referring first to fig1 the intruder alarm system comprises one or more electro - mechanical transducers , represented by the block 1 , which are arranged to provide electric signals in response to mechanical or acoustic vibrations incident upon said transducers . these transducers , sometimes known as geophones , may be attached to posts or walls or buried in the ground within the area to be protected by the alarm system , and may be connected either in common or individually to broadband amplifiers represented by the block 2 . in response to footsteps or other causes of ground - borne or acoustic vibrations within or near the protected area the transducers 1 receive vibrations and derive analogous electric signals , extending over a range of frequencies , the signals of interest for intruder detection ranging from say 15 to 150 hertz . the electric signals , after amplification , are applied to two narrow - band filters 3 and 4 having pass - bands some 12 and 37 hertz wide respectively centred on 33 hertz and 100 hertz respectively . the output signals from these filters are envelope detected , or halfwave rectified , at 5 and 6 respectively and the rectified signals applied to respective circuits 7 and 8 for detecting elements in these signals which indicate a human footstep , one of these circuits being shown diagrammatically in fig2 . the rectified signals in general comprise a succession of spikes representing a background of vibrations incident upon the transducers 1 , each spike made up of or extending over a few half - cycles of the selected frequency . rectified signals including components originating from an impact , such as a footstep , on the surface of the ground within range of the transducers 1 have superimposed on this background a pulse signal having a steep leading edge and a duration typically of the order of 30 to 100 milliseconds , and the system may be arranged so that two such pulse signals occurring within an interval of , say , 5 seconds gives rise to an alarm indication . referring now to fig2 which shows one of the circuits 7 and 8 , rectified signals are applied directly to a unity - gain stage 9 and , by way of voltage dividing resistors 10 and 11 , to an input of an envelope shaping circuit 12 . the stage 9 is followed by a d . c . restorer and compressor stage 13 . the envelope shaping circuit 12 comprises an amplifier 14 having a feedback path to its inverting input by way of a resistor 15 and a connection to that inverting input by way of a resistor 16 from a negative bias voltage source , such that the output signals of the amplifier 14 , at the cathode of a diode 17 in its output path , are set at a d . c . level some 30 to 50 millivolts positive with respect to earth . the proportion , some two thirds , of the rectified signal voltage that is applied to the circuit 12 is superimposed on this d . c . level at the cathode of the diode 17 , and the composite signal is applied by way of a resistor 18 to charge a capacitor 19 . the time constant of the resistor 18 with the capacitor 19 is several times as long as the rise time of any significant component of the rectified signals , and the effect is to produce a voltage waveform across the capacitor 19 which tends to follow that at the cathode of the diode 17 but with slowed rise - times . when the instantaneous value of the composite waveform falls , the capacitor 19 discharges by way of a resistor 10 which has a value an order of magnitude greater than that of the resistor 18 , so that the fall times tend to be slowed more than the rise times . it will be appreciated therefore that when a pulse signal occurs the voltage waveform applied by the circuit 12 to the inverting input of a differential amplifier 21 , which amplifier acts as a comparator , will start from a higher base level but rise at a slower rate than the original signal , as represented by the output signal from the stage 13 . the instantaneous value of the waveform from the circuit 12 will therefore be higher than that of the original signal except when this original signal rises quickly for more than a predetermined time , the rate of rise being restricted of course by the narrow bandwidth . these exceptions are characteristic of the vibration patterns which are of interest in detecting intruders . if the comparator amplifier 21 detects one of these high - rising transients the resulting voltage changeover at its output applies a positive potential to an integrator circuit 22 by way of a diode 23 and resistor 24 whereby the output voltage of the integrator circuit 22 commences going negative . if the trnsient is the leading edge of a pulse of longer than a given duration , which would be characteristic of signals arising from a human footstep , the output voltage of the integrator 22 passes the voltage set by divider resistors 25 and 26 in an input circuit of a second comparator 27 , and this comparator 27 thereupon changes its output state to register the event . when the pulse ends the integrator 22 is returned to its initial condition by current flow through a resistor 28 and a diode 29 into the output of the amplifier 21 . referring again to fig1 it has been found that human footsteps within a given range of the transducers 1 tend to give rise to these longer pulses within both the 30 hertz band and the 100 hertz band , whereas other sources of vibration do not . apart from slight dispersion effects which cause the 100 hertz components to arrive at the transducer 1 before the 33 hertz components , a human footstep within limited range of the transducers therefore gives rise to coincident events , as detected by the respective comparators 27 , and a circuit 30 is provided to detect and count such coincidences . this circuit 30 is shown in more detail in fig3 . since just two events in 5 seconds may be sufficient to justify an alarm being given the circuit 30 comprises a monostable circuit 31 which resets itself after a 5 second delay . a first event is then arranged to set this monostable circuit 31 and a second event occurring before the monostable circuit has reset is detected by an &# 34 ; and &# 34 ; gate 32 , which is arranged to apply an alarm signal to an indicator 35 , which may for example comprise a warning lamp and an audible alarm . to overcome the dispersion effect a 100 hertz event is arranged to set a monostable circuit 33 , which resets itself after a period off 150 milliseconds . if a 33 hertz event occurs within this period a coincident event output is provided by an &# 34 ; and &# 34 ; gate 34 . it will be appreciated that the transducers 1 may be arranged either to detect footsteps within a general area or within a strip - like area defining the perimeter of a protected area , the two events in 5 seconds facility being particularly aimed at detecting intruders crossing a perimeter strip .