There are two main categories of training whistles; audible ones that both people and dogs can hear and inaudible ones that seem to be silent, but in actuality, they discharge a sound frequency that is too high for the human ear to perceive, but dogs can. Dogs can hear frequencies at least three times higher than humans. They respond to sound, so verbal commands, clickers and whistles are effective for training. Whistles, especially inaudible ones, are the least exhausting training tool for instructors and are the least irritating to people around them. You can alter the frequency of some whistles to distinguish between various commands, and dogs are highly receptive to the changes and can associate each sound with its respective instruction.