Review on Hoont Electronic Dog Repellent and Trainer with LED Flashlight - Powerful Sonic + Ultrasonic Dog Deterrent and Bark Stopper by Kara Hans

I work in a residential home and deal with a lot of dogs and the device works about half the time. I can hear him whistling, so I have to imagine he's not big enough. And it's unlikely to work on a large aggressive dog. You seem to be hearing and noticing, but it's not at all what my dad had 20 years ago. My dad's ultrasound machine was completely inaudible and EVERY dog would wag its tail and run from a great distance, through fences and windows. This device does NONE of those things. They must be 5 feet away with no obstacles. I still have to have an aggressive dog jumper with me.