unit 40 receives input signals from the ultrasonic receiving unit 10 and from the low frequency unit 30 the microcontroller 41 is configured according to the preferred embodiment of the present invention to determine whether said input signals were detected within an assigned time period and to generate an alarm signal 42 only if both of said received input signals were received within an assigned time period wherein said time period is preferably less than 2 seconds and more preferably is less than 1 second and even more preferably is less than 0 5 second according to another preferred embodiment of the present invention the microcontroller 41 is configured to determine if an input signal from the low frequency unit 30 was received before an input signal from the ultrasonic receiving unit 10 and to determine if both of said received input signals were detected within an assigned time period and to generate the alarm signal 42 only if the input signal from said low frequency unit 30 was received before the input signal from said ultrasonic receiving unit 10 and both of said received input signals were detected within an assigned time period wherein said time period is preferably less than 2 seconds and more preferably is less than 1 second and even more preferably is less than 0 5 second referring now to fig2 a flowchart 200 illustrates intrusion detection in accordance with the preferred embodiment of the present invention the flowchart 200 commences with a protected space being monitored for an intrusion of foreign objects as shown in step 201 in the case of the intrusion a lf acoustic signal and a doppler signal i e an ultrasonic acoustic signal with a doppler shift are generated step 202 and if both of them.