RMS and Peak Levels
RMS is the average loudness of an audio file over time. Peak is the level of its single loudest instant. Audiobook distributors check both, because a file can sit at a comfortable average and still contain one moment loud enough to clip, or hit safe peaks while being too quiet to listen to in a car.
Both are measured in dBFS, decibels relative to full scale. Full scale is 0, and every usable level is a negative number. Quieter means further from zero, so -23 dBFS is quieter than -18 dBFS.
The numbers ACX enforces
| Measure | Requirement | What it is protecting against |
|---|---|---|
| RMS | -23 to -18 dBFS | Files too quiet to hear in a car, or so loud they fatigue over ten hours |
| Peak | -3 dBFS or lower | Clipping and distortion on consumer playback hardware |
The RMS window is deliberately narrow. A five-decibel band across an entire audiobook means the loudness has to be consistent from chapter one to the end, not merely correct on average. This is the single most common reason a technically clean recording is rejected: each chapter is fine in isolation, but chapter fourteen was recorded on a different day and sits two decibels hotter.
Why -3 dBFS and not 0
The three decibels between the peak ceiling and full scale are headroom. Lossy encoding to MP3 or AAC can push individual samples slightly above their original level, so a file that peaks at exactly 0 dBFS before encoding can clip after it. Leaving the gap means the encoded file that the listener actually receives stays clean.
RMS is not LUFS
Both describe average loudness and they are not interchangeable. RMS is a straight mathematical average of signal power. LUFS applies a perceptual weighting that reflects how human hearing responds across frequencies, which is why streaming platforms normalise to LUFS while ACX still specifies RMS. A file can meet an LUFS target and miss the ACX RMS window. If you are submitting to ACX, measure what ACX measures.
What this means when you are buying production
Meeting these two numbers is not an optional polish step, it is the difference between a file that distributes and one that is rejected before a human hears it. Ask any producer what RMS and peak they deliver at. A producer who cannot answer in numbers is not measuring.