Security on board

Ok , before I get shot down in flames for bringing up the subject of on-board security that no megayacht owner will ever reveal, I remember reading an article a while back where a passenger ship was cruising off the coast of Somalia was nearly boarded by a band of marauding, and heavily armed, pirates. The captain made use of the ship's so-called "decibel gun" that soon had the pirates wondering what had hit their eardrums and they sped off.

Anyone heard of a decibel gun?