The Sonic Fence

Painful sound waves could keep wetsuit-clad terrorists away from ships

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Since the bombing of the U.S.S. Cole in 2000, protecting docked ships-both military and commercial-has been a big priority in the fight against terrorism. The Department of Homeland Security has already awarded $489 million to help guard the nation´s ports, spurring a number of innovative ideas, the latest of which is an underwater system that blasts enemy swimmers with painful acoustic waves. Patented by the Raytheon Corporation last October, the system is the brainchild of former Navy and Raytheon acoustics expert Frederick Di Napoli. His scheme is simple: Generate a region of high-pressure, low-frequency sound around the ship, creating a sort of sonic fence that â€shocks†anything that swims through it. Although a diver would probably flee from pain, Di Napoli says, "you could really dial up the pressure and make it lethal if you had to."

Raytheon has no plans to pursue the idea but hasn´t ruled out the possibility of building a version of it in the future. Here, how the fence works.

How It Works:

  1. Find Enemy: New advanced sonar systems, such as the SM2000 from Konigsberg Mesotech in Vancouver, Canada, distinguish among divers, large fish and submersibles. In this case, sonar picks up the intruder, tracks his path, and relays this information back to the ship.
  2. Test Water: After the threat is identified and the intruder´s path predicted, the sonic fence system kicks in. First, an acoustic projector tests the water conditions, sending a low-pressure signal to a hydrophone below the ship.
  3. Adjust Signal: After receiving the test signal, the hydrophone relays it to a "waveform" processor aboard the ship, which analyzes it to determine how water conditions such as salin-
    ity or tem-perature modified the signal. The operator must account for these variables to ensure that the sound "peaks" in the right location.
  4. Blast Enemy: As the diver gets closer, the operator activates the system. The acoustic generator pumps out high-pressure sound waves, creating a sonic fence about 65 feet in diam-eter around the ship. Inaudible to humans, the frequency of the sound is tuned to produce nausea and even vomiting in humans