Question 1

What are the ACX audio requirements?

Accepted Answer

ACX (Audible) requires every audiobook file to have a peak level no higher than −3 dBFS, an average (RMS) level between −23 and −18 dBFS, and a noise floor at or below −60 dBFS. Files must also be submitted as 192 kbps or higher MP3, but the three level measurements are where most automated rejections happen.

Question 2

How do I check my audio against ACX requirements for free?

Accepted Answer

Drop a WAV, MP3, M4A, FLAC, or OGG file into VoxPrompter’s free ACX checker at vox-prompter.com/acx-check. It measures peak, RMS, and noise floor in your browser and compares them to the ACX limits. Nothing is uploaded — the analysis runs entirely on your own machine.

Question 3

How is the noise floor measured?

Accepted Answer

The same way Audacity’s ACX Check plugin measures it: the RMS of the quietest half-second window anywhere in the file. That window is your room tone, so it is the honest measure of how quiet your recording space is.

Question 4

Why did ACX reject my audiobook audio?

Accepted Answer

The most common causes are peaks above −3 dB (lower your interface gain), RMS outside −23 to −18 dB (normalize loudness toward −20 dB RMS), and a noise floor above −60 dB (treat the room, eliminate fans and computer noise, or apply gentle noise reduction). A free level check before submission catches all three.

Free ACX check — test your audiobook audio against Audible's requirements

What ACX requires

Record takes that pass the first time