An ultrasonic Dog & Fox repeller creates an uncomfortable environment that a Dog or Fox wants to get away from quickly. Most Dog & Fox repellers have an PIR motion detector, that when the animal moves infront of it, activates the ultrasonic noise that a Dogs & Foxes hates. It's not painful or cruel, just annoying to them. The ultrasonic sound produced is beyond that of Humans, so we can't hear it but animals can. For more information on animal hearing ranges, click here.
The most common reason for purchasing such a device is to stop Dogs & Foxes fouling on your lawn or flower bed. If you aim the ultrasonic repeller in the general direction of where the pest is pooing, it should work fine.
There are a few points to mention about positioning the ultrasonic Dog & Fox repeller. They don't work through bushes, shrubs, hedges or anything else that is an obstacle. The unit needs a clear line-of-sight for both the PIR motion sensor to be able to detect movement and the ultrasonic tone to be heard by the Dog or Fox.