What happens to each file
- Low cut. An 80 Hz high-pass filter removes rumble, handling noise and some hum, which also stops them counting towards the peaks.
- Loudness. The file is measured, then gained so its RMS sits at your target. A few short stretches are run through the whole chain first to learn how much loudness the limiter will take, and the gain is corrected for it.
- Peaks. A limiter holds every peak under the ceiling (−3.5 dB by default), half a decibel under ACX’s −3 dB so the MP3 encoder can’t push one over.
- Pauses. When the background noise, raised by the gain, would come near ACX’s −60 dB, a gentle gate turns the pauses down. The voice isn’t touched.
- Room tone. ACX asks for 1 to 5 seconds of room tone before the narration and after it. Missing room tone is added from the quietest half-second of your own recording; too much is trimmed.
- Format. 192 kbps constant bitrate MP3 at 44.1 kHz, mono or stereo for the whole book.
ACX’s audio requirements
| Check | ACX asks for | Mastering aims for |
|---|---|---|
| RMS loudness | −23 to −18 dB | −20 dB |
| Peak level | −3 dB or lower | −3.5 dB |
| Noise floor | −60 dB or lower | Pauses gated when needed |
| Room tone | 1–5 s at the start and at the end | 1.5 s and 3 s |
| Format | 192 kbps or higher CBR MP3, 44.1 kHz | 192 kbps CBR, 44.1 kHz |
Mastering sets levels; it doesn’t remove mouth clicks, loud breaths, harsh “s” sounds or plosive pops, which ACX listens for too. Run narration clean-up on your edited chapters first, then master them.
Mastering in Audacity instead? The same chain, step by step with its settings, is in Audacity for audiobooks.
Not sure what’s failing? Run your files through the ACX checker first; it shows every check and where the loud peaks are. audiobook.tools is not affiliated with ACX or Audible.