Not sure, I never could hear one. I expect it depends more on the trainer than the whistle. To me it is silence, to the dog, noise. The different tweets or noises the whistle makes has to have some meaning attached to it and it is up to the trainer to provide that meaning. I remember seeing a family where the father used a whistle to control his children but that was some kind of navy whistle. It worked on kids so it is bound to work on dogs provided the trainer knows what he is doing. But you knew all this didn't you and also knew I was being a smart so and so. I did or have trained three dogs to be good boys and girls. I bought a dog whistle but it was more confusing to me than the dogs, I suspect and I tossed the whistle in my junk drawer and used hand signals and rewards. People did comment how smart my dog was but that was probably only after they had talked to me.