A NEW sonic weapon being developed for the Pentagon makes use of one of the most fearsome sounds known to humans: a baby crying.

Once aimed at the enemy, a focused beam of the familiar noise, played backwards, will be painful enough to make enemy soldiers run for their lives, according to its developers. They call it the ‘‘sonic bullet’’.

There are 50 soundtracks to choose from, and they are played at 140 decibels. That is akin to standing beneath a passenger jet as it takes off. Hardly surprising, then, that the weapon causes an intense headache in its victim.

For years armies have used sound, speech, or radio broadcasts as a propaganda weapon. US troops invading Panama blasted the besieged General Manuel Noriega with loud rock music. Nazi Germany used sound in torture.

But the "hypersonic sound system" beams sound along two ultrasonic signals to produce noise only when they hit the target. The weapon’s user doesn’t hear a thing.

It has been suggested for use in crowd-control, to clean al-Qaeda terrorists out of caves, or as a ship-to-ship device that goes one better than the shot across the bows.

A hand-held version of the weapon - a thin tube about a yard long, dubbed a "directed stick radiator" - might in theory be used against hijackers in a plane cockpit, though the bouncing beams could deafen passengers as well.

The firm which developed the system, American Technology Corporation, has won approval from the US Food and Drug Administration to market it. It also hopes to offer the device for use in drinks machines. Someone in front of one would hear the sound of a can opening with an enticing fizz.