Re:Triangulation (Score 1)

I'm sure 3kHz is slow enough for interferometry to work. If we can do it with visible light sound is probably not an issue. Also most scratchy hissy sounds are around the 16kHz mark and I'm sure there are at least a few overtones in the ultrasonic range (still wavelength of 5-80cm). I'd imagine echoes from the borders of the table would be involved and other non-trivial bits, so I'd say a drawing surface is plausible (maybe not for detailed sketches but for something like a whiteboard).