Habituation
While many pests and animals can adapt to the frequency emitted by sonic repellers, this is not true with ultrasonic repellers. Ultrasonic repellers are a constant irritation to pests because the high-pitched noise affects their central nervous systems and causes them physical discomfort. Though not harmful, it does make it unpleasant enough to leave the treated areas.
Humans
Some humans with good high-frequency hearing "hear" the emissions as an annoying high-pitched or clicking constant "background" sound. Generally the emissions are silent to most humans.
Pets
Ultrasonic repellers designed to drive pests crazy may also annoy family dogs, cats and hamsters; most devices recommend placement away from pets.
Safe Houses
Ultrasonic sounds can't travel around furniture or structures or through drapes, establishing safe zones where pests can congregate. Try to target pests in specific areas such as attics, pantries, kitchens, basements or sheds, ensuring that most of the area is covered.