ACX check free, in your browser
ACX rejects chapters that are too quiet, peak too high, hiss in the pauses or come in the wrong MP3 format. Choose a chapter to see each requirement as pass, check or fails, with the number behind it.
Measuring runs on your device. Up to 200 MB and 60 minutes. It can't check that each file is one chapter or that your credits are right; ACX's reviewers listen for those. ACX's requirements.
What to expect
- Cost and privacy
- Free, with no account. Processing runs on your device; your audio is never uploaded.
- Use it for
- Test a finished audiobook chapter before you upload it to ACX for Audible: average loudness, peaks, noise floor, room tone at both ends, MP3 format, length and channels.
- Inputs
- The exported MP3 is best, since ACX's format rules apply to it. WAV, M4A and other files your browser can decode work for the level checks.
- Limits
- Up to 200 MB and 60 minutes, subject to available browser memory.
- Keep in mind
- It measures and grades. It does not change your file, and it can't hear whether each file is one chapter or whether the credits are right.
How to check a chapter for ACX, in three steps
- STEP 01
Export the chapter as you'll upload it
Export one chapter as a 192 kbps constant bitrate MP3 at 44.1 kHz, then drop that MP3 onto the check. It is read on your device and nothing is uploaded.
- STEP 02
Read each line
The top sentence says whether anything would fail. Each requirement below is marked Pass, Check or Fails, with a plain sentence and the measured number.
- STEP 03
Fix the fails, then check the new export
Change the level, noise or room tone in your editor, export again and check the new MP3. Lines marked Check aren't failures but deserve a listen.
What ACX checks, and how this measures it
ACX, the platform that puts audiobooks on Audible, reviews every chapter by ear and by meter before it goes on sale. The measurable part is short: an average level between -23 and -18 dB RMS, peaks no higher than -3 dB, a noise floor at -60 dB RMS or lower, and a constant bitrate MP3 of 192 kbps or more at 44.1 kHz. Each file holds one chapter, starts and ends with a little room tone, and runs no longer than 120 minutes.
RMS here is the plain average level of the whole file, not LUFS. A full-scale sine wave reads -3 dB on this scale, the same convention Audacity's ACX check uses. The noise floor is the RMS of the quietest half second that isn't pure digital silence, which is how ACX's own guidance describes it. Room tone is the quiet before the first word and after the last.
ACX gives the peak limit as -3 dB without saying how to measure it. We grade the sample peak, which is what most tools read, and warn when the wave between samples goes over, because an MP3 encoder can push those peaks up.
The format comes from the file's own bytes, since decoding hides the original bitrate and sample rate. That is why the exported MP3 is the file to check. Everything runs on your device in a Worker, so the chapter is never uploaded.
ACX audio requirements
A summary in our words. ACX's own page is the rule book, so read it before your first upload: ACX audio submission requirements.
CHECKED OCTOBER 22, 2026
| RULE | REQUIREMENT | WE CHECK IT |
|---|---|---|
| Average loudness (RMS) | -23 to -18 dB | YES |
| Peak | -3 dB or lower | YES |
| Noise floor | -60 dB RMS or lower | YES |
| File format | MP3, 192 kbps or higher, constant bitrate (CBR), 44.1 kHz | YES |
| Room tone at the start and end | 1 to 5 seconds recommended, never more than 5 | YES |
| Length | 120 minutes or less per file | YES |
| Channels | All files mono, or all stereo | YES |
| Content | One chapter or section per file | NO, LISTEN |
| Credits | Opening credits name the title, author and narrator; closing credits say it's the end | NO, LISTEN |
Questions
What are ACX's audio requirements?
Each file is one chapter, a constant bitrate MP3 of 192 kbps or higher at 44.1 kHz, no longer than 120 minutes. It measures between -23 and -18 dB RMS, peaks no higher than -3 dB, and has a noise floor at -60 dB RMS or lower, with 1 to 5 seconds of room tone at each end. ACX's page has the full list.
Is this the same as ACX's own check?
No. ACX's reviewers listen to your files and look at more than numbers, such as editing, mouth noise and credits. This measures the same numbers ACX publishes, so a pass here means the meters should agree, not that the book is approved.
Why does my file fail RMS but pass the peak?
It is too quiet on average while its loudest moments are already near -3 dB. Plain gain would push the peaks over. Use a compressor or limiter in your editor to lift the quiet parts while holding the peaks just under -3 dB.
What is the noise floor?
The level of the background when nobody is speaking: hiss, hum, air conditioning, computer fans. ACX wants it at -60 dB RMS or lower. Recording in a quieter, softer room helps most; a noise remover can close the rest of the gap.
Why does the format say Check for my WAV?
ACX accepts MP3 only. A WAV is fine for editing, and the level checks still apply, but export the final chapter as a 192 kbps constant bitrate MP3 at 44.1 kHz and check that file.
Does it change or upload my file?
No. It reads and measures a copy in your browser. Nothing is uploaded and nothing is saved over your chapter.