ACX audiobook check

What ACX requires, and how this check measures it

RMS is the average level of the recording. ACX publishes it as an RMS range, not a LUFS target, and there is no official LUFS equivalent. This check uses the same convention as the ACX Check plugin and FFmpeg, in which a full-scale sine wave reads −3.01 dB RMS. Meters that follow the AES17 convention read about 3 dB higher, which is one reason ACX warns that readings can differ between tools.

Peak. ACX does not say whether its −3 dB limit means sample peak or true peak. The pass mark here uses the sample peak, as ACX Check does, and the true peak is shown for information. Because MP3 encoding can raise peaks slightly, mastering to a true peak around −3.5 dB and re-checking the exported MP3 leaves a safe margin. LufsReady's ACX preset does exactly that.

Noise floor is the level of the quietest moment, usually the room tone between sentences. The ACX Check documentation describes it as the quietest 500 ms. The plugin's code uses a 400 ms window moved in 100 ms steps, and so does this check. A floor below about −90 dB usually means gated or digitally silent pauses, which ACX Check flags because dead silence sounds unnatural.

Also required by ACX (not checked here)

• Each file must be a 192 kbps or higher constant bit rate MP3 at 44.1 kHz. Joint stereo is not accepted.
• All files must use the same channel format: all mono or all stereo.
• One chapter or section per file, no longer than 120 minutes. Opening and closing credits are separate files.
• ACX recommends 1 to 5 seconds of room tone at the beginning and end of each file, and gaps of room tone must not exceed 5 seconds.
• A retail sample of 5 minutes or less, taken from the first five minutes of the audiobook.

Requirements are taken from ACX's Audio Submission Requirements page, checked on 23 October 2026. LufsReady is not affiliated with ACX or Audible. For other platforms, see the loudness targets reference, and if a chapter has distortion, run it through the clipping detector.