a transducer switching system for , in effect , removing electrical energy from a transducer without breaking the electrical circuit . a resistor is in series with the transducer . an electro - mechanical switch shunts the resistor . the switch is within the enclosure containing the transducer . when the acoustic connector of a use - element , such as a stethoscope type headset for a listener , is inserted into the enclosure the switch is changed from its normally open state to a closed state , thereby shorting out the resistor and allowing an effective level of electrical power to operate the transducer . two systems may be simultaneously actuated for stereophonic sound .