During this year’s Transcontinental Race I was chased by countless packs of dogs and was very traumatized by it all. There is nothing worse than having food poisoning, being exhausted from cycling 24+ h, and trying to dodge dogs. I fantasized about what to do with them but some of my more effective solutions—like taking my rifle with me—are not feasible. Let along legal. So I’ve decided to try a very, very loud horn—at around biting level.
After doing a lot of research I found the Hornet 140 decibel horn which claims to be the loudest bike horn you can purchase. It is seriously loud. To the point where it hurts. I ordered the horn with an extra-long trigger and that allowed me to mount the horn anywhere on my bike. I put the trigger up at the handlebar. The photos below show my setup. Will it work? I have no idea. Hopefully I will not need to find out. But one thing I do know, is that any dog that has a go at me will at least have trouble hearing cyclists in the future!
Chris Bennett's Bikepacking and Triathlon Blog 
There are a few horns that are ultrasonic and designed specifically for dogs, I read about this one on bikerumor a couple of years ago: http://www.bikerumor.com/2013/10/22/ib13-sound-defense-humanely-keeps-dogs-at-bay/, and Amazon has something similar: http://www.amazon.com/Dog-Dazer-II-Ultrasonic-Deterrent/dp/B000IBRI2Y. I haven’t personally used them, but they seem like a good idea. Does this mean that you’re preparing for another TCR? I don’t remember having any dog encounters when riding in NZ.
I got a ‘Dog Dazer’ but the issue with both of these is that you have to have your hands off the handlebars. The ‘Sound Defense’ looks interesting – I might see if I can do a hack. No, we don’t have dog problems in NZ but I was so traumatized after the TCR that I thought I’d put in place a plan for when I do something similar in the future. Hopefully another TCR one day …