Cheap, compact sound weapons: This has inspired engineers at the US Anti-Terrorism Afloat programme, based in Washington DC, to develop a system that used sound waves both to spot intruders and to stop them. Their aim was to make the technology cheap and compact enough to deploy from patrol boats or place on the seabed and control remotely. Conventional ship-borne sonar systems were too cumbersome and power-hungry to be much use here. So the researchers have borrowed some ideas from geologists, who have used sound generators for underwater seismic surveys.
Undersea shockwaves: One system being assessed uses a high-voltage discharge between electrodes – basically a giant underwater spark plug to produce a high-pressure bubble of plasma. As the bubble collapses it sends out an intense acoustic shock wave. The other device is an “air gun” that creates a shock wave by the sudden release of compressed air. The plasma generator could be used to produce a continuous stream of pulses, while the simpler air system produces a sharp burst of noise. One of the main challenges was to control the effect the weapon on a diver. Ideally, the device would emit an audible, low-powered warning when an intruder was sighted but it should also be capable of creating progressively more severe effects if an intruder persists. “At high enough intensity, it would probably cause people to surface.”
New Scientist, 30/4/2007, p.34-35
Source: Erisk Net