This unit has been designed to release an ultrasonic wave that will transmit 20feet. The ultrasonic sound emitted by Dog Off Trainer gets dogs attention and stops them in their tracks. As a defensive measure against an attacking dog, when the attacking dog approaches, hold the repellent button down and aiming directly at it. The sound is irritating to dogs, but is completely safe, humane, and inaudible to humans. Dog Off Trainer can be handy for joggers, postal workers and meter readers