Abstract: The sound of breaking glass comprises a low frequency or thump sound at the moment of breakage, followed by a tinkle sound caused by collision of the glass fragments, this sound being of lower amplitude.The invention turns on the discovery that, irrespective of the size and shape of the glass and the characteristics of the surroundings, the thump has substantial low frequency components at about 350 Hz, and the tinkle has substantial high frequency components at about 6.5 KHz. The method of the invention lies in identifying the high and low frequency components in that order and separated by a short time interval.Apparatus according to the invention comprises a microphonefeeding into a high frequency and a low frequency channel. The low frequency channel has a bandpass filter centered on 350 Hz, the output of which triggers a time delay establishing a time window during which a signal is applied to one input of an AND gate. The high frequency channel includes a bandpass filter centered on 6.

Abstract: An alarm bell assembly wherein the working parts are located beneath and protected by the gong, and preferably also a cover for the assembly, being secured by means comprising a detector such as a micro-switch by which alarm drive is actuable when the bell is tampered with. A rechargeable battery is connected to an external control circuit such that the battery remains fully charged and supplies the alarm drive if the control circuit connection is severed.