Accidentally set it off in my garage (I thought it was off) and it hurt my ears it was so loud.FYI: Make sure to securely fasten the battery tray (ie superglue, duct tape, screws) because an intruder could easily knock off the battery panel and turn is off and you want to make sure that does not happen or at least it takes them a long enough time for you or neighbors to hear.If your home security system is WIFI (which goes out if the WIFI goes out or gets hacked like mine did last month) this is a good old school back up. I just received my second one and will be ordering more.