DogAway

Dog Away Repeller Ultrasonic Whistle Pro+ is a self-protection application that keeps away stray or aggressive dogs.

This unique application generates an invisible shield between you and any dog on the loose. The shield is merely an ultrasonic high frequency sound unheard by the human ear but extremely unpleasant to dogs. Dogs seek to run away from that ultrasonic sound, with no harm done to them or to you.
It may be used for teaching your dog prohibitive commands as well.

This application provides multiple frequencies. Usually the higher the frequency the stronger the deterring effect upon dogs, regardless that at one point the human ear stops detecting the ultrasonic sound, i.e. the less you hear the more repelling it is to dogs.