Abstract: A standing wave field is generated within a protected space, the standing wave field comprising a plurality of signals traveling along a plurality of propagation paths within the protected space, the propagation paths including reflections of the signals off of objects within the protected space. A change in a predetermined characteristic of the standing wave field is detected upon the changing of a physical characteristic of an object within said protected space, and an alarm signal is generated when said change in the standing wave field is greater than a predetermined threshold value.