What the checker measures
These are the technical requirements in the ACX audio submission rules. Most audiobook distributors, including Spotify for Authors (formerly Findaway Voices), ask for the same or very similar numbers.
RMS loudness
RMS is the average level across the whole file. Below -23 dB listeners turn the volume up and hear the noise; above -18 dB the narration sounds squashed. Around -20 dB is the comfortable middle. If you are too quiet, raise the gain or use a loudness normalizer, then put a limiter last so the peaks stay legal.
Peak level
The single loudest sample must stay at -3 dB or below. A limiter with its ceiling at -3.5 dB is the usual fix, and the extra half decibel covers the small overshoot that MP3 encoding adds.
Noise floor
The noise floor is the level of your room when nobody is speaking. The checker reports the quietest half second of the file, which is how most ACX tools read it. If it is above -60 dB, record a few seconds of room tone, use it as the noise profile for gentle noise reduction, and remember that every decibel of gain you add to fix RMS also raises the noise.
Room tone at the start and end
Each file needs 1 to 5 seconds of quiet room sound before the first word and after the last. Use real room tone rather than digital silence, which sounds like a dropout. The checker counts everything below -50 dB as quiet.
Format, sample rate and length
ACX takes MP3 files at 192 kbps or higher with a constant bit rate (CBR) and a 44.1 kHz sample rate, one chapter per file, up to 120 minutes each. Mono or stereo are both fine, but every file in the book must use the same. If you drop several files at once, the checker compares them for you.
What a pass does not cover
The numbers are the part a machine can check. ACX also listens for mouth clicks, plosives, background sounds, edits you can hear and chapters that sound different from each other. It also asks for opening and closing credits as separate files and a retail sample of up to five minutes. Treat a pass here as the first gate, then listen to every chapter on headphones.
Common questions
Are my files uploaded?
No. The checker reads and measures the files inside your browser. Nothing is sent to VoicedBook or anyone else, so it also works for unpublished books.
Which files can I check?
MP3 and WAV files. You can drop a whole folder of chapters at once, and the checker also compares the files with each other for channel format, sample rate, bit rate and loudness.
If every check passes, will ACX accept my audiobook?
It means the technical numbers fit. ACX also listens for quality, such as mouth noise, clipped words, edits you can hear and consistent sound between chapters, and it has rules for credits, samples and file names.
Why does my result differ a little from another checker?
Tools measure the noise floor and room tone in slightly different ways. That is why the checker marks values within 0.5 dB of a limit as close. Aim for the middle of each range and small differences will not matter.
Does Spotify for Authors use the same specs?
Its audio requirements are very close to ACX: MP3 at 192 kbps constant bit rate, 44.1 kHz, one chapter per file, with similar levels and room tone. A file that passes here is in good shape for most audiobook distributors.
Does ACX accept AI narration?
ACX requires narration by a human, except for the virtual voice program Amazon runs itself. VoicedBook makes AI narrated audiobooks for Spotify for Authors, Google Play Books and Kobo, which accept digital narration with disclosure. This checker works for any audiobook file, including books you record yourself.
VoicedBook is not affiliated with ACX, Audible or Amazon. Specs as published by ACX in November 2026.