Google the hornet, it's 12v's and whistles all the time.
The insurance companies have tested the air powered ones, and say they're ineffective.
As for the
Hornet I try to watch out for deer, and when they're close to the road, I see them look at me, a large number will actively leave, I've seen them change directions as I got closer. I think it gives them good auditory input so they know where you are, and if you're heading towards them, or they're heading towards you.
On the other hand IMO they won't save you at 100Mph (maybe if you install 2 to give it more range), as you get there to fast. But I have them on both bikes, and I still use them after 7+ years.