Abstract: A structure-bone sound detector suppresses unwanted signals in a frequency range to be monitored, to enhance the immunity of the detector to false alarms. The output signal from a sound sensor undergoes preprocessing, and is then fed to a pair of comb filters that are connected in parallel. The comb filters have mirror image filtering characteristics. The filtered output signals from the two comb filters are provided to a minimum value stage, which selects the smaller of the two output signals. This selected signal is further processed to detect an alarm condition. With this arrangement, broad band signals of interest will produce approximately the same outputs from each of the two comb filters, and therefore be passed on for further processing. In contrast, a narrow band interference signal will be suppressed by one of the two comb filters, and therefore not selected for further processing.

Abstract: The detector contains a structure-borne sound microphone connected to an electronic evaluation unit with a piezoelectric sensor (18) for structure-borne sound vibrations. This sensor has a vibrating bar (28), which is attached by means of hybrid technology to a carrier (27). The vibrating bar (28) is formed by a bimorph element and the carrier by an electrically conductive ceramic wafer.