Presumably, a sound at the
resonant frequency of the
human colon, about 11
Hz. When played at high
volume, it can act as a kind of
bowel disruptor. The
french police looked into making a
riot control weapon out of one of these in the late 1960s. I suspect that even if it worked, it didn't take on because
- There is no easy way to guarantee that only the mob and not other police will be affected.
- Colons probably differ enough from person to person for this device to be very selective.
May be an urban legend.