ACX audiobook audio requirements, explained.

Audio level and format requirements

ACX currently requires each uploaded file to be between −23 dB and −18 dB RMS, with peaks no higher than −3 dB, and a noise floor no higher than −60 dB RMS. It requires consistent mono or stereo files and MP3 at 192 kbps or higher, constant bitrate, at 44.1 kHz. These refer to RMS and peak measurements, not interchangeable LUFS targets. Verify the latest rules before uploading.

Source: ACX audio submission requirements.

Technical compliance is not the whole job

Even if an automated checker reports passing levels, a listener may still hear mouth clicks, claps, repeated takes, misreads, page turns or distracting room noise. Professional editing and a proof listen against the manuscript address a different part of quality control.

Chapters and required credits

ACX requires a separate file for each section or chapter, with section headers. Opening and closing credits and a retail sample also form part of submission. Check that credits align with the audiobook title, author and narrator metadata. ACX also specifies chapter/file-length limits and room-tone guidance.

Human narration and publishing authorization

ACX states that audio submitted through its service must be human-narrated unless otherwise authorized. Distribution requirements can vary across retailers and programs, so confirm eligibility for the particular release and platform rather than assuming every platform follows ACX rules.

A useful pre-submission checklist

• All words match the approved manuscript, including pickups
• No repeated takes, click markers or spoken recording instructions remain
• Chapter heads and tails have consistent room tone and spacing
• Files meet the stated technical format, RMS, peak and noise-floor checks
• Credits and metadata are consistent
• Final files have been listened to, not just checked by software

Learn about editing and mastering or human proof listening.