ACX requirements explained
ACX checks every file against measurable rules before a book reaches Audible. The complete list is on the ACX audio requirements page, and the essentials are below.
Loudness and peaks
Each file must measure between minus 23 dB and minus 18 dB RMS, with peaks no higher than minus 3 dB. These two values keep volume consistent between books.
Noise floor
Background noise must sit at or below minus 60 dB RMS. It also cannot be artificially silent, so gating a noisy room to digital zero will fail review.
File format
Files are delivered as MP3 at 192 kbps or higher, constant bit rate, 44.1 kHz, with a short stretch of room tone at the head and tail of each chapter.