Reply to post: And because there's very little background noise in the near-ultrasonic range

And because there's very little background noise in the near-ultrasonic range

Interesting.

I've heard that when TV's had ultrasound remote controls they could be fooled by jingling coins. Presumably the "chink" produced noise that was broadband and loud enough to spoof the (very simple) channel changing protocol used.

I imagine your system is much more resistant.