Free ACX Check
Drop in a chapter and see exactly which ACX spec failed, loudness, peak, noise floor, or the head and tail silence Audacity cannot report, then master it to pass in your browser. No re-recording. Nothing gets uploaded.
The ACX targets at a glance
ACX wants your narration between -23 and -18 dBFS RMS, your true peak at or below -3 dBFS, and your noise floor at or below -60 dBFS, delivered as a 44.1 kHz, 192 kbps CBR MP3. Test your own file against every target below, see exactly which spec failed, then master it to pass in your browser. No re-recording.
| Spec | ACX target |
|---|---|
| RMS loudness | -23 to -18 dBFS |
| True peak | At or below -3 dBFS |
| Noise floor | At or below -60 dBFS |
| Sample rate | 44.1 kHz |
| Channels | Mono or stereo, consistent across the book |
| Format and bitrate | MP3, 192 kbps CBR |
| Head silence | 0.5 to 1 second |
| Tail silence | 1 to 5 seconds |
Source: ACX: Audio Submission Requirements. For the full breakdown of what each number means, read the ACX audio specs guide.
The free ACX audio check uses a browser audio engine that needs a laptop or desktop. Come back on a bigger screen to check your file.
Drop your chapter files here
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Common questions
What does this tool measure?
RMS loudness, peak level, noise floor, head and tail silence, sample rate, channel count, and duration. Every value is checked against Audible's ACX submission spec, and each row shows pass or fail alongside your measured number and the target range.
Does my audio leave my computer?
No. The file is decoded and analysed inside your browser using ChapterPass's audio engine. Nothing is uploaded, stored, or sent to a server.
Why does my file still fail when Audacity says it is fine?
Audacity's built-in meters do not include the head and tail silence check, which is one of the most common reasons an ACX submission gets flagged. Your loudness, peak, and noise floor can all be in range while the file still fails because of the quiet lead-in or run-out.
Which file formats can I upload?
WAV, MP3, FLAC, AIFF, and M4A. If your browser cannot decode a file, a short error message will tell you which format to try instead.
Is the peak value the same as Audacity's ACX Check plugin?
Yes. We measure peak the same way the ACX Check Audacity plugin does, so the number you see here matches the number you would see inside Audacity.
What do I do if a check fails?
Start with the recording itself: reduce background noise by turning off HVAC and isolating from fans, re-record any clipped takes, and adjust input levels so the narration sits a few decibels below peak. The ACX rejection guide walks through each issue one by one. If you would rather skip the manual fix, ChapterPass will master the file to ACX spec in one step.
Do I need an account to use this?
No. No sign-up, no email, no card. Drop in a file and read the report.