Sound

Narrower sounds are gentler; the full band gives the sharpest peak. Longer bursts and more repeats help in noisy rooms. Use speakers, not headphones (the microphone has to hear it).

How long sound takes to go out of the speaker and back in through the microphone: the delay you'd correct for when recording along to playback. A burst of noise (a maximum-length sequence) plays, and the recording is searched for it by cross-correlation, to a fraction of a sample. Each repeat of the sequence is one measurement, and runs add up, so the spread shows how steady the audio path is. The browser's own figure covers only the output side.

The room is measured within the sound's band (reverberation differs by frequency). More repeats lower the noise. Keep still and quiet while it plays.

What the room does to a click. A burst of noise plays and the recording is matched against it, giving the sound arriving directly, then reflections off walls, floor, ceiling and furniture, then a fading tail. Each reflection's delay is extra distance travelled; with the speaker and microphone together, the surface is about half that away. The tail gives the reverberation time (RT60), how long the room takes to go 60 dB quieter.

Only the code has to match on both devices (leave it at 1 unless another pair nearby uses it). The beacon's sound doesn't: the listener finds the code in any band. Make the beacon the device with the louder speaker.

How far apart two devices are, by sound alone: one plays a code, the other listens. Start the two side by side, start listening, and hold still for a few seconds while the listener measures how far the two clocks drift apart; then move. The code only repeats every 234 m of path, so every reading is absolute; each extra 0.1 ms of delay is 3.4 cm. The listener follows the earliest strong arrival (the direct sound, even when a reflection is louder) and skips readings that jump further than anyone could move.