Sensitivity variations in linear microphonic cables

Linear microphonic cables are used as the sensor for many Perimeter Intrusion Detection Systems (PIDS). These cables transform vibrational energy into electrical energy which is then analysed to detect an intrusion attempt. Usually the cables are attached to a fence to sense the vibrations generated by cutting through or climbing over the fence fabric. This paper will not refer to the application where microphonic cables are buried in the ground. It is important that the response from microphonic cables is uniform. Manufacturers of cables often stress the linearity of response of their cables (in terms of variation along the cable's length.) Any areas of above-average sensitivity will lead to a propensity to false alarms. Conversely, any stretches which have below-average sensitivity will be more susceptible to being defeated. Although cable manufacturers say that their products are linear along their length, some anomalous, unexpected results are found once the cable is attached to a welded-mesh fence and the response is tested. If physical impacts are made repeatedly, in exactly the same place, using a "standard" impact device, then the electrical response generated seems to vary at that point by only around 5%. This implies that the standard impact device is reasonably repeatable. However, if the response is tested along a zone then the variation is much greater. This variation has always been seen, to some degree, when microphonic cables, of all types, are attached to fences. The variation is the subject of the paper. The variation seems to be relatively random, with only a little correlation in the response between adjacent fence bays. Although it seems unlikely that an attacker would be able to take advantage of the variation, it seems that it will still lead to difficulties in obtaining optimum performance from the system. If the variation could be eliminated then presumably some false alarms caused by the bays that are oversensitive could be eliminated, and some of the missed detections in the bays, that are under-sensitive could be captured.