Comment by tripzilch

I'm not very good with electronics, but someone who is showed me this cool device they built themselves. It was something like a "frequency-lowerer" (NO idea how that works, I know DSP but not the analog stuff) and the intended use was to be able to hear the screeches of bats. One way to test it was to jingle a bunch of keys in front of its mic, the metal clanging has a lot of ultrasound.

I dunno how hard they are to build (that guy was pretty skilled), but I think you can also just buy these devices ready made. Probably called "bat detectors" or something.