Abstract: A behind the detector sounder has a base plate and walls which define mechanical horns having throats which open into a space at the centre of the base plate. The mouths of the mechanical horns are at the outer periphery of the base plate and the walls bound sound paths which are longer than the straight line distance from the throat of each mechanical horn to the mouth of that mechanical horn.

Abstract: A behind the detector sounder has a base plate and walls which define mechanical horns having throats which open into a space at the centre of the base plate. The mouths of the mechanical horns are at the outer periphery of the base plate and the walls bound sound paths which are longer than the straight line distance from the throat of each mechanical horn to the mouth of that mechanical horn.

Abstract: A portable alarm device which includes a casing containing one or more of the following; means for detecting that the device has been moved, means for detecting a person in the vicinity of the device and means for detecting changes in ambient light level. The device further includes a scrolling system which makes each of said means available in turn for receiving commands that are inputted manually. The device has a code accepting unit which receives an inputted code to bring the device to a condition in which a selected one of said means can be activated. The device is deactivated by insertion of said code for a second time. By providing the device with a sound generator and a sound receiver it is possible to interconnect the devices so that activating or deactivating one is used to activate or deactivate a series. Likewise the establishment of an alarm condition at one device can be used to trigger alarm conditions at other devices.

Abstract: A sounder is disclosed which has walling (36, 38) defining a first air column (26) leading to a first sound aperture (34) through which the first column opens to atmosphere, and walling (40, 44) defining a second air column (60) leading to a second aperture (64) through which the second column opens to atmosphere. A diaphragm (16) is between the air columns and means are provided for causing said diaphragm to vibrate.