A security apparatus comprising a plurality of sensing elements, each adapted to detect intrusion into protected premises, each sensing element outputs a sensing signal representing a detected event, a signal processing section for examining each sensing signal and outputting a signature for each sensing signal, a computing section for translating each signature into a normalized threat value, ranging from “0” to “1”, modifying each normalized threat values by multiplying a weighting coefficient corresponding to a type of sensing element, storing for a temporary period of time, each modified normalized threat value, and an alarm generating section for adding each of the stored modified normalized threat values, outputting an aggregate threat value and generating an alarm enable signal based upon an analysis of the aggregate threat value.