FAQ / Meeting ACX audio standards
How do I meet ACX audiobook audio standards?
ACX wants each chapter as a single MP3 file at 192 kbps or higher (CBR), 44.1 kHz, with RMS between -23 dB and -18 dB, peaks no higher than -3 dB, a noise floor no louder than -60 dB RMS, room tone at head and tail, and consistent loudness across files. Get the room right, record clean paragraphs, and the export step handles the rest.
ACX requirements have shifted over time and may be different by the time you submit. Always check the current ACX submission guidelines before mastering. Treat the numbers below as a working baseline, not a substitute for the official spec.
Targets you can actually hit
- File format: 192 kbps CBR MP3, 44.1 kHz, mono is allowed. Export from VoiceOverAndOver as MP3 to land here.
- RMS (average level): -23 to -18 dB. Loudness equalization at -16 to -18 LUFS in the merge step puts you in the right neighborhood.
- Peak ceiling: -3 dB. Stay away from peaks above this; a brick-wall limiter is not strictly required but a small headroom safety margin during merge is wise.
- Noise floor: -60 dB RMS or quieter. This is a room problem, not a software problem. Treat the room.
- Room tone: 1 to 5 seconds of silence at the start and end. Record this once and reuse.
- Consistency: every chapter file at roughly the same RMS. Use the same loudness target for every export.
Get the room under -60 dB
If you hear traffic, the fridge compressor, or your computer fan in a quiet section of your recording, you are above the noise floor. Fix the room first: close windows, shut HVAC off during takes, move the computer out of the booth or into a sound-isolated box. Software noise reduction is a last resort and almost always audible on close listening.
Record paragraph by paragraph for safety
ACX rejections almost always trigger on one bad chapter, not the whole book. VoiceOverAndOver records each paragraph as its own file. If chapter 7 paragraph 12 is the problem, re-record paragraph 12, remerge chapter 7, resubmit. You do not have to touch the rest of the book.
Use the same export settings every time
On every chapter, export to MP3 at the same quality and use the same loudness target in the merge step. Save the project. If ACX comes back with a tweak, you can rebuild the same chapter with the same settings on the first try.
Head and tail room tone
Record one paragraph of pure room tone at the start of each session. Drop it as a paragraph at the start and end of each chapter. The merge will glue it in place at the right level. Same room tone every chapter keeps your noise floor consistent.