ACX audiobook checker

Will every chapter pass ACX’s levels?

Drop in one chapter or the whole book. Each file’s RMS, peak, noise floor, room tone and format are checked against what ACX requires, with the gain that fixes a file that’s out, and the book is checked for mixed mono and stereo, uneven levels and duplicate files. Files are read a piece at a time in your browser. Nothing is uploaded.

Drop one chapter, or every file in the book

MP3 and WAV at any length; FLAC and other formats up to 64 MB a file. Measured one file at a time in your browser; nothing is uploaded.

What ACX requires

Every file must measure between −23 and −18 dB RMS, with peaks no higher than −3 dB and a noise floor no higher than −60 dB RMS. Files must be 192 kb/s or higher constant bitrate MP3s at 44.1 kHz, no longer than 120 minutes, one chapter or section each, and all mono or all stereo.

Room tone changed in ACX’s current requirements (dated 15 April 2026): ACX recommends 1 to 5 seconds at the beginning and end of each file, and room tone must not exceed 5 seconds. Older pages and many guides still say half a second to a second at the head. Spotify for Authors’ guide still does, and the tool checks each platform against its own figure. The two only agree on exactly 1 second at the head (and 1 to 5 at the tail), so that suits both.

How it’s measured

ACX doesn’t publish how it measures; its own FAQ says RMS can be measured several ways and that your DAW may disagree with its analysis tool. So this tool measures the way most narrators already do, with the method of Audacity’s ACX Check plug-in, read from its source: RMS over every sample of the file, unweighted, with both channels counted together; peak as the largest sample; noise floor as the quietest 0.4 seconds. It agrees with ffmpeg to 0.01 dB on WAV files.

Because another meter can read a file a fraction differently, anything within 0.2 dB of a limit is marked “check” rather than passed or failed. Room tone is our own measure, since no platform says how it measures room tone: narration starts at the first 10 ms that’s louder than halfway (in dB) between the file’s noise floor and its RMS.

RMS here is the whole file’s, room tone included, as in ACX Check. That’s why a chapter can read a decibel or so below what your meter showed during the narration itself.

What a meter can’t tell you

ACX’s QA team also listens for plosives, mouth noise, clicks, outtakes, missing chapter headers and uneven sound. ACX’s own Audio Lab doesn’t check the noise floor either, which this tool does. Passing here means the numbers are right, not that the book will pass review.