I have a three level home and I have 5 of these devices. They wirelessly link up. If the detector is activated on the lowest level, I will hear it on the 3rd level. I do recommend this product. I do think it is expensive though. It is of good quality, but I did have two of them over the past 7 years become defective. First alert with replaced a defective one that was within the warranty period.
Great alarm! I really like and recommend interconnected alarms alerting in all areas of home. This is a non hardwired alarm making installation very easy in a older home. Plus you have added bonus of CO alarm in every room instead of just the areas with potential CO hazard. Highly recommended by this firefighter.