[Dual Function]: If someone tries to open the door, this product will prevent the door from opening and sound a 120dB alarm. You can also use it as a door stopper when you turn off the ON/OFF switch. Its highly shock-absorbing rubber bearing can prevent the door from sliding and prevent bad guys from entering. Security Alarm 120dB Loudness: The alarm will sound when someone opens the door, the 120dB continuous siren is a great deterrent to thieves or intruders. Easy installation: 1) Insert a 9V battery; 2) Select the sensitivity level: M (medium), L (low), H (high); 3) Turn on the alarm; 4) Insert the alarm under the door. The gap between the bottom of the door and the floor should be between 10mm (0.38 in) and 35mm (1.35 in). 3 Sensitivity Levels: With an adjustable sensitivity switch on the side, 3 sensitivity levels are optional, M (medium), L (low), H (high). Multiple Applications: Easy to carry, size 5.3*1.8*1.4inch(13.5*4.5*3.5cm) Net weight 4.2oz(120g).Great for travel;ideal for bedrooms, hotels, apartments, dorms, front doors, etc;peace of mind when traveling or working alone.