The Sabre Window Glass Alarm helps prevent intrusions through glass windows and glass sliding doors. The alarm detects vibrations on the glass, which is how it can tell when the window is tampered with or broken. A 100-decibel alarm then sounds to alert you and deter would-be intruders. High- and low-sensitivity settings are available to help prevent false triggering on windy days. Arm or disarm the alarm with a simple on/off switch. For indoor use only, with a warning sign visible outside. Easy to install. Battery test button. Batteries (included): 2 x CD2032 per alarm.