3-Pack Safety Wedge Door Stop Alarm Stopper Blocking Travel Home Security Alert 120db (pack of 3) H-M-L, 3 Levels of Sensitivity adjustment, stands for High-Middle-Low Also works as a wedge to prevent door from opening when powered off. No installation required. Requires one 9 volt battery. Portable and easy way to add piece of mind when traveling or working alone. Extra loud alarm. 120 decibel alarm sounds when pressure is applied to door stop.