The PestBye Whole House rat & mouse repeller combines both ultrasonic and electro-magnetic technology. Both of these help to create environments where rats and mice can't be comfortable and so are discouraged from entering your home. Though inaudible to human ears the repeller will emit loud sounds at a frequency registered by rats and mice; it's a bit like having to listen to a smoke alarm going off constantly - we wouldn't stick around long if we had to put up with that!