These plug-in devices emit high-frequency sound undetectable to humans but supposedly confuse pests so they can’t feed, breed or communicate and flee the premises. But laboratory tests have shown that the majority of such devices do not work as advertised, in violation of Federal Trade Commission guidelines. What’s more, some scientists are concerned about the effect of ultrasound on humans. Ultrasonic pest repellers can also affect the signal of your telephone, burglar alarm and hearing aids. Also walls and other physical impediments diminish their effectiveness.