1. A low-power security and monitoring system based on variation measurement of a sound transfer characteristic, comprising: a sound source generating device outputting a sound wave of an audible frequency band having a predetermined frequency band within a predetermined monitoring space; and a sound measuring device calculating a acoustic wave transfer function which is a unique characteristic value of the monitoring space by receiving the sound wave outputted from the sound source generating device, wherein the sound measuring device judges intrusion or no intrusion by comparing a acoustic wave transfer function measured in a monitoring mode with a acoustic wave transfer function measured in an initial setting mode.