ultrasonic device pairing

one device broadcasts a six-character session id at 18500–19500 hz; the other listens. the carrier sits above the typical limit of human hearing — most adults won’t hear anything, while phone microphones still capture it cleanly. requires speakers and a microphone on the same physical room.

some laptop speakers cannot reproduce above ~17 khz. if discovery fails, switch which device broadcasts.

session ids are 6 chars from a 32-char alphabet · 30 bits of entropy

• cross-device tracking via ultrasound is real. multiple ad-tech vendors use the technique in tv ads to identify nearby phones — research papers have catalogued dozens of such products in the wild.
• no permission for emit, mic permission for receive. any tab can play any frequency through your speakers. the asymmetry is the danger.
• this demo deliberately stays inaudible to most adults. if you can hear this, your ears are unusual — kids, dogs, and many cats will hear it clearly.