Dual-sensor smoke alarm features photoelectric & ionization sensors for the best overall protection. Ionization Sensor may detect invisible fire particles (associated with flaming fires) sooner than photoelectric alarms. Photoelectric Sensor may detect visible particles (associated with smoldering fires) sooner than ionization alarms. Hush feature temporarily silences nuisance alarms. Battery-operated smoke alarm provides continuous protection even during power outages. Easy-to-install smoke alarm includes a 9 volt battery. No wiring is required.