Abstract: A security system for not enabling, enabling or disabling electrical devices for deterring theft, or preventing unauthorized use of such devices. The security system comprises a plurality of electrical devices which are operationally linked via a bi-directional communication medium, which may be via a mains power supply or a microwave or radiowave medium. Programmable means are provided for controlling operation of the operative parts of the electrical devices. The programmable means may have a signal transmitting and receiving means associated with it for transmitting and receiving control signals over the communication medium.

Abstract: A method of compensating for a reduction in output sound pressure level of an electrostatic loudspeaker, caused by front to back cancellation effects, using a resonant circuit and filter connected between the output of the audio power amplifier (8) and the input of the electrostatic loudspeaker transformer (1). The resonant circuit includes an inductor (3), capacitor (4) and resistor (5), the values of which are chosen so that the shape of the tuned circuit response curve closely mnatches the shape of that part of the loudspeaker response curve that requires compensation. The resonant circuit and filter may be combined with a metal oxide varistor or similar non-linear device to limit over-voltages.

Abstract: A security system (20) for not enabling, enabling or disabling electrical devices for deterring theft, or preventing unauthorised use, of such devices. The security system comprises a plurality of electrical devices (22, 24) which are operationally linked via a bi-directional communication medium, which may be via a mains power supply (26) or a microwave or radiowave medium. Each electrical device includes a programmable means (28, 30) for controlling operation of the operative parts (38, 40) of the electrical devices. Each programmable means has a signal transmitting and receiving means (32) associated with it for transmitting and receiving control signals over the communication medium (26), with the programmable means (28) of one of the electrical devices (22) being programmed as a controller for the other electrical device(s) (24).