Audiobook Glossary
Plain definitions of the terms that appear across our guides without being explained. Each entry is a reference page: what the term means, the numbers attached to it where there are any, and how it affects an author publishing an audiobook. For the narrative version of any topic, follow the links out to the guides.
Production & Audio Specs
RMS and peak levels
Average loudness and loudest instant. ACX wants -23 to -18 dBFS RMS and peaks at or below -3 dBFS.
Noise floor
The constant background sound under a recording. It has to sit at -60 dBFS or lower, and it cannot be fixed afterwards.
Words per finished hour
The unit audiobooks are priced and scheduled in. Roughly 9,300 words of manuscript make one finished hour.
File Formats & Delivery
M4B
The MPEG-4 audiobook container: one file, chapter markers, and a player that remembers where you stopped.
Chapter markers
Named time offsets that give a player a navigable chapter list. They come from your manuscript's structure, not from the audio.
Distribution & Rights
ACX
Amazon's Audiobook Creation Exchange, the marketplace behind Audible and the source of the industry's de facto audio spec.
Wide vs exclusive distribution
One retailer at a higher royalty, or every retailer at a lower one. A rights decision, and only one direction is easy to reverse.
Royalty share
No upfront production cost in exchange for half your royalty, usually for seven years. Cheap if the book sells modestly.
ISBN for audiobooks
The audiobook is its own edition. Most authors never buy an ISBN, and the reason to is portability across retailers.