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.
The problem with commercially available ultrasonic dog barking deterrents are they lack sufficient intensity and range to be effective against a neighbors barking dog. Although the dog next door might be able to hear the device, it would be the equivelent to a human hearing a wind-chime. The dog might respond at first, but merely out of curiosity then simply ignore the sound in some cases even become accustomed to it and enjoy it.
Commercially available ultrasonic dog barking deterrent = pleasant wind-chime to the dog next door.