ACX Requirements
The technical audio specifications that Audible/ACX requires for all audiobook submissions, covering sample rate, bit depth, loudness, noise floor, peak level, and file format.
ACX (Audiobook Creation Exchange) is Amazon and Audible’s platform for producing and distributing audiobooks. All audiobooks submitted through ACX must meet specific technical requirements, and files that fail these checks are rejected during quality review. Understanding and consistently meeting these specs is essential for any narrator working with ACX.
The current ACX technical requirements are: sample rate of 44.1 kHz, bit depth of 16-bit, RMS level between -23dB and -18dB, noise floor of -60dB or lower, peak level no higher than -3dB, and final delivery in MP3 format at 192 kbps constant bit rate (CBR). Each chapter must be between 1 and 120 minutes long. Files must include a few seconds of room tone at the beginning and end. The opening credits file must include the title, author, and narrator name spoken aloud, and the closing credits must include an end statement.
Beyond the automated technical checks, ACX also conducts human quality review for issues like background noise, inconsistent audio quality, mispronunciations, and excessive mouth sounds. Meeting the technical specs gets your files past the automated gate, but producing professional-quality narration is what gets them through final review. Many narrators find it helpful to run their own quality checks before submitting, using metering tools to verify RMS, peak, and noise floor on each chapter.