This is a 2N1 PERSONAL/DOOR ALARM. It acts as a burglar alarm and a personal alarm. Hang it on any door knob (non-metallic doors) and it instantly and automatically sounds when an intruder touches the door knob. Carry it with you and just pull out the metal chain to sound the alarm and deter an assailant. The alarm is 120 decibels. Requires one 9 volt battery. 4 1/4 " x 2 1/4 " x 1"