The device is fixed to a fence or part of a building, so that a good acoustic coupling is achieved. A sensor (2), e.g. a piezo crystal, is fixed to the casing of the device, but electrically isolated from it. The sensor converts mechanical oscillations of the fence into electrical signals. These signals are amplified by a pre-amplifier (3) and fed to an adjustable damping element (4). The signal is amplified by another amplifier (5) and transmitted by a cable to a central alarm office. There the signal is monitored by a watchman. Several alarms can be connected in a chain. Individual devices can be tested by activating an oscillator (9) via a reed switch (7) with a magnet (8). Feedback form the amplifier to a threshold switch (6) then causes an audible signal to be generated.
