Frist Alert, 6 Pack, Hardwire Photoelectric Smoke Alarm With Battery Back Up, Contractor Pack, 120V Single Or Multiple Station Smoke Alarm Designed For Residential Home Use & Institutional Locations Such As Hospitals, Hotels, Motels, Dormitories & Other Multiple Family Dwellings, Interconnect able & Can Be Installed & Linked With Up To 18 Compatible Devices, Up To 12 Smoke Alarms & 6 Compatible Devices Like Bells, Horns & Repeaters, So That When One Triggers, All Alarms Sound, Emits Loud 85DB Audible Alarm, Photoelectric Smoke Sensor, Which Is Particularly Effective At Detecting Slow Smoldering Fires, Single Test/Silence Button Silences A Non-Threatening Alarm Or Low Battery Warning, Plus Allows You To Test The Unit's Functionality Quickly.