The Big Cheese Ultrasound Mouse and Rat Repeller works by producing over 100 decibels of ultrasound at constantly changing frequencies. The loud noises at these frequencies are very unpleasant for rodents but the frequencies used are too high for humans, dogs or cats to hear. The Big Cheese Ultrasound Mouse and Rat Repeller is designed to protect a house or similar sized building. The four units supplied need to be plugged directed into power sockets spread out around the building. Each unit will protect an area of about 400 square feet from rodent infestation. The information with the unit states that it may not work on rodents in a building ceiling, but we have found this not to be true, and they work especially well if placed in a loft. Do not use near rodents pets such as rabbits, hamsters, gerbils etc!