Just attach the main unit to the glass surface and watch the window opening and closing from intrusion. When you detect either side, the alarm will sound loud (about 90 decibels of sound pressure) for about 25 seconds to protect your home from unlawful intrusion. The mounting surface of the main unit is visible from the outside, so it has a suppression effect. Size: Main unit size (thickness x height x width): 0.4 x 3.9 x 2.0 inches (9 x 100 x 50 mm); Package size: 4.3 x 7.1 inches (110 x 180 mm); Weight: Approx. 1.4 oz (40 g) (main unit only); Sound pressure: Approx. 3 years; Material: ABS resin.