Spec library.

The technical specs we work to, kept in one place. Every project also has its own Script and Spec folder, because publishers often add their own limits on top. Where they do, theirs win.

ACX and Audible audiobooks

What ACX checks before it accepts a finished audiobook.

File format	MP3, constant bit rate, 192 kbps or higher, 44.1 kHz (256 or 320 kbps also accepted)
Channels	All files mono, or all files stereo. Don’t mix them in one book
Loudness	RMS between −23 dB and −18 dB
Peaks	Below −3 dB
Noise floor	Below −60 dB RMS
Room tone	1 to 5 seconds at the start and end of every file, and no gap longer than 5 seconds
File length	Under 120 minutes each
Structure	One chapter or section per file, each starting with its section header. Opening and closing credits and the retail sample are separate files
Retail sample	5 minutes or less
Narration	Human narration only

Download as PDF

From the ACX audio submission requirements (page last updated 15 April 2026). ACX can change them, so check their page before a big delivery.

More specs are on the way. Working to a different publisher or platform? Tell us and we’ll check it for you.