A dog whistle is a type of whistle used in the training of dogs. The frequency range of a dog whistle is largely out of the range of human hearing. Typically, a dog whistle is within the range of 5800 Hz to 12400 Hz. Some dog whistles have adjustable sliders for active control of the frequency produced.
Depending on the way the whistle is used, a trainer may simply gather a dog's attention or inflict pain for the purpose of behavior modification.
The name dog whistle is often used for both lung-powered whistles as well as electronic devices that emit ultrasonic sound via piezoelectric emitters. The electronic variety are sometimes coupled with bark detection circuits in an effort to curb barking behavior.
I've seen a look in dogs' eyes, a quickly vanishing look of amazed contempt, and I am convinced that basically dogs think humans are nuts.
Paper and canvas prints of "Growing Up Chinese Shar-Pei" by Barbara Keith are available online.