SOUND PATTERN DISCRIMINATION SYSTEM

Apparatus for the selective monitoring and recognition of sounds of different frequencies including a first detector for recognising signals of a first frequency and producing an output signal indicative thereof, at least one further sound detector for recognising a second sound signal of different frequency and producing an output signal indicative thereof, a time delay circuit responsive to the output signal from the first detector, and producing a time delayed enable signal to enable a further circuit connected to the output of the further detector to pass signals from the further detector through the further circuit in response to the enable signal and the output of the further circuit being coupled to a control circuit to produce a control signal only in response to the receipt of signals of said first and different frequency in a sequential time pattern established by the time delay circuit and said further circuit.