Why It Reaches What Nothing Else Could
When he falls asleep, the muscles holding his lower jaw forward go slack. The jaw slides backward, the tongue follows, and the airway behind them collapses into a narrow slit. Air rushing through that slit is what makes the snore. Earplugs, white noise, nasal strips, sleeping in another room — none of them can reach a collapse that's happening behind his teeth.
The mouthpiece sits gently over his bottom teeth and holds the lower jaw a few millimeters forward of where it would otherwise slide. The airway stays open, the air flows past the soft tissue smoothly instead of shaking it, and the noise has nowhere to come from.