Ultrasonic repellent devices have been marketed for decades as a control for rodents pests such as mice. These devices produce high frequency (15 to 19 kilohertz) or ultrasonic (above 19 kilohertz) sound waves in an effort to repel rodents. Theories on how they work range from causing fear, disorientation and physical pain to mimicking alarm signals.
Effects of sound
Extremely loud sounds (120 to 150 decibels) can cause seizures and even death in laboratory mice. However, noise at this level is also damaging to human ears and cannot be used in repelling mice. Noise repellents use higher frequencies than humans can hear in attempts to repel mice. Sound also dissipates rapidly away from its source. Noise 60 cm (2 feet) away from a source is only one-quarter the strength of the noise 30 cm (1 foot) away from the source. At 1.2 m (4 feet), the sound is only one-sixteenth as strong.
Wild vs laboratory mice
Tests in laboratory species such as mice and rats have shown that noise is a more effective repellent of laboratory animals than wild animals. Wild Norway rats, for instance, are only temporarily repelled by ultrasonic devices, as reported in the 1984 study at the University of Nebraska. They eventually overcome their fear or apprehension of the repellent and go about their normal business.
Habituation is a psychology term that refers to an organism "getting used to" something and overcoming fear or apprehension. This seems to occur when ultrasonic repellent devices are used to repel mice under normal conditions. The initial use of the device is usually successful in displacing mice, but, as they get used to the noise, they overcome their fear of it and may reinhabit areas that were previously abandoned.
Humans can hear sound at frequencies up to about 23,000 Hz (Hertz). Louisiana State University researchers report that mice and rats are sensitive to sound up to 60,000 and to about 76,000 Hz, respectively. However, common house pets such as cats and dogs can also hear ultrasonic frequencies. Cats can perceive sound up to about 64,000 Hz, and dogs perceive sounds up to 45,000 Hz. You should consider not using noise as a repellent for mice if you have pets.
Discovery News reports that mice communicate with high frequency sounds beyond the range of human hearing. Ultrasonic repellents may work by disrupting the ability of mice to communicate by drowning out their sounds. Male mice sing ultrasonic mating songs, and ultrasonic devices might interfere with the reproduction of mice. Mice also communicate distress or alarm calls ultrasonically. Mice subjected to ultrasonic repellent devices may interpret the sound as a distress or alarm call and leave the area.