A knock on the bedroom door
Headphones on, door closed, world off — and you still catch the knock.
The WAP's mic doesn't record; it listens for shape. Three or more evenly spaced raps inside two seconds become a Pattern: Knock signal in Home Assistant, and the app chimes in your headphones. Turn on the opt-in flag (FEATURE_ACOUSTIC_TRANSIENTS) when you flash.
The mic path reduces sound to three loudness figures per 20 ms and zeroes the buffer. The word "knock" is the only thing that ever leaves the device.
A coin-cell vibration sensor on the door itself? That board doesn't exist yet — vote it forward.