ACX Audiobook Mastering — Meet Audible Standards Online

ACX requires MP3 files at 192 kbps CBR or higher, 44.1 kHz sample rate, mono channel, RMS between -23 and -18 dB, peaks no higher than -3 dB, and noise floor below -60 dB. ACX recommends 1 to 5 seconds of room tone at the start and end of each chapter, and no more than 5 seconds.

Yes. The Auto-Master feature normalizes RMS to -20 dB (the sweet spot within ACX range), limits peaks to -3.5 dB with a safety margin, applies noise reduction if needed, adds room tone, and converts to the required format — all automatically.

Yes. Upload all your chapter files: each one is mastered on its own, and any chapter can be made the reference so the others match its loudness, noise and tone. The finished files keep their names and download as one ZIP file.

Yes, with the Reference block. If the reference is your own approved chapter, the tone is matched fully so the chapters sound the same. If it is another narrator, the tone is matched only a third of the way and only above 400 Hz: the low end of a recording is the voice itself, and full matching would carry over someone else’s timbre (we checked this on 13 narrators). Loudness, noise in pauses, loudness spread and the silence at the edges are matched in both cases, within ACX limits. Pace and pauses are shown for information and left unchanged.

It fixes the three things that are a matter of level: loudness is brought to −20 dB RMS, peaks are limited under the ceiling, and the noise floor is pushed down. What it cannot fix is a room. If the failure is the noise floor and the recording was made next to a fridge or in a bare room with an echo, the level work only makes the noise louder along with the voice — that one has to be solved before the microphone, not after.

The tool uses the lightest noise reduction possible to achieve -60 dB. You can preview the result with A/B comparison before saving, and adjust the strength from Light to Heavy. If your recording environment is already quiet, no noise reduction may be needed.