The use of high frequency audio at a relatively intense level to influence a dog to cease barking.
Dogs are capable of hearing higher frequencies than humans. At sufficient volumes, frequencies above 25,000 Hz become irritating for dogs. The louder and higher those sounds are, the more uncomfortable for the dog they become. Dogs may whimper, whine and run away if confronted with a sufficiently loud and high-frequency sound.
Dog whistles can be in the range of 23,000 Hz or higher, however, lack the level of intensity to deter a dog from barking. Such a device is only useful for signaling a dog to perform simple acts with training. Sounds of a frequency between 23,000-25,000 Hz are inaudible to humans, but are tolerable for dogs.