ultrasonic presence detector

the device emits a continuous tone just above human hearing (~19.8 kHz). when nearby objects move, they shift the reflected sound back slightly higher or lower via the doppler effect. the microphone picks up both the direct tone and the reflected component — the difference becomes visible as deviation in the spectrum below. wave your hand in front of the device for an instant signal.

start below 20 kHz if your speakers lack high-frequency response · some dogs will object above 20 kHz

• no radar hardware needed. a commodity speaker and microphone in a phone are sufficient to detect human presence through a room.
• commercial applications include presence sensors in smart home devices (google home, meta portal) that use inaudible sonar to detect whether someone is in the room.
• privacy researchers demonstrated in 2021 that an ultrasonic sonar running in a browser could be used for cross-device tracking without any permissions on older browsers.
• sensitivity is real. on a quiet day, breathing from 1–2 metres away is sometimes measurable as a low-frequency modulation on the deviation metric.