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
  1. There is no easy way to guarantee that only the mob and not other police will be affected.
  2. Colons probably differ enough from person to person for this device to be very selective.

May be an urban legend.

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