Do ultrasonic dog repellers actually work against strays and other loose dogs?

Question

In answers to a recent question, ultrasonic dog repellers were suggested as a means of self defence against stray dogs rather than weapons. I'm also concerned with loose farm dogs, and live somewhere with very strict weapons laws (so bear spray is out of the question). A trekking pole I or kayak paddle may have some value in fending off a dog but are rather unwieldy if you're on a bike (for example). The omnidirectional sort are what I have in mind, as they could be clipped to gear/taped to handlebars and used immediately.

Most of what I can find online is arguments for and against using them to train or discipline your own dog or a noisy nuisance dog. So do they work for stopping (feral) dogs from approaching? Do they make dogs back off?

Answer 1

Steve Gibson's experience seems to be yes, but only at close range.

From “The Quiet Canine” Electronic Dog Whistle - Acoustic Management of Unruly Dogs from a Distance:

But in no event was this able to function as any sort of barking deterrent. Dogs heard it, and at any distance, they didn't care. We soon came to appreciate that my own original “point blank” blasting of the original “Portable Dog Killer” (as I named my first device when I was in high school) was required for the device's effectiveness. No dog next door, let alone down the block, will care about a high pitched sound. It needs to be blasted directly into the dog's face at a very short distance.

This means that while this device would not be useful for silencing dogs at a distance, it would likely be extremely useful and effective as a personal defense device for walkers, postal workers on foot and joggers who are harassed and threatened by overly aggressive canines on the loose. Although we cannot and do not offer any specific guarantees, it is difficult to see how any attacking dog would not be stopped in its tracks by a close blast of incredibly loud and high pitched sound.

(Emphasis mine)

Background story going back to 1971 (Youtube video)