Good day Horatio:
Regarding your range specification. It seems to me that the two most important factors to keep the dogs away would be the frequency of the sound, and the decibel level of the sound.
I have read that dogs normally respond to ultrasonic frequencies between 12-25 KHz, depending on the age of the dog. As you can reason, older dogs may be less sensitive to sounds based upon what ear damage has already occurred.
As far as the decibel level, some of the ultrasonic systems that I have seen, operate around 80-90 decibels, which isn't entirely too loud.
Consider this: Normal human conversation is between 30-60 decibels. A quiet recording studio would be around 10 decibels. Also, power tools and aircraft are usually around 110-130 decibels. (Source:
www.coolmath.com/decibels1.htm).
So with that frame of reference, 80-100 decibels might be a pretty good range, especially since dogs ears are more sensitive than human ears. The human ear threshold for pain is around 130 decibels, so I'm assuming that dogs may have a lower threshold for pain as well. An adjustable output decibel level would be preferred.
One final note: I'm not sure how close your neighbors are, and if they have pets. I imagine that if they have pets, and are close by, the sound may cause these pets to go into a frenzy. My point, you may be driving the dogs away, but you may also be driving the neighbors pet insane! I suggest checking this out with your neighbors.