1. A security monitoring system using beamforming acoustic imaging, comprising: an acoustic generating device that generates acoustic signals; a plurality of acoustic measuring devices that receive the acoustic signals; an acoustic image processing device that generates an acoustic image using a beamforming algorithm from the acoustic signals received in the plurality of acoustic measuring devices and determines a position of a trespasser by comparing the acoustic image before the trespasser is detected and the acoustic image after the trespasser is detected.