A communication device comprises a processing circuit having at least two modes, a sleep mode and an awake mode, a wireless communications circuit that can wirelessly send a message as to whether an alarm has been triggered, and a passive sensor, powered by audio signals impinging on the passive sensor, that provides at least an approximation of an audio signal to the processing circuit so as to cause the processing circuit to switch between the at least two modes. The communication device can be housed in a housing sized to fit into a battery compartment.