The phone emits a short upward chirp from the speaker and listens for echoes on the mic. Cross-correlation against the chirp template turns the recording into a distance trace — point it at a wall and watch the peak slide.
It clicks rapidly. The chirp sweeps 4–14 kHz over 8 ms and repeats up to ~4×/sec. Best in a quiet room, hard surfaces, phone held vertical with the speaker pointing at the target. Headphones defeat the demo — this needs the speaker to put real sound into the room.
3.0 Hz
55%
6.0 m
0 m3 m6 m
Waterfall: time scrolls down. Bright stripes = persistent echoes (walls). Wandering stripes = a moving object (a hand, a person walking).