Magnetism: The alarm works thanks to the magnetism between the alarm transmitter and the magnetic receiver. The 90 dB siren will be triggered when the equipped door or window is opened (triggering distance: greater than 10 mm). Simple: the alarm sensor and the magnetic receiver are delivered with Scotch tape. No tools are required, just stick them on the door frame. Technical details: Transmitter dimensions: 9x3x2cm; Magnetic receiver: 5 x 1 x 1.4 cm - Batteries: 2 AA batteries per alarm (not included) Volume: 90 dB MULTIPLE USES: Increase security at your front door, on your patio or balcony, in your bedroom or in your children's room - this set of 4 alarms and receivers gives you the choice. Make sure your apartment or house has ideal conditions to set up a security system without accidentally triggering it. Note: The data is all measured manually, please allow 1-3 error cm Due to differences in light and display, there may be color differences, please refer to the actual item received.