What “ACX-spec” actually covers, the two limits you can break by writing rather than by mixing, and why meeting a file spec is not the same as being allowed to sell.
Lesson 4 · advanced · 6 min read
Open Audiobook StudioThere are two different questions hiding inside “will this be accepted”, and they have different answers. The first is whether the FILE is right — the encoding, the loudness, the silence at each end. The second is whether the SHOP allows a book narrated by a machine. The export settles the first completely and cannot settle the second at all.
Every chapter in an ACX package is encoded to the same spec, and none of it is a setting you touch.
The audio spec applied to every chapter during packaging.
Tip
Do not master the audio yourself first. Normalization, the peak ceiling and the room tone are applied during the export, so a file you have already loudness-matched is more likely to end up out of spec than in it.
Almost every requirement above is the encoder's job. Two are yours, because they are decided by how you split the book rather than how it is read — and neither is shown anywhere in the studio.
Careful
The usual casualty is front matter. A title page, a dedication or an epigraph detected as its own chapter narrates in a few seconds and falls under the floor. Merge short front matter into the chapter that follows it — the chapter list in import is where to fix it, before you narrate rather than after.
A package that meets every line of the table above can still be declined, because acceptance of AI narration is a policy each shop sets for itself. Audible currently routes most synthesized narration through its own Voice Replica programme rather than taking third-party AI narration, so the largest audiobook shop is also the one most likely to refuse a file that is technically perfect.
AI narration is broadly accepted on Spotify for Authors, Apple Books and Google Play Books, each of which asks you to disclose it — which is what the ai voice disclosure option in the export sheet writes into the package. Retailer policy moves faster than any guide, so check the current terms on the day you submit rather than trusting this paragraph.
Tip
Disclose it. Records in the package that the narration was synthesized. On by default, and worth leaving on: retailers increasingly ask. The failure mode for hiding it is not a refused submission — it is a published title pulled later, which costs far more than picking the right shop in the first place.
Questions