This 2 in 1 personal door alarm acts as a burglar alarm and a personal alarm. The personal door alarm can hang on a doorknob ( non-metallic doors) and it instantly and automatically sounds when an intruder touches the doorknob. With the personal door alarm you can can carry it with you and just put out a metal chain to sound the alarm to deter an assailant. The alarm has a 120 dB noise level and requires one 9 V battery. Dimensions are 4 1/4" X 2 1/4" x 1".