The ones I have listen for the thud of something hitting the glass followed by the higher frequencies of falling glass. A ziplock bag full of Legos tossed on the floor sets them off every time. This is one of my son's favorite things to do. Also, loud banging sounds will sometimes set them off. I was assembling a metal shelving system last night and pounded on it with a hammer, and that set it off.
I have mine on 24 hour zones. If someone breaks a window when I'm home, I want it to go off whether or not the alarm is armed.
I hadn't thought about having them on 24 hour zones. Do you have it contact the central station if they go off? Any good wireless ones that work with the Elk M1XRF2G?
Thanks,
Kevin