity, and that has not changed in the current 3.x releases, so a fresh install will not have it under Analyze. Second, the same .ny file works on Windows and on macOS. Nyquist plugins are plain text rather than compiled binaries, so there is no separate Mac download to hunt for. Copies of the file are mirrored in various GitHub repositories and they generally work, but take yours from the Audacity plugins page.
Installing it takes a minute.[3]
- Download
ACX-Check.nyfrom the analysis plugins page. - In Audacity, open Tools > Nyquist Plugin Installer.
- Click File Browser, select the file you downloaded, then click Open, Apply, and OK.
- Restart Audacity. ACX Check now sits under the Analyze menu alongside the built-in analysers.
If it does not appear after the restart, open Tools > Plugin Manager, find ACX Check in the list, select it, and click Enable. On Windows you can also install it by hand: drop the .ny file into %AppData%\audacity\Plug-ins and restart.
To run it, select the part of your track you want to measure (usually the whole chapter, with the room tone left in), then choose Analyze > ACX Check. The three results appear in a small panel you can read at a glance.
One expectation to set before you install it: ACX Check measures, it does not fix. It reads your selection and reports three numbers, and every correction is still yours to make in the effects below, where each fix moves the numbers the other two depend on. Audacity's own page for the plugin puts it plainly: it is an aid, and a straight pass is no guarantee of ACX acceptance.[3] chapterpass takes the other half of the job, mastering the file to the ACX targets in your browser and reporting each measured value against its spec, free for the first file.
What does the ACX Check plugin measure?
Run the plugin on a selection and it reports three numbers, each against its ACX target.[1] It reads your audio, it does not change it, so you use it to confirm a fix, then run it again after the next change.
- Peak level. The loudest single sample in the selection. ACX wants this no louder than -3 dB, which leaves headroom for the listener's device.
- RMS level. The average loudness across the selection, what ACX calls performance loudness. ACX wants this between -23 and -18 dB.
- Noise floor. The RMS level of the quietest half second the plugin can find in the selection. ACX wants this no louder than -60 dB.[3]
Worth knowing up front: these are three of the eight specs ACX measures. The plugin says nothing about head and tail silence, sample rate, channel count, or file format, all of which ACX also rejects for. A file can pass the ACX Check plugin and still be returned. The full ACX audio requirements list all eight.
Which Audacity setting fixes each ACX target?
This is the mapping most narrators want: a value failed, which control brings it back. Each setting below is the one Audacity's own audiobook guidance recommends.[2]
| ACX target | Value | Audacity control | Setting |
|---|---|---|---|
| RMS loudness | -23 to -18 dB | Effect > Volume and Compression > Loudness Normalization | Normalize RMS to -20 dB[4] |
| Peak level | Below -3 dB | Effect > Volume and Compression > Limiter | Ceiling -3.5 dB[5] |
| Noise floor | Below -60 dB | Effect > Noise Removal and Repair > Noise Reduction | Start at 6, 6, 6, with a captured noise profile |
| Tone and clarity | Not measured | Effect > EQ and Filters > Filter Curve EQ | Preset: Low roll-off for speech |
A few notes that the table cannot hold:
RMS at -20 dB is deliberate. ACX accepts anything from -23 to -18 dB. Normalising to -20 dB lands you in the centre of that window, so a chapter that drifts a decibel either way during the rest of your processing still passes.[2]
The peak ceiling is set to -3.5 dB, not -3.0 dB, on purpose. Audacity measures the loudest sample. The true peak between samples sits higher, and ACX does not publish which of the two readings it takes,[1] so the half-decibel of margin is a hedge against that gap. The reasoning, and where the limiter belongs in the chain, is in the full Audacity workflow.
Noise floor is mostly a recording problem. Noise reduction in Audacity needs a captured noise profile, and pushed too hard it leaves your voice sounding processed. If the room tone itself is above -60 dB, treat the room before you reach for the effect. The audiobook noise floor guide covers the recording side and the gain-budget trap that makes loudness and noise floor fight each other.
The order you apply these in matters, because each step changes the numbers the next one reads. The Audacity audiobook mastering workflow sets out the full chain.
What does a failing result look like?
A first run on a raw recording might read peak -2.1 dB, RMS -24.6 dB, noise floor -58 dB. That is three fails, and the order you clear them in is the whole game.
- Peak -2.1 dB is too hot, above the -3 dB ceiling. The limiter brings it down.
- RMS -24.6 dB is too quiet, below the -23 dB floor. Loudness Normalization to -20 dB raises it. The catch: lifting the whole signal lifts the noise floor with it.
- Noise floor -58 dB is already over the -60 dB line, and normalising the RMS upward only pushes it further out. This is the gain-budget trap. You cannot raise a quiet recording loud enough for ACX without dragging its noise up too, so the noise floor has to be won first, at the recording stage or with noise reduction, before you normalise.
Run ACX Check again after each change, not just at the end. A clean pass reads something like peak -3.4 dB, RMS -20.1 dB, noise floor -64 dB: every value inside its window with a little room to spare, which is exactly what survives ACX review.
Where the ACX Check plugin can mislead you
The plugin is honest about what it sees, but what it sees has limits worth knowing.
Noise floor reads the quietest half second, wherever it is. If your selection contains a stretch of truly digital silence, an edit gap or a trimmed breath, the plugin measures that instead of your real room tone and reports a noise floor far better than your recording actually has.[3] Select a passage with natural pauses, not surgical silence, for a reading you can trust.
Peak is sample peak, not true peak. A file showing -3.1 dB in the plugin can still be above -3 dB on a true-peak meter, and ACX does not publish which reading it takes.[1] This is why the limiter ceiling sits at -3.5 dB rather than right on the line.
Silence and format are out of scope. The plugin never looks at your room tone (ACX recommends 1 to 5 seconds at the beginning and end of each file, with a hard cap of 5 seconds), your sample rate, or your bitrate. Those are common rejection reasons, and you have to confirm them separately. It does raise a warning when the selection is not at 44,100 Hz or runs past ACX's 120 minute file limit, which is useful, but a warning is all you get.[3]
A quiet number is not always a clean number. Some noise is worse than its level suggests. The 1000 Hz whine common to USB audio interfaces can sit below -60 dBFS, pass the plugin, and still draw an ACX rejection, because ACX's audio review team listens as well as measures.[3] Listen to your room tone on headphones as well as reading the number.
For how this plugin's blind spots compare with the official ACX Audio Lab and the free browser tools, see what every free ACX tool measures and misses.
What export settings does ACX need?
Once the three measured values pass, the file still has to leave Audacity in the right container. ACX wants MP3 at 192 kbps constant bit rate, 44,100 Hz.[1] Channel format can be mono or stereo, but every file in the title must use the same one, and mono is the usual choice for a single narrator. Audacity does not default to constant bit rate, so set it by hand in the export dialogue: choose MP3, set the bit rate mode to Constant, and select 192 kbps. The export step is covered in detail in the Audacity audiobook mastering workflow.
A faster path when the numbers will not settle
Mapping each spec to a setting works, but it is iterative: normalise, limit, run ACX Check, find the noise floor moved, adjust, run it again. On a long book, across dozens of chapters, that adds up.
chapterpass masters your recording to every ACX spec in one step, including the head and tail silence the ACX Check plugin does not measure, and reports each value against its target so you can see exactly what changed.
Ready to submit?
Run the free ACX check to see exactly where your file stands, or let chapterpass master it to spec in one step.