Abstract: A multimode perimeter intrusion detection system comprises at least one elongated electret tape mounted on a barrier, such as a fence or wall, along the perimeter of an area to be protected and operated simultaneously in both active and passive modes to detect intruders in proximity to, as well as in contact with, the barrier. The tape is directly mechanically coupled to the barrier above and parallel to the ground or floor and is electrically connected to a sound wave generator and to signal processing apparatus which detects signals produced on the tape by intruders and activates an alarm. In the active mode, the tape radiates sound waves, preferably in the ultrasonic range, outwardly from the barrier and receives doppler-shifted reflections from a person moving in the sound field for transmission to the signal processing apparatus.