First a clarification of terms (and notation). Ultrasonic, or ultrasound, waves are sound waves that have a frequency of over 20 kHz or (20000 Hz); hence its name "Ultra" ~ to go beyond others, and "sonic" or sound. 20 kHz is the upper bound for the average human ear, and thus waves with a frequency higher than 20kHz cannot be heard by the typical human. Humans, however, are not the most sensitive of auditory creatures. Many animals, such as dogs, cats, mice and other animals, can hear sounds with frequencies much higher than 20kHz. These animals can hear these ultrasonic waves and will typically find the sound to be painful. A good example is that of a dog whistle and the almost immediate reaction it will incur in dogs (and the other animals listed). A human example would be the equivalent to blowing a high pitched whistle in your home for extended periods of time. Will these ultrasonic waves repel pests? Typically yes. However care should be taken in choosing a device to generate these ultrasonic waves as those which only produce a single frequency will result in only temporary results unless you turn the device on and off. The reason for this is typically referred to as habituation. Habituation is the neurobiological result of over stimulation of a sensory organ. Habituation happens in all animals, and can be simply described as "learning to ignore". Pests will "learn to ignore" the sound produced by a single frequency device, much like people "learn to ignore" or "tune out" constant noise, such as those produced by your computer, refrigerator, or annoying next door neighbor. Devices which generate random patterns of high frequency noises will perform much better, through preventing the animal from habituation to any single frequency. Are the waves harmful to people? No, not at the levels normally associated with pest deturance. All waves can be harmful to people in high enough concentrations and prolonged exposure, but these waves will have no more effect on a human than normal sound does, only now you can't hear it. Ultrasonic waves are fairly safe, and are used in many medical procedures such as ultrasound images of infant children still in their mothers womb. Extra tidbit: A sound wave of less than 20Hz has a period (length from peak to peak) of over 17.19 meters long. Thats roughly 57 feet! A sound wave that long has to be generated by something extremely large, such as a boeing 747 taking off or landing.

Id like to start with my credentials for the right to answer this question. Ive been for many years and currently am a Pest Control Technician, I also have a degree in Urban and Industrial Pest Management/Entomology from Purdue University. To date there has been no research that im aware of showing positive results of the use of such electronic devices to repel or prevent pests or insects from entering your home. Ill just leave it at that.