Abstract: A switch function is incorporated into an audible alarm by sensing obstruction of a sound emitting orifice opening into the chamber of an acoustic impedance matching network of the alarm. The alarm has an electromechanical sound transducer in acoustic communication with the chamber and an electronic drive circuit for driving the transducer. The circuit senses a change beyond a selected threshold of the sound pressure wave in the chamber caused by the obstruction. Some means by which the sound pressure change can be sensed are by sensing voltage, current or phase changes of the impedance of the transducer or by directly sensing the sound pressure wave by a microphone or the use of a third terminal on a piezoelectric transducer. The invention can also be used with acoustic frequencies outside the range of frequencies that are perceivable by human hearing and can be used as a proximity switch that switches in response to an obstruction or a gas or fluid stream.