The CLIX "Silent" Whistle is a lightweight, compact whistle, ideal for dog training, indoors or out. Ultrasonic whistles are particularly suited for training noise-sensitive breeds such as border collies. Although termed as a 'silent' whistle, the whistle has to be slightly audible to the human ear, in order for you to be able to effectively control your dog - as otherwise you would not be able to gauge the effect that it is having. Compact design. Great for noise-sensitive breeds. Adjustable pitch. Once adjusted please ensure the lock nut is fully tightened.