The search for an easy way to do away with pests like rodents, insects and marauding deer and wandering cats and dogs can be ongoing. The ultrasonic repeller has several inconvenient side effects in addition to its questionable effectiveness.
Habituation
Pests can adapt to the frequency emitted by the repellers, creating a generation of resistant bugs; many devices include a frequency "sweeper" that varies frequency modulation and adds to its expense.
Humans
Some humans with good high-frequency hearing "hear" the emissions as an annoying high-pitched or clicking constant "background" sound.
Pets
Ultrasonic repellers designed to drive pests crazy may also annoy family dogs, cats and hamsters; most devices recommend placement away from pets.
Indiscriminate Repellant
Emissions may drive beneficial insects and creatures (like pollinating bees and soil-conditioning worms) away as effectively as it does annoying pests.
Safe Houses
Ultrasonic sounds can't travel around furniture or structures or through drapes, establishing safe zones where pests can congregate.
Hidden Costs
Providing electrical wiring and speaker placement for an adequate number of units may end up costing more money than the original devices and clutter recreational areas.