Andy, we are neighbours.
I have just tried my amplifier and the ebay tweeter on my cat. Frequency 60KHz, I rms = 158mA, V = 6V rms , total power on tweater = 0.9W.
The cat will run away at about 2-4 metres and directly in the view of the tweater. She will sit in adjoning rooms quite happily.
The tweater says 150W, but anything over the power I have described above and it starts to make audible, crackling noises. Maybe I need to buy a proper horn tweater, not that cheap ebay crap.
Anyway, the cat is now scared and out of the house. I presume I need much much more power to cover the distance I need (neighbours dog, at least 50 metres away).