The Big Cheese Mouse and Rat Repeller 3 pack uses ultrasonic sound waves to create a distressing noise audible to rodents only. The nature of sound waves mean that any obstruction creates a barrier and the waves cannot go any further. Mice tend to venture back into areas where they may find food. To avoid this, best practise before using the repeller is to thoroughly clean the area and remove all possible food sources. The repellers are a deterrent only and so vermin may venture back occasionally against the distressing noise. You may also wish to consider alternative methods of control such as catch and release traps or traditional baits and traps. Warning: Never use sonic repellers around rodent pets such as gerbils, hamsters and rabbits. Safe to use near children, cats and dogs. No poisons, no traps.