Bat Detectors.
Above is from this design

using a circuit board from
but reverting most of the changes back to the muto design and vastly increasing the range and highest osc pitch, swapping the 40mhz transducer for a telephone pick-up / jack for plugging anything else into it, and making the lm386 200x gain (switchable). The case is some kind of acupuncture machine given to me by Nagako Takechi / MSBR. The meter doesn't really give any useful reading, but the needle moves with the amplitude.
A lot of spooks lurk around in the feedback, not just up there with the bats, as the first mp3 shows. The second mp3 demonstrates heterodyning feedback using a tiny plastic clockwork sushi given to me by Rudolf