Andy those devices are called motion detectors and when you are trapped under something such as a fallen beam and can not move (even though you can scream and make a lot of noise) it will put out a loud piercing alarm. The way it looks you make a lot of noise with very little motion.   Duane Brandt   retired member Galva Fire Dept.                         <!-- s;) --><img src="{SMILIES_PATH}/icon_wink.gif" alt=";)" title="Wink" /><!-- s;) -->