Abstract: A dynamic loudspeaker is driven by chopped d.c. from the collector of a transistor. A free wheeling diode is connected in parallel with the speaker circuit to dissipate the inductive energy in the speaker voice coil when the transistor is turned off. The transistor is controlled by a voltage controlled oscillator which, under control of other elements, can be made to produce a wide variety of cyclically varying output signals for producing a wide range of sounds, tones and signals from the loudspeaker. This permits selection of a tone which differs sufficiently from the ambient noise to assure attracting attention. By the use of chopped d.c. to drive the speaker, the expense, weight and bulk of an impedance matching transformer is eliminated. A plurality of individually operable switches are provided for switching circuit elements in and out of circuit to modify wave shapes and alter sound output.

Abstract: An electro-acoustic transducer, adapted to constitute part of an alarm unit, includes a magnetic diaphragm adjacent the core of the transducer. The case for the transducer is so configured that it forces the diaphragm into a concave shape, whereby the diaphragm operates extremely efficiently and provides other advantages over a planar diaphragm.

Abstract: A housing for a horn or like electro-acoustic element is provided, including a cover which will snap on the horn or the like by the application of slight pressure; however, removal of such cover from the horn requires significantly greater effort. Accordingly, a firm and long-lasting panel mounting is enabled.

Abstract: An electronic circuit including a transistor which is biased to conduction is provided to initiate actuation of an electromagnetic transducer. The transducer has first and second coils one of which actuates the sounder. A potential is induced in the other coil and this induced potential is used to cut off the transistor. The next cycle of operation is initiated when the induced potential falls below a predetermined value in response to the reduction of the magnetic flux which, in turn, was the result of the transistor having been cut off.