The Elro Blue Line CBFS58R Duo Pack is a set of 2 linkable smoke detectors that ensure you receive a warning in case of fire on 2 floors or throughout an entire apartment. You can connect the detectors to each other without needing a Wi-Fi signal. Both detectors will go off simultaneously if there is smoke detected anywhere in the house. The smoke detectors have a replaceable battery that lasts for 1 year, so you do not need to replace the smoke detector itself. The smoke detectors emit an alarm signal of 85 decibels, allowing you to hear the alarm clearly from the floor where it is installed.