Alan B. asked if glass break detectors are a good addition to a monitored home security system, since I am not a fan of window contacts.
For residential applications, I am not a fan of glass break detectors. Just like with window contacts, every installation is unique, and there are instances where either or both devises may be appropriate.
In general glass break detection is a good part of a security system in a retail application where there is a threat of a “smash and grab” scenario. The burglar can break the glass of the store and take merchandise without entering the premise.
In a residential application, it is unlikely that someone will break a window, reach in to grab something, and leave without entering the home. So when they do enter the home with your alarm system armed in the “away” mode, they will be picked up by your motion detector and immediately send the system into alarm. If you are in the home and the system is armed in the “stay” mode without your motion detection active, you will hear the very loud sound of glass breaking and hit the panic button on your system, which will sound your alarm and bring law enforcement to your home.
The problem with glass break detection in homes is that it is the biggest cause of false alarms. The device is tripped by sound that is within the decibel and frequency range that is created when glass is broken. The problem is two fold. First, if you drop a glass or plate in your home while your system is armed, in addition to the startling affect of dropping something, you now have the added startling affect of your security system siren activating. Second, there are many noises like dogs barking, children screaming and motorcycles driving by your home that will set your alarm off. If you live by one of our military bases that are practicing blowing things up, your alarm is likely to trip.
When folks experience multiple false alarms, they get fined by their local municipality, and tend to stop using their system. So whatever benefit is gained by these devices are lost if you stop arming your system.