since I have a unit that I use on the dogs next door when they continuously bark. Works on the small one, but only makes the large dog bark more! Works just the opposite on the 2 across the street. The big one immediately heads for the back yard and the little one continues with the barking. I guess different dog breeds have different hearing frequencies too. My loss is a result of 1) being old 2) having a silver blast firecracker blow up inches from my ears in my teens and 3) operating power tools with no ear protection. no. 1 is not my fault:). 2&3 can be contributed to being stupid:) It's all coming back to haunt me now. What???