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....