Ultrasonic devices emit sound waves that are above the range of human hearing, but they can be effective in deterring squirrels. These devices work by creating a loud, irritating noise that will scare away the squirrels. However, some people find that ultrasonic devices are ineffective and simply create a nuisance. If you are having trouble with squirrels in your yard, you may want to try a different type of squirrel repellent.