An acoustic device called Boomerang and developed by BBN Technologies, Cambridge, Mass. (www.bbn.com), is protecting soldiers traveling in convoys against snipers. Using an array of five acoustic sensors, it picks up the supersonic shock wave created by the firing rifle, as well as the muzzle blast, which travels at the speed of sound. Boomerang compares the two signals and determines the azimuth from which the shot was fired to within 15°. U.S. soldiers use this data to target their weapons.

Boomerang's five sensors are mounted on an aluminum pole on about 50 Humvees in Iraq. A dashboard-mounted display gives visual and audio warnings of where the incoming rounds are coming from.

Boomerang detects AK-47 fire and rocket-propelled grenades launched from 50 to 150 m and takes only a second to come up with a heading. It works in urban environments and even while Humvees are moving at up to 60 mph over rough terrain.

The systems cost about $10,000, but BBN is trying to get that down to $3,000. Eventually, the military plans to network several Boomerang-equipped Humvees and precisely triangulate enemy positions.