Sound produced at a frequency of 20kHz or above (very high pitched) is usually referred to as ultrasonic sound. Since humans cannot hear sounds above 20kHz in frequency, it is considered to be silent.
Many creatures, however, are perfectly able to hear frequencies much higher than the human hearing range. One that is very well known is the Bat, which will hunt its prey using echolocation - a system of detection using ultrasonic sound.
Other animals such as dogs, cats, foxes, badgers, rats and mice can also hear ultrasonic sound and, as such, it can be a very effective deterrent.
Powerful blasts of ultrasonic sound, directed at the creature, can cause intense discomfort or irritation, discouraging it from entering the area where the sound is being emitted.
For best effect, the area must be completely clear and unobstructed as the high-frequency sound is easily blocked or deflected. In addition, the sound becomes weaker the further it travels, so any devices employing ultrasonic technology will have a maximum effective distance (range) of up to 50-70 feet (15-20 metres).
When used indoors, the ultrasonic sound will be contained within the room, as it is unable to pass through the walls, floors and ceilings and is reflected back into the room.
Ultrasonic deterrents can be used against many types of pests and solutions take many forms: simple plug in units like the Pest Stop Single Room device, a mouse and rat repeller for use indoors, and outdoor units such as the versatile Pest Stop Outdoor, a repeller suitable for a range of different pests.
Used as a pest repeller, ultrasonic sound can also be used in conjunction with other effects. Products such as the Pest-Stop 4000 combine ultrasonic sound with an electromagnetic deterrent and the MP2B bird repeller includes a strobe light as an extra visual repellent effect for pigeons.