The ultrasonic, or sonic, repellant is a battery-powered or plug-in device that is worn or set near people to repel mosquitoes by emitting sounds to frighten away the female insects (only the females bite humans). However, these devices have not been scientifically proven to work, nor are the sounds proven to scare away the female. In fact, the Federal Trade Commission charged one manufacturer with making false and unsubstantiated claims because it had no evidence supporting the product claims.