Abstract: An intruder alarm system of the type used to detect mechanical vibrations in a security fence or the like. Vibrations are assessed for alarm potential by detecting peak amplitudes against a dynamic vibration reference. A sufficient ratio between the two indicates an intrusion.

Abstract: The effects of wind noise signals from fence-mounted transducers are overcome by basing alarm indications on signal components in at least two narrow frequency bands centered on, say, 120 Hz and 240 Hz.

Abstract: Electric signals in two narrow frequency bands, centered on thirty-three and one hundred Hertz, arising from ground or airborne vibrations incident on one or more geophones are processed to give an alarm when signal components in the two bands exhibiting a rapid rise to greater than a predetermined amplitude level, characteristic of human footsteps within a protected area, occur at more than a predetermined repetition rate.

Abstract: Electric signals in two narrow frequency bands arising from ground or airborne vibrations incident on one or more geophones at the perimeter of an area to be protected are monitored and an alarm given if signal components that are indicative of human footsteps near the geophones occur in both frequency bands two or more times within a predetermined period. These signal components comprise bursts of oscillations having steep leading edges and durations of some thirty to one hundred milliseconds, and there are provided means to rectify these bursts and to detect resulting pulse waveforms.